-#LyX 1.6.5 created this file. For more info see http://www.lyx.org/
+#LyX 1.6.7 created this file. For more info see http://www.lyx.org/
\lyxformat 345
\begin_document
\begin_header
\end_layout
\begin_layout Date
-
-\change_deleted 0 1283307542
-26-July
-\change_inserted 0 1284423485
-14-September
-\change_unchanged
--2010
+1-December-2010
\end_layout
\begin_layout Abstract
\begin_layout Subsubsection
Proposed Solution
-\change_inserted 0 1284422789
-
\begin_inset CommandInset label
LatexCommand label
name "attributes"
\end_inset
-\change_unchanged
-
\end_layout
\begin_layout Standard
of the union.
\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Complete.
+\end_layout
+
\begin_layout Subsection
tdb_traverse Makes Impossible Guarantees
\end_layout
You can prevent changes by using a transaction or the locking API.
\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Complete.
+ Delete-during-traverse will still delete every record, too (assuming no
+ other changes).
+\end_layout
+
\begin_layout Subsection
Nesting of Transactions Is Fraught
\end_layout
-obscure case.
\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Incomplete; nesting flag is still defined as per tdb1.
+\end_layout
+
\begin_layout Subsection
Incorrect Hash Function is Not Detected
\end_layout
hash function produces the same answer, or fail the tdb_open call.
\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Complete.
+\end_layout
+
\begin_layout Subsection
tdb_set_max_dead/TDB_VOLATILE Expose Implementation
\end_layout
tuning, but initially will become a no-op.
\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Incomplete.
+ TDB_VOLATILE still defined, but implementation should fail on unknown flags
+ to be future-proof.
+\end_layout
+
\begin_layout Subsection
\begin_inset CommandInset label
LatexCommand label
an API.
\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Incomplete.
+\end_layout
+
\begin_layout Subsection
TDB API Is Not POSIX Thread-safe
\end_layout
\begin_layout Standard
The aim is that building tdb with -DTDB_PTHREAD will result in a pthread-safe
version of the library, and otherwise no overhead will exist.
-
-\change_inserted 0 1284016998
Alternatively, a hooking mechanism similar to that proposed for
\begin_inset CommandInset ref
LatexCommand ref
\end_inset
could be used to enable pthread locking at runtime.
-\change_unchanged
+\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Incomplete.
\end_layout
\begin_layout Subsection
It also keeps the complexity out of the API, and in ctdbd where it is needed.
\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Incomplete.
+\end_layout
+
\begin_layout Subsection
tdb_chainlock Functions Expose Implementation
\end_layout
\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Incomplete.
+\end_layout
+
\begin_layout Subsection
The API Uses Gratuitous Typedefs, Capitals
\end_layout
the API/ABI.
\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Complete.
+\end_layout
+
\begin_layout Subsection
Various Callback Functions Are Not Typesafe
\end_layout
See CCAN's typesafe_cb module at http://ccan.ozlabs.org/info/typesafe_cb.html
\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Incomplete.
+\end_layout
+
\begin_layout Subsection
TDB_CLEAR_IF_FIRST Must Be Specified On All Opens, tdb_reopen_all Problematic
\end_layout
\end_inset
.
-\change_inserted 0 1284015637
+\end_layout
+\begin_layout Subsubsection
+Status
\end_layout
-\begin_layout Subsection
+\begin_layout Standard
+Incomplete, TDB_CLEAR_IF_FIRST still defined, but not implemented.
+\end_layout
-\change_inserted 0 1284015716
+\begin_layout Subsection
Extending The Header Is Difficult
\end_layout
\begin_layout Standard
-
-\change_inserted 0 1284015906
We have reserved (zeroed) words in the TDB header, which can be used for
future features.
If the future features are compulsory, the version number must be updated
\end_layout
\begin_layout Subsubsection
-
-\change_inserted 0 1284015637
Proposed Solution
\end_layout
\begin_layout Standard
-
-\change_inserted 0 1284016114
The header should contain a
\begin_inset Quotes eld
\end_inset
\end_layout
\begin_layout Enumerate
-
-\change_inserted 0 1284016149
The lower part reflects the format variant understood by code accessing
the database.
\end_layout
\begin_layout Enumerate
-
-\change_inserted 0 1284016639
The upper part reflects the format variant you must understand to write
to the database (otherwise you can only open for reading).
\end_layout
\begin_layout Standard
-
-\change_inserted 0 1284016821
The latter field can only be written at creation time, the former should
be written under the OPEN_LOCK when opening the database for writing, if
the variant of the code is lower than the current lowest variant.
\end_layout
\begin_layout Standard
-
-\change_inserted 0 1284016803
This should allow backwards-compatible features to be added, and detection
if older code (which doesn't understand the feature) writes to the database.
-\change_deleted 0 1284016101
+\end_layout
+\begin_layout Subsubsection
+Status
\end_layout
-\begin_layout Subsection
+\begin_layout Standard
+Incomplete.
+\end_layout
-\change_inserted 0 1284015634
+\begin_layout Subsection
Record Headers Are Not Expandible
\end_layout
\begin_layout Standard
-
-\change_inserted 0 1284015634
If we later want to add (say) checksums on keys and data, it would require
another format change, which we'd like to avoid.
\end_layout
\begin_layout Subsubsection
-
-\change_inserted 0 1284015634
Proposed Solution
\end_layout
\begin_layout Standard
-
-\change_inserted 0 1284422552
We often have extra padding at the tail of a record.
If we ensure that the first byte (if any) of this padding is zero, we will
have a way for future changes to detect code which doesn't understand a
not present on that record.
\end_layout
-\begin_layout Subsection
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Incomplete.
+\end_layout
-\change_inserted 0 1284422568
+\begin_layout Subsection
TDB Does Not Use Talloc
\end_layout
\begin_layout Standard
-
-\change_inserted 0 1284422646
Many users of TDB (particularly Samba) use the talloc allocator, and thus
have to wrap TDB in a talloc context to use it conveniently.
\end_layout
\begin_layout Subsubsection
-
-\change_inserted 0 1284422656
Proposed Solution
\end_layout
\begin_layout Standard
-
-\change_inserted 0 1284423065
The allocation within TDB is not complicated enough to justify the use of
talloc, and I am reluctant to force another (excellent) library on TDB
users.
\end_layout
\begin_layout Standard
-
-\change_inserted 0 1284423042
This would form a talloc heirarchy as expected, but the caller would still
have to attach a destructor to the tdb context returned from tdb_open to
close it.
All TDB_DATA fields would be children of the tdb_context, and the caller
would still have to manage them (using talloc_free() or talloc_steal()).
-\change_unchanged
+\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Deferred.
\end_layout
\begin_layout Section
point.
\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Incomplete; TDB_CLEAR_IF_FIRST still defined, but does nothing.
+\end_layout
+
\begin_layout Subsection
TDB Files Have a 4G Limit
\end_layout
be erased and initialized as a fresh tdb!)
\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Complete.
+\end_layout
+
\begin_layout Subsection
TDB Records Have a 4G Limit
\end_layout
).
\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Complete.
+\end_layout
+
\begin_layout Subsection
Hash Size Is Determined At TDB Creation Time
\end_layout
\end_layout
\begin_layout Subsubsection
-
-\change_inserted 0 1283336713
\begin_inset CommandInset label
LatexCommand label
name "sub:Hash-Size-Solution"
\end_inset
-
-\change_unchanged
Proposed Solution
\end_layout
, it became clear that it is hard to beat a straight linear hash table which
doubles in size when it reaches saturation.
-
-\change_deleted 0 1283307675
-There are three details which become important:
-\end_layout
-
-\begin_layout Enumerate
-
-\change_deleted 0 1283307675
-On encountering a full bucket, we use the next bucket.
-\end_layout
-
-\begin_layout Enumerate
-
-\change_deleted 0 1283307675
-Extra hash bits are stored with the offset, to reduce comparisons.
-\end_layout
-
-\begin_layout Enumerate
-
-\change_deleted 0 1283307675
-A marker entry is used on deleting an entry.
-\end_layout
-
-\begin_layout Standard
-
-\change_deleted 0 1283307675
-The doubling of the table must be done under a transaction; we will not
- reduce it on deletion, so it will be an unusual case.
- It will either be placed at the head (other entries will be moved out the
- way so we can expand).
- We could have a pointer in the header to the current hashtable location,
- but that pointer would have to be read frequently to check for hashtable
- moves.
-\end_layout
-
-\begin_layout Standard
-
-\change_deleted 0 1283307675
-The locking for this is slightly more complex than the chained case; we
- currently have one lock per bucket, and that means we would need to expand
- the lock if we overflow to the next bucket.
- The frequency of such collisions will effect our locking heuristics: we
- can always lock more buckets than we need.
-\end_layout
-
-\begin_layout Standard
-
-\change_deleted 0 1283307675
-One possible optimization is to only re-check the hash size on an insert
- or a lookup miss.
-
-\change_inserted 0 1283307770
Unfortunately, altering the hash table introduces serious locking complications
: the entire hash table needs to be locked to enlarge the hash table, and
others might be holding locks.
\end_layout
\begin_layout Standard
-
-\change_inserted 0 1283336187
Thus an expanding layered hash will be used: an array of hash groups, with
each hash group exploding into pointers to lower hash groups once it fills,
turning into a hash tree.
\end_layout
\begin_layout Standard
-
-\change_inserted 0 1283336586
Note that bits from the hash table entries should be stolen to hold more
hash bits to reduce the penalty of collisions.
We can use the otherwise-unused lower 3 bits.
bits are valid.
This means we can choose not to re-hash all entries when we expand a hash
group; simply use the next bits we need and mark them invalid.
-\change_unchanged
+\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Complete.
\end_layout
\begin_layout Subsection
\begin_layout Subsubsection
Proposed Solution
-\change_deleted 0 1283336858
-
\end_layout
\begin_layout Standard
This implies that the number of free lists is related to the size of the
hash table, but as it is rare to walk a large number of free list entries
we can use far fewer, say 1/32 of the number of hash buckets.
-\change_inserted 0 1283336910
-
\end_layout
\begin_layout Standard
-
-\change_inserted 0 1283337052
It seems tempting to try to reuse the hash implementation which we use for
records here, but we have two ways of searching for free entries: for allocatio
n we search by size (and possibly zone) which produces too many clashes
for our hash table to handle well, and for coalescing we search by address.
Thus an array of doubly-linked free lists seems preferable.
-\change_unchanged
-
\end_layout
\begin_layout Standard
) but it's not clear this would reduce contention in the common case where
all processes are allocating/freeing the same size.
Thus we almost certainly need to divide in other ways: the most obvious
- is to divide the file into zones, and using a free list (or set of free
+ is to divide the file into zones, and using a free list (or table of free
lists) for each.
This approximates address ordering.
\end_layout
\begin_layout Standard
-Note that this means we need to split the free lists when we expand the
- file; this is probably acceptable when we double the hash table size, since
- that is such an expensive operation already.
- In the case of increasing the file size, there is an optimization we can
- use: if we use M in the formula above as the file size rounded up to the
- next power of 2, we only need reshuffle free lists when the file size crosses
- a power of 2 boundary,
-\emph on
-and
-\emph default
-reshuffling the free lists is trivial: we simply merge every consecutive
- pair of free lists.
+Unfortunately it is difficult to know what heuristics should be used to
+ determine zone sizes, and our transaction code relies on being able to
+ create a
+\begin_inset Quotes eld
+\end_inset
+
+recovery area
+\begin_inset Quotes erd
+\end_inset
+
+ by simply appending to the file (difficult if it would need to create a
+ new zone header).
+ Thus we use a linked-list of free tables; currently we only ever create
+ one, but if there is more than one we choose one at random to use.
+ In future we may use heuristics to add new free tables on contention.
+ We only expand the file when all free tables are exhausted.
\end_layout
\begin_layout Standard
\end_layout
\begin_layout Enumerate
-Identify the correct zone.
+Identify the correct free list.
\end_layout
\begin_layout Enumerate
\end_layout
\begin_layout Enumerate
-Re-check the zone (we didn't have a lock, sizes could have changed): relock
+Re-check the list (we didn't have a lock, sizes could have changed): relock
if necessary.
\end_layout
\begin_layout Enumerate
-Place the freed entry in the list for that zone.
+Place the freed entry in the list.
\end_layout
\begin_layout Standard
\end_layout
\begin_layout Enumerate
-Pick a zone either the zone we last freed into, or based on a
-\begin_inset Quotes eld
-\end_inset
-
-random
-\begin_inset Quotes erd
-\end_inset
-
- number.
+Pick a free table; usually the previous one.
\end_layout
\begin_layout Enumerate
\end_layout
\begin_layout Enumerate
-Re-check the zone: relock if necessary.
+If the top entry is -large enough, remove it from the list and return it.
\end_layout
\begin_layout Enumerate
-If the top entry is -large enough, remove it from the list and return it.
+Otherwise, coalesce entries in the list.If there was no entry large enough,
+ unlock the list and try the next largest list
\end_layout
\begin_layout Enumerate
-Otherwise, coalesce entries in the list.If there was no entry large enough,
- unlock the list and try the next zone.
+If no list has an entry which meets our needs, try the next free table.
\end_layout
\begin_layout Enumerate
\end_layout
\begin_layout Standard
-I anticipate that the number of entries in each free zone would be small,
- but it might be worth using one free entry to hold pointers to the others
- for cache efficiency.
-\change_inserted 0 1283309850
-
-\end_layout
-
-\begin_layout Standard
-
-\change_inserted 0 1283337216
-\begin_inset CommandInset label
-LatexCommand label
-name "freelist-in-zone"
-
-\end_inset
-
-If we want to avoid locking complexity (enlarging the free lists when we
- enlarge the file) we could place the array of free lists at the beginning
- of each zone.
- This means existing array lists never move, but means that a record cannot
- be larger than a zone.
- That in turn implies that zones should be variable sized (say, power of
- 2), which makes the question
-\begin_inset Quotes eld
-\end_inset
-
-what zone is this record in?
-\begin_inset Quotes erd
-\end_inset
-
- much harder (and
-\begin_inset Quotes eld
-\end_inset
-
-pick a random zone
-\begin_inset Quotes erd
-\end_inset
-
-, but that's less common).
- It could be done with as few as 4 bits from the record header.
-\begin_inset Foot
-status open
-
-\begin_layout Plain Layout
-
-\change_inserted 0 1284424151
-Using
-\begin_inset Formula $2^{16+N*3}$
-\end_inset
-
-means 0 gives a minimal 65536-byte zone, 15 gives the maximal
-\begin_inset Formula $2^{61}$
-\end_inset
-
- byte zone.
- Zones range in factor of 8 steps.
- Given the zone size for the zone the current record is in, we can determine
- the start of the zone.
-\change_unchanged
-
-\end_layout
-
-\end_inset
-
-
-\change_unchanged
-
+Each free entry has the free table number in the header: less than 255.
+ It also contains a doubly-linked list for easy deletion.
\end_layout
\begin_layout Subsection
it reduces 99.9% of false memcmp).
As an aside, as the lower bits are already incorporated in the hash table
resolution, the upper bits should be used here.
-
-\change_inserted 0 1283336739
Note that it's not clear that these bits will be a win, given the extra
bits in the hash table itself (see
\begin_inset CommandInset ref
\end_inset
).
-\change_unchanged
-
\end_layout
\begin_layout Enumerate
\end_layout
\begin_layout LyX-Code
- uint32_t magic : 16,
+ uint32_t used_magic : 16,
\end_layout
\begin_layout LyX-Code
- prev_is_free: 1,
+
\end_layout
\begin_layout LyX-Code
\end_layout
\begin_layout LyX-Code
- top_hash: 10;
+ top_hash: 11;
\end_layout
\begin_layout LyX-Code
\end_layout
\begin_layout LyX-Code
- uint32_t free_magic;
+ uint64_t free_magic: 8,
\end_layout
\begin_layout LyX-Code
- uint64_t total_length;
-\change_inserted 0 1283337133
-
+ prev : 56;
\end_layout
\begin_layout LyX-Code
-\change_inserted 0 1283337139
- uint64_t prev, next;
-\change_unchanged
+\end_layout
+\begin_layout LyX-Code
+ uint64_t free_table: 8,
\end_layout
\begin_layout LyX-Code
- ...
+ total_length : 56
\end_layout
\begin_layout LyX-Code
- uint64_t tailer;
+ uint64_t next;;
\end_layout
\begin_layout LyX-Code
\begin_layout Standard
-\change_inserted 0 1283337235
-We might want to take some bits from the used record's top_hash (and the
- free record which has 32 bits of padding to spare anyway) if we use variable
- sized zones.
- See
-\begin_inset CommandInset ref
-LatexCommand ref
-reference "freelist-in-zone"
-
-\end_inset
-
-.
+\change_deleted 0 1291206079
+
\change_unchanged
+Note that by limiting valid offsets to 56 bits, we can pack everything we
+ need into 3 64-byte words, meaning our minimum record size is 8 bytes.
+\end_layout
+
+\begin_layout Subsubsection
+Status
+\end_layout
+\begin_layout Standard
+Complete.
\end_layout
\begin_layout Subsection
a transaction in progress; we need only check for recovery if this is set.
\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Deferred.
+\end_layout
+
\begin_layout Subsection
\begin_inset CommandInset label
LatexCommand label
\end_layout
\begin_layout Subsubsection
-Proposed Solution
-\change_deleted 0 1284423472
-
-\end_layout
-
-\begin_layout Standard
-None.
+Proposed SolutionNone.
At some point you say
\begin_inset Quotes eld
\end_inset
\begin_inset Quotes erd
\end_inset
-
-\change_inserted 0 1284423891
-
-\change_deleted 0 1284423891
-.
-
-\change_inserted 0 1284423901
(but see
\begin_inset CommandInset ref
LatexCommand ref
\end_inset
).
-\change_unchanged
-
\end_layout
\begin_layout Standard
\begin_layout Standard
We could then implement snapshots using a similar method, using multiple
different hash tables/free tables.
-\change_inserted 0 1284423495
+\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Deferred.
\end_layout
\begin_layout Subsection
\end_layout
\begin_layout Standard
-
-\change_inserted 0 1284424201
None (but see
\begin_inset CommandInset ref
LatexCommand ref
\end_inset
).
-
-\change_unchanged
-We could solve a small part of the problem by providing read-only transactions.
+ We could solve a small part of the problem by providing read-only transactions.
These would allow one write transaction to begin, but it could not commit
until all r/o transactions are done.
This would require a new RO_TRANSACTION_LOCK, which would be upgraded on
commit.
\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Deferred.
+\end_layout
+
\begin_layout Subsection
Default Hash Function Is Suboptimal
\end_layout
hash bombing.
\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Complete.
+\end_layout
+
\begin_layout Subsection
\begin_inset CommandInset label
LatexCommand label
.
\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Complete.
+\end_layout
+
\begin_layout Subsection
Fcntl Locking Adds Overhead
\end_layout
free lists (perhaps when the array of top-level pointers filled).
On crash, tdb_open() would examine the array of top levels, and apply the
transactions until it encountered an invalid checksum.
-\change_inserted 0 1284423555
-
\end_layout
\begin_layout Subsection
-
-\change_inserted 0 1284423617
Tracing Is Fragile, Replay Is External
\end_layout
\begin_layout Standard
-
-\change_inserted 0 1284423719
The current TDB has compile-time-enabled tracing code, but it often breaks
as it is not enabled by default.
In a similar way, the ctdb code has an external wrapper which does replay
\end_layout
\begin_layout Subsubsection
-
-\change_inserted 0 1284423864
Proposed Solution
\begin_inset CommandInset label
LatexCommand label
\end_layout
\begin_layout Standard
-
-\change_inserted 0 1284423850
Tridge points out that an attribute can be later added to tdb_open (see
\begin_inset CommandInset ref
) to provide replay/trace hooks, which could become the basis for this and
future parallel transactions and snapshot support.
-\change_unchanged
+\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Deferred.
\end_layout
\end_body
-head 1.10;
+head 1.13;
access;
symbols;
locks; strict;
comment @# @;
+1.13
+date 2010.12.01.12.22.08; author rusty; state Exp;
+branches;
+next 1.12;
+
+1.12
+date 2010.12.01.12.20.49; author rusty; state Exp;
+branches;
+next 1.11;
+
+1.11
+date 2010.12.01.11.55.20; author rusty; state Exp;
+branches;
+next 1.10;
+
1.10
date 2010.09.14.00.33.57; author rusty; state Exp;
branches;
@
-1.10
+1.13
log
-@Tracing attribute, talloc support.
+@Merged changes.
@
text
-@#LyX 1.6.5 created this file. For more info see http://www.lyx.org/
+@#LyX 1.6.7 created this file. For more info see http://www.lyx.org/
\lyxformat 345
\begin_document
\begin_header
\end_layout
\begin_layout Date
-
-\change_deleted 0 1283307542
-26-July
-\change_inserted 0 1284423485
-14-September
-\change_unchanged
--2010
+1-December-2010
\end_layout
\begin_layout Abstract
\begin_layout Subsubsection
Proposed Solution
-\change_inserted 0 1284422789
-
\begin_inset CommandInset label
LatexCommand label
name "attributes"
\end_inset
-\change_unchanged
-
\end_layout
\begin_layout Standard
of the union.
\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Complete.
+\end_layout
+
\begin_layout Subsection
tdb_traverse Makes Impossible Guarantees
\end_layout
You can prevent changes by using a transaction or the locking API.
\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Complete.
+ Delete-during-traverse will still delete every record, too (assuming no
+ other changes).
+\end_layout
+
\begin_layout Subsection
Nesting of Transactions Is Fraught
\end_layout
-obscure case.
\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Incomplete; nesting flag is still defined as per tdb1.
+\end_layout
+
\begin_layout Subsection
Incorrect Hash Function is Not Detected
\end_layout
hash function produces the same answer, or fail the tdb_open call.
\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Complete.
+\end_layout
+
\begin_layout Subsection
tdb_set_max_dead/TDB_VOLATILE Expose Implementation
\end_layout
tuning, but initially will become a no-op.
\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Incomplete.
+ TDB_VOLATILE still defined, but implementation should fail on unknown flags
+ to be future-proof.
+\end_layout
+
\begin_layout Subsection
\begin_inset CommandInset label
LatexCommand label
an API.
\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Incomplete.
+\end_layout
+
\begin_layout Subsection
TDB API Is Not POSIX Thread-safe
\end_layout
\begin_layout Standard
The aim is that building tdb with -DTDB_PTHREAD will result in a pthread-safe
version of the library, and otherwise no overhead will exist.
-
-\change_inserted 0 1284016998
Alternatively, a hooking mechanism similar to that proposed for
\begin_inset CommandInset ref
LatexCommand ref
\end_inset
could be used to enable pthread locking at runtime.
-\change_unchanged
+\end_layout
+
+\begin_layout Subsubsection
+Status
+\end_layout
+\begin_layout Standard
+Incomplete.
\end_layout
\begin_layout Subsection
It also keeps the complexity out of the API, and in ctdbd where it is needed.
\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Incomplete.
+\end_layout
+
\begin_layout Subsection
tdb_chainlock Functions Expose Implementation
\end_layout
\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Incomplete.
+\end_layout
+
\begin_layout Subsection
The API Uses Gratuitous Typedefs, Capitals
\end_layout
the API/ABI.
\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Complete.
+\end_layout
+
\begin_layout Subsection
Various Callback Functions Are Not Typesafe
\end_layout
See CCAN's typesafe_cb module at http://ccan.ozlabs.org/info/typesafe_cb.html
\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Incomplete.
+\end_layout
+
\begin_layout Subsection
TDB_CLEAR_IF_FIRST Must Be Specified On All Opens, tdb_reopen_all Problematic
\end_layout
\end_inset
.
-\change_inserted 0 1284015637
+\end_layout
+\begin_layout Subsubsection
+Status
\end_layout
-\begin_layout Subsection
+\begin_layout Standard
+Incomplete, TDB_CLEAR_IF_FIRST still defined, but not implemented.
+\end_layout
-\change_inserted 0 1284015716
+\begin_layout Subsection
Extending The Header Is Difficult
\end_layout
\begin_layout Standard
-
-\change_inserted 0 1284015906
We have reserved (zeroed) words in the TDB header, which can be used for
future features.
If the future features are compulsory, the version number must be updated
\end_layout
\begin_layout Subsubsection
-
-\change_inserted 0 1284015637
Proposed Solution
\end_layout
\begin_layout Standard
-
-\change_inserted 0 1284016114
The header should contain a
\begin_inset Quotes eld
\end_inset
\end_layout
\begin_layout Enumerate
-
-\change_inserted 0 1284016149
The lower part reflects the format variant understood by code accessing
the database.
\end_layout
\begin_layout Enumerate
-
-\change_inserted 0 1284016639
The upper part reflects the format variant you must understand to write
to the database (otherwise you can only open for reading).
\end_layout
\begin_layout Standard
-
-\change_inserted 0 1284016821
The latter field can only be written at creation time, the former should
be written under the OPEN_LOCK when opening the database for writing, if
the variant of the code is lower than the current lowest variant.
\end_layout
\begin_layout Standard
-
-\change_inserted 0 1284016803
This should allow backwards-compatible features to be added, and detection
if older code (which doesn't understand the feature) writes to the database.
-\change_deleted 0 1284016101
+\end_layout
+
+\begin_layout Subsubsection
+Status
+\end_layout
+\begin_layout Standard
+Incomplete.
\end_layout
\begin_layout Subsection
-
-\change_inserted 0 1284015634
Record Headers Are Not Expandible
\end_layout
\begin_layout Standard
-
-\change_inserted 0 1284015634
If we later want to add (say) checksums on keys and data, it would require
another format change, which we'd like to avoid.
\end_layout
\begin_layout Subsubsection
-
-\change_inserted 0 1284015634
Proposed Solution
\end_layout
\begin_layout Standard
-
-\change_inserted 0 1284422552
We often have extra padding at the tail of a record.
If we ensure that the first byte (if any) of this padding is zero, we will
have a way for future changes to detect code which doesn't understand a
not present on that record.
\end_layout
-\begin_layout Subsection
+\begin_layout Subsubsection
+Status
+\end_layout
-\change_inserted 0 1284422568
+\begin_layout Standard
+Incomplete.
+\end_layout
+
+\begin_layout Subsection
TDB Does Not Use Talloc
\end_layout
\begin_layout Standard
-
-\change_inserted 0 1284422646
Many users of TDB (particularly Samba) use the talloc allocator, and thus
have to wrap TDB in a talloc context to use it conveniently.
\end_layout
\begin_layout Subsubsection
-
-\change_inserted 0 1284422656
Proposed Solution
\end_layout
\begin_layout Standard
-
-\change_inserted 0 1284423065
The allocation within TDB is not complicated enough to justify the use of
talloc, and I am reluctant to force another (excellent) library on TDB
users.
\end_layout
\begin_layout Standard
-
-\change_inserted 0 1284423042
This would form a talloc heirarchy as expected, but the caller would still
have to attach a destructor to the tdb context returned from tdb_open to
close it.
All TDB_DATA fields would be children of the tdb_context, and the caller
would still have to manage them (using talloc_free() or talloc_steal()).
-\change_unchanged
+\end_layout
+
+\begin_layout Subsubsection
+Status
+\end_layout
+\begin_layout Standard
+Deferred.
\end_layout
\begin_layout Section
point.
\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Incomplete; TDB_CLEAR_IF_FIRST still defined, but does nothing.
+\end_layout
+
\begin_layout Subsection
TDB Files Have a 4G Limit
\end_layout
be erased and initialized as a fresh tdb!)
\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Complete.
+\end_layout
+
\begin_layout Subsection
TDB Records Have a 4G Limit
\end_layout
).
\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Complete.
+\end_layout
+
\begin_layout Subsection
Hash Size Is Determined At TDB Creation Time
\end_layout
\end_layout
\begin_layout Subsubsection
-
-\change_inserted 0 1283336713
\begin_inset CommandInset label
LatexCommand label
name "sub:Hash-Size-Solution"
\end_inset
-
-\change_unchanged
Proposed Solution
\end_layout
, it became clear that it is hard to beat a straight linear hash table which
doubles in size when it reaches saturation.
-
-\change_deleted 0 1283307675
-There are three details which become important:
-\end_layout
-
-\begin_layout Enumerate
-
-\change_deleted 0 1283307675
-On encountering a full bucket, we use the next bucket.
-\end_layout
-
-\begin_layout Enumerate
-
-\change_deleted 0 1283307675
-Extra hash bits are stored with the offset, to reduce comparisons.
-\end_layout
-
-\begin_layout Enumerate
-
-\change_deleted 0 1283307675
-A marker entry is used on deleting an entry.
-\end_layout
-
-\begin_layout Standard
-
-\change_deleted 0 1283307675
-The doubling of the table must be done under a transaction; we will not
- reduce it on deletion, so it will be an unusual case.
- It will either be placed at the head (other entries will be moved out the
- way so we can expand).
- We could have a pointer in the header to the current hashtable location,
- but that pointer would have to be read frequently to check for hashtable
- moves.
-\end_layout
-
-\begin_layout Standard
-
-\change_deleted 0 1283307675
-The locking for this is slightly more complex than the chained case; we
- currently have one lock per bucket, and that means we would need to expand
- the lock if we overflow to the next bucket.
- The frequency of such collisions will effect our locking heuristics: we
- can always lock more buckets than we need.
-\end_layout
-
-\begin_layout Standard
-
-\change_deleted 0 1283307675
-One possible optimization is to only re-check the hash size on an insert
- or a lookup miss.
-
-\change_inserted 0 1283307770
Unfortunately, altering the hash table introduces serious locking complications
: the entire hash table needs to be locked to enlarge the hash table, and
others might be holding locks.
\end_layout
\begin_layout Standard
-
-\change_inserted 0 1283336187
Thus an expanding layered hash will be used: an array of hash groups, with
each hash group exploding into pointers to lower hash groups once it fills,
turning into a hash tree.
\end_layout
\begin_layout Standard
-
-\change_inserted 0 1283336586
Note that bits from the hash table entries should be stolen to hold more
hash bits to reduce the penalty of collisions.
We can use the otherwise-unused lower 3 bits.
bits are valid.
This means we can choose not to re-hash all entries when we expand a hash
group; simply use the next bits we need and mark them invalid.
-\change_unchanged
+\end_layout
+
+\begin_layout Subsubsection
+Status
+\end_layout
+\begin_layout Standard
+Complete.
\end_layout
\begin_layout Subsection
\begin_layout Subsubsection
Proposed Solution
-\change_deleted 0 1283336858
-
\end_layout
\begin_layout Standard
This implies that the number of free lists is related to the size of the
hash table, but as it is rare to walk a large number of free list entries
we can use far fewer, say 1/32 of the number of hash buckets.
-\change_inserted 0 1283336910
-
\end_layout
\begin_layout Standard
-
-\change_inserted 0 1283337052
It seems tempting to try to reuse the hash implementation which we use for
records here, but we have two ways of searching for free entries: for allocatio
n we search by size (and possibly zone) which produces too many clashes
for our hash table to handle well, and for coalescing we search by address.
Thus an array of doubly-linked free lists seems preferable.
-\change_unchanged
-
\end_layout
\begin_layout Standard
) but it's not clear this would reduce contention in the common case where
all processes are allocating/freeing the same size.
Thus we almost certainly need to divide in other ways: the most obvious
- is to divide the file into zones, and using a free list (or set of free
+ is to divide the file into zones, and using a free list (or table of free
lists) for each.
This approximates address ordering.
\end_layout
\begin_layout Standard
-Note that this means we need to split the free lists when we expand the
- file; this is probably acceptable when we double the hash table size, since
- that is such an expensive operation already.
- In the case of increasing the file size, there is an optimization we can
- use: if we use M in the formula above as the file size rounded up to the
- next power of 2, we only need reshuffle free lists when the file size crosses
- a power of 2 boundary,
-\emph on
-and
-\emph default
-reshuffling the free lists is trivial: we simply merge every consecutive
- pair of free lists.
+Unfortunately it is difficult to know what heuristics should be used to
+ determine zone sizes, and our transaction code relies on being able to
+ create a
+\begin_inset Quotes eld
+\end_inset
+
+recovery area
+\begin_inset Quotes erd
+\end_inset
+
+ by simply appending to the file (difficult if it would need to create a
+ new zone header).
+ Thus we use a linked-list of free tables; currently we only ever create
+ one, but if there is more than one we choose one at random to use.
+ In future we may use heuristics to add new free tables on contention.
+ We only expand the file when all free tables are exhausted.
\end_layout
\begin_layout Standard
\end_layout
\begin_layout Enumerate
-Identify the correct zone.
+Identify the correct free list.
\end_layout
\begin_layout Enumerate
\end_layout
\begin_layout Enumerate
-Re-check the zone (we didn't have a lock, sizes could have changed): relock
+Re-check the list (we didn't have a lock, sizes could have changed): relock
if necessary.
\end_layout
\begin_layout Enumerate
-Place the freed entry in the list for that zone.
+Place the freed entry in the list.
\end_layout
\begin_layout Standard
\end_layout
\begin_layout Enumerate
-Pick a zone either the zone we last freed into, or based on a
-\begin_inset Quotes eld
-\end_inset
-
-random
-\begin_inset Quotes erd
-\end_inset
-
- number.
+Pick a free table; usually the previous one.
\end_layout
\begin_layout Enumerate
\end_layout
\begin_layout Enumerate
-Re-check the zone: relock if necessary.
+If the top entry is -large enough, remove it from the list and return it.
\end_layout
\begin_layout Enumerate
-If the top entry is -large enough, remove it from the list and return it.
+Otherwise, coalesce entries in the list.If there was no entry large enough,
+ unlock the list and try the next largest list
\end_layout
\begin_layout Enumerate
-Otherwise, coalesce entries in the list.If there was no entry large enough,
- unlock the list and try the next zone.
+If no list has an entry which meets our needs, try the next free table.
\end_layout
\begin_layout Enumerate
\end_layout
\begin_layout Standard
-I anticipate that the number of entries in each free zone would be small,
- but it might be worth using one free entry to hold pointers to the others
- for cache efficiency.
-\change_inserted 0 1283309850
-
-\end_layout
-
-\begin_layout Standard
-
-\change_inserted 0 1283337216
-\begin_inset CommandInset label
-LatexCommand label
-name "freelist-in-zone"
-
-\end_inset
-
-If we want to avoid locking complexity (enlarging the free lists when we
- enlarge the file) we could place the array of free lists at the beginning
- of each zone.
- This means existing array lists never move, but means that a record cannot
- be larger than a zone.
- That in turn implies that zones should be variable sized (say, power of
- 2), which makes the question
-\begin_inset Quotes eld
-\end_inset
-
-what zone is this record in?
-\begin_inset Quotes erd
-\end_inset
-
- much harder (and
-\begin_inset Quotes eld
-\end_inset
-
-pick a random zone
-\begin_inset Quotes erd
-\end_inset
-
-, but that's less common).
- It could be done with as few as 4 bits from the record header.
-\begin_inset Foot
-status open
-
-\begin_layout Plain Layout
-
-\change_inserted 0 1284424151
-Using
-\begin_inset Formula $2^{16+N*3}$
-\end_inset
-
-means 0 gives a minimal 65536-byte zone, 15 gives the maximal
-\begin_inset Formula $2^{61}$
-\end_inset
-
- byte zone.
- Zones range in factor of 8 steps.
- Given the zone size for the zone the current record is in, we can determine
- the start of the zone.
-\change_unchanged
-
-\end_layout
-
-\end_inset
-
-
-\change_unchanged
-
+Each free entry has the free table number in the header: less than 255.
+ It also contains a doubly-linked list for easy deletion.
\end_layout
\begin_layout Subsection
it reduces 99.9% of false memcmp).
As an aside, as the lower bits are already incorporated in the hash table
resolution, the upper bits should be used here.
-
-\change_inserted 0 1283336739
Note that it's not clear that these bits will be a win, given the extra
bits in the hash table itself (see
\begin_inset CommandInset ref
\end_inset
).
-\change_unchanged
-
\end_layout
\begin_layout Enumerate
\end_layout
\begin_layout LyX-Code
- uint32_t magic : 16,
+ uint32_t used_magic : 16,
\end_layout
\begin_layout LyX-Code
- prev_is_free: 1,
+
\end_layout
\begin_layout LyX-Code
\end_layout
\begin_layout LyX-Code
- top_hash: 10;
+ top_hash: 11;
\end_layout
\begin_layout LyX-Code
\end_layout
\begin_layout LyX-Code
- uint32_t free_magic;
+ uint64_t free_magic: 8,
\end_layout
\begin_layout LyX-Code
- uint64_t total_length;
-\change_inserted 0 1283337133
-
+ prev : 56;
\end_layout
\begin_layout LyX-Code
-\change_inserted 0 1283337139
- uint64_t prev, next;
-\change_unchanged
+\end_layout
+\begin_layout LyX-Code
+ uint64_t free_table: 8,
\end_layout
\begin_layout LyX-Code
- ...
+ total_length : 56
\end_layout
\begin_layout LyX-Code
- uint64_t tailer;
+ uint64_t next;;
\end_layout
\begin_layout LyX-Code
\begin_layout Standard
-\change_inserted 0 1283337235
-We might want to take some bits from the used record's top_hash (and the
- free record which has 32 bits of padding to spare anyway) if we use variable
- sized zones.
- See
-\begin_inset CommandInset ref
-LatexCommand ref
-reference "freelist-in-zone"
-
-\end_inset
-
-.
+\change_deleted 0 1291206079
+
\change_unchanged
+Note that by limiting valid offsets to 56 bits, we can pack everything we
+ need into 3 64-byte words, meaning our minimum record size is 8 bytes.
+\end_layout
+
+\begin_layout Subsubsection
+Status
+\end_layout
+\begin_layout Standard
+Complete.
\end_layout
\begin_layout Subsection
a transaction in progress; we need only check for recovery if this is set.
\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Deferred.
+\end_layout
+
\begin_layout Subsection
\begin_inset CommandInset label
LatexCommand label
\end_layout
\begin_layout Subsubsection
-Proposed Solution
-\change_deleted 0 1284423472
-
-\end_layout
-
-\begin_layout Standard
-None.
+Proposed SolutionNone.
At some point you say
\begin_inset Quotes eld
\end_inset
\begin_inset Quotes erd
\end_inset
-
-\change_inserted 0 1284423891
-
-\change_deleted 0 1284423891
-.
-
-\change_inserted 0 1284423901
(but see
\begin_inset CommandInset ref
LatexCommand ref
\end_inset
).
-\change_unchanged
-
\end_layout
\begin_layout Standard
\begin_layout Standard
We could then implement snapshots using a similar method, using multiple
different hash tables/free tables.
-\change_inserted 0 1284423495
+\end_layout
+
+\begin_layout Subsubsection
+Status
+\end_layout
+\begin_layout Standard
+Deferred.
\end_layout
\begin_layout Subsection
\end_layout
\begin_layout Standard
-
-\change_inserted 0 1284424201
None (but see
\begin_inset CommandInset ref
LatexCommand ref
\end_inset
).
-
-\change_unchanged
-We could solve a small part of the problem by providing read-only transactions.
+ We could solve a small part of the problem by providing read-only transactions.
These would allow one write transaction to begin, but it could not commit
until all r/o transactions are done.
This would require a new RO_TRANSACTION_LOCK, which would be upgraded on
commit.
\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Deferred.
+\end_layout
+
\begin_layout Subsection
Default Hash Function Is Suboptimal
\end_layout
hash bombing.
\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Complete.
+\end_layout
+
\begin_layout Subsection
\begin_inset CommandInset label
LatexCommand label
.
\end_layout
+\begin_layout Subsubsection
+Status
+\end_layout
+
+\begin_layout Standard
+Complete.
+\end_layout
+
\begin_layout Subsection
Fcntl Locking Adds Overhead
\end_layout
free lists (perhaps when the array of top-level pointers filled).
On crash, tdb_open() would examine the array of top levels, and apply the
transactions until it encountered an invalid checksum.
-\change_inserted 0 1284423555
-
\end_layout
\begin_layout Subsection
-
-\change_inserted 0 1284423617
Tracing Is Fragile, Replay Is External
\end_layout
\begin_layout Standard
-
-\change_inserted 0 1284423719
The current TDB has compile-time-enabled tracing code, but it often breaks
as it is not enabled by default.
In a similar way, the ctdb code has an external wrapper which does replay
\end_layout
\begin_layout Subsubsection
-
-\change_inserted 0 1284423864
Proposed Solution
\begin_inset CommandInset label
LatexCommand label
\end_layout
\begin_layout Standard
-
-\change_inserted 0 1284423850
Tridge points out that an attribute can be later added to tdb_open (see
\begin_inset CommandInset ref
) to provide replay/trace hooks, which could become the basis for this and
future parallel transactions and snapshot support.
-\change_unchanged
+\end_layout
+
+\begin_layout Subsubsection
+Status
+\end_layout
+\begin_layout Standard
+Deferred.
\end_layout
\end_body
@
+1.12
+log
+@Add status, some fixes, linked freelists.
+@
+text
+@d53 1
+a53 7
+
+\change_deleted 0 1291204535
+14-September
+\change_inserted 0 1291204533
+1-December
+\change_unchanged
+-2010
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+a1432 2
+
+\change_inserted 0 1291205029
+a1436 2
+
+\change_inserted 0 1291206020
+a1437 2
+\change_unchanged
+
+a1492 2
+\change_inserted 0 1291205043
+
+a1495 2
+
+\change_inserted 0 1291205043
+a1499 2
+
+\change_inserted 0 1291205057
+a1500 2
+\change_unchanged
+
+a1547 2
+\change_inserted 0 1291205062
+
+a1550 2
+
+\change_inserted 0 1291205062
+a1554 2
+
+\change_inserted 0 1291205062
+a1555 2
+\change_unchanged
+
+a1584 2
+\change_inserted 0 1291205072
+
+a1587 2
+
+\change_inserted 0 1291205073
+a1591 2
+
+\change_inserted 0 1291205073
+a1592 2
+\change_unchanged
+
+a1632 4
+
+\change_deleted 0 1291204504
+
+\change_unchanged
+a1657 2
+\change_inserted 0 1291205079
+
+a1660 2
+
+\change_inserted 0 1291205080
+a1664 2
+
+\change_inserted 0 1291205080
+a1665 2
+\change_unchanged
+
+a1791 2
+\change_inserted 0 1291205090
+
+d1827 2
+a1828 7
+ is to divide the file into zones, and using a free list (or
+\change_inserted 0 1291205498
+table
+\change_deleted 0 1291205497
+set
+\change_unchanged
+ of free lists) for each.
+a1829 2
+\change_inserted 0 1291205203
+
+a1832 2
+
+\change_inserted 0 1291205358
+a1848 21
+\change_unchanged
+
+\end_layout
+
+\begin_layout Standard
+
+\change_deleted 0 1291205198
+Note that this means we need to split the free lists when we expand the
+ file; this is probably acceptable when we double the hash table size, since
+ that is such an expensive operation already.
+ In the case of increasing the file size, there is an optimization we can
+ use: if we use M in the formula above as the file size rounded up to the
+ next power of 2, we only need reshuffle free lists when the file size crosses
+ a power of 2 boundary,
+\emph on
+and
+\emph default
+reshuffling the free lists is trivial: we simply merge every consecutive
+ pair of free lists.
+\change_unchanged
+
+d1857 1
+a1857 7
+Identify the correct
+\change_inserted 0 1291205366
+free list
+\change_deleted 0 1291205364
+zone
+\change_unchanged
+.
+d1865 2
+a1866 7
+Re-check the
+\change_inserted 0 1291205372
+list
+\change_deleted 0 1291205371
+zone
+\change_unchanged
+ (we didn't have a lock, sizes could have changed): relock if necessary.
+d1870 1
+a1870 5
+Place the freed entry in the list
+\change_deleted 0 1291205382
+ for that zone
+\change_unchanged
+.
+d1879 1
+a1879 15
+Pick a
+\change_deleted 0 1291205403
+zone either the zone we last freed into, or based on a
+\begin_inset Quotes eld
+\end_inset
+
+random
+\begin_inset Quotes erd
+\end_inset
+
+ number.
+\change_inserted 0 1291205411
+free table; usually the previous one.
+\change_unchanged
+
+a1883 10
+\change_deleted 0 1291205432
+
+\end_layout
+
+\begin_layout Enumerate
+
+\change_deleted 0 1291205428
+Re-check the zone: relock if necessary.
+\change_unchanged
+
+d1892 1
+a1892 7
+ unlock the list and try the next
+\change_inserted 0 1291205455
+largest list
+\change_deleted 0 1291205452
+zone.
+\change_inserted 0 1291205457
+
+a1895 2
+
+\change_inserted 0 1291205476
+a1896 2
+\change_unchanged
+
+a1924 2
+\change_inserted 0 1291205542
+
+a1927 2
+
+\change_inserted 0 1291205591
+a1929 70
+\change_unchanged
+
+\end_layout
+
+\begin_layout Standard
+
+\change_deleted 0 1291205539
+I anticipate that the number of entries in each free zone would be small,
+ but it might be worth using one free entry to hold pointers to the others
+ for cache efficiency.
+\change_unchanged
+
+\end_layout
+
+\begin_layout Standard
+
+\change_deleted 0 1291205534
+\begin_inset CommandInset label
+LatexCommand label
+name "freelist-in-zone"
+
+\end_inset
+
+If we want to avoid locking complexity (enlarging the free lists when we
+ enlarge the file) we could place the array of free lists at the beginning
+ of each zone.
+ This means existing array lists never move, but means that a record cannot
+ be larger than a zone.
+ That in turn implies that zones should be variable sized (say, power of
+ 2), which makes the question
+\begin_inset Quotes eld
+\end_inset
+
+what zone is this record in?
+\begin_inset Quotes erd
+\end_inset
+
+ much harder (and
+\begin_inset Quotes eld
+\end_inset
+
+pick a random zone
+\begin_inset Quotes erd
+\end_inset
+
+, but that's less common).
+ It could be done with as few as 4 bits from the record header.
+\begin_inset Foot
+status collapsed
+
+\begin_layout Plain Layout
+Using
+\begin_inset Formula $2^{16+N*3}$
+\end_inset
+
+means 0 gives a minimal 65536-byte zone, 15 gives the maximal
+\begin_inset Formula $2^{61}$
+\end_inset
+
+ byte zone.
+ Zones range in factor of 8 steps.
+ Given the zone size for the zone the current record is in, we can determine
+ the start of the zone.
+\end_layout
+
+\end_inset
+
+
+\change_inserted 0 1291205139
+
+d2176 1
+a2176 5
+ uint32_t
+\change_inserted 0 1291205758
+used_
+\change_unchanged
+magic : 16,
+a2180 4
+\change_deleted 0 1291205693
+ prev_is_free: 1,
+\change_unchanged
+
+d2188 1
+a2188 7
+ top_hash: 1
+\change_inserted 0 1291205704
+1
+\change_deleted 0 1291205704
+0
+\change_unchanged
+;
+d2212 1
+a2212 9
+ uint
+\change_inserted 0 1291205725
+64
+\change_deleted 0 1291205723
+32
+\change_unchanged
+_t
+\change_inserted 0 1291205753
+free_magic: 8,
+a2215 2
+
+\change_inserted 0 1291205746
+a2220 24
+\change_deleted 0 1291205749
+free_magic;
+\change_unchanged
+
+\end_layout
+
+\begin_layout LyX-Code
+ uint64_t
+\change_inserted 0 1291205786
+free_table: 8,
+\end_layout
+
+\begin_layout LyX-Code
+
+\change_inserted 0 1291205788
+
+\change_unchanged
+total_length
+\change_inserted 0 1291205792
+ : 56
+\change_deleted 0 1291205790
+;
+\change_unchanged
+
+d2224 1
+a2224 7
+ uint64_t
+\change_deleted 0 1291205801
+prev,
+\change_unchanged
+next;
+\change_deleted 0 1291205811
+
+d2228 1
+a2228 3
+
+\change_deleted 0 1291205811
+ ...
+d2232 1
+a2232 5
+
+\change_deleted 0 1291205808
+ uint64_t tailer
+\change_unchanged
+;
+d2241 5
+a2245 16
+\change_deleted 0 1291205827
+We might want to take some bits from the used record's top_hash (and the
+ free record which has 32 bits of padding to spare anyway) if we use variable
+ sized zones.
+ See
+\begin_inset CommandInset ref
+LatexCommand ref
+reference "freelist-in-zone"
+
+\end_inset
+
+.
+
+\change_inserted 0 1291205885
+ Note that by limiting valid offsets to 56 bits, we can pack everything
+ we need into 3 64-byte words, meaning our minimum record size is 8 bytes.
+a2248 2
+
+\change_inserted 0 1291205886
+a2252 2
+
+\change_inserted 0 1291205886
+a2253 2
+\change_unchanged
+
+a2343 2
+\change_inserted 0 1291205894
+
+a2346 2
+
+\change_inserted 0 1291205894
+a2350 2
+
+\change_inserted 0 1291205902
+a2351 2
+\change_unchanged
+
+a2373 4
+
+\change_deleted 0 1291204504
+
+\change_unchanged
+a2403 2
+\change_inserted 0 1291205910
+
+a2406 2
+
+\change_inserted 0 1291205910
+a2410 2
+
+\change_inserted 0 1291205914
+a2411 2
+\change_unchanged
+
+a2443 2
+\change_inserted 0 1291205919
+
+a2446 2
+
+\change_inserted 0 1291205919
+a2450 2
+
+\change_inserted 0 1291205922
+a2451 2
+\change_unchanged
+
+a2491 2
+\change_inserted 0 1291205929
+
+a2494 2
+
+\change_inserted 0 1291205929
+a2498 2
+
+\change_inserted 0 1291205929
+a2499 2
+\change_unchanged
+
+a2536 2
+\change_inserted 0 1291205932
+
+a2539 2
+
+\change_inserted 0 1291205933
+a2543 2
+
+\change_inserted 0 1291205933
+a2544 2
+\change_unchanged
+
+a2682 2
+\change_inserted 0 1291205944
+
+a2685 2
+
+\change_inserted 0 1291205945
+a2689 2
+
+\change_inserted 0 1291205948
+a2690 2
+\change_unchanged
+
+@
+
+
+1.11
+log
+@Merge changes
+@
+text
+@d53 7
+a59 1
+14-September-2010
+d587 16
+d644 18
+d716 16
+d753 16
+d813 18
+d883 16
+d953 16
+d1084 16
+d1181 16
+d1273 16
+d1328 16
+d1381 16
+d1447 19
+a1465 2
+ if older code (which doesn't understand the feature) writes to the database.Reco
+rd Headers Are Not Expandible
+d1484 16
+d1546 16
+d1617 16
+d1680 16
+d1725 16
+d1810 16
+d1951 8
+a1958 3
+Proposed SolutionThe first step is to remove all the current heuristics,
+ as they obviously interact, then examine them once the lock contention
+ is addressed.
+d1989 7
+a1995 2
+ is to divide the file into zones, and using a free list (or set of free
+ lists) for each.
+d1997 2
+d2002 25
+d2039 2
+d2049 7
+a2055 1
+Identify the correct zone.
+d2063 7
+a2069 2
+Re-check the zone (we didn't have a lock, sizes could have changed): relock
+ if necessary.
+d2073 5
+a2077 1
+Place the freed entry in the list for that zone.
+d2086 3
+a2088 1
+Pick a zone either the zone we last freed into, or based on a
+d2097 4
+d2105 2
+d2110 2
+d2113 2
+d2123 15
+a2137 1
+ unlock the list and try the next zone.
+d2166 11
+d2180 2
+d2185 2
+d2190 2
+d2223 1
+a2223 1
+status open
+d2243 2
+d2491 5
+a2495 1
+ uint32_t magic : 16,
+d2499 2
+d2502 2
+d2511 7
+a2517 1
+ top_hash: 10;
+d2541 29
+a2569 1
+ uint32_t free_magic;
+d2573 11
+a2583 1
+ uint64_t total_length;
+d2587 7
+a2593 1
+ uint64_t prev, next;
+d2597 2
+d2603 5
+a2607 1
+ uint64_t tailer;
+d2615 2
+d2628 18
+d2736 16
+d2808 16
+d2856 16
+d2912 16
+d2965 16
+d3119 16
+@
+
+
+1.10
+log
+@Tracing attribute, talloc support.
+@
+text
+@d1 1
+a1 1
+#LyX 1.6.5 created this file. For more info see http://www.lyx.org/
+d53 1
+a53 7
+
+\change_deleted 0 1283307542
+26-July
+\change_inserted 0 1284423485
+14-September
+\change_unchanged
+-2010
+a472 2
+\change_inserted 0 1284422789
+
+a479 2
+\change_unchanged
+
+a838 2
+
+\change_inserted 0 1284016998
+a846 2
+\change_unchanged
+
+a1194 2
+\change_inserted 0 1284015637
+
+a1197 2
+
+\change_inserted 0 1284015716
+a1201 2
+
+\change_inserted 0 1284015906
+a1210 2
+
+\change_inserted 0 1284015637
+a1214 2
+
+\change_inserted 0 1284016114
+a1227 2
+
+\change_inserted 0 1284016149
+a1232 2
+
+\change_inserted 0 1284016639
+a1237 2
+
+\change_inserted 0 1284016821
+a1243 2
+
+\change_inserted 0 1284016803
+d1245 2
+a1246 9
+ if older code (which doesn't understand the feature) writes to the database.
+\change_deleted 0 1284016101
+
+\end_layout
+
+\begin_layout Subsection
+
+\change_inserted 0 1284015634
+Record Headers Are Not Expandible
+a1249 2
+
+\change_inserted 0 1284015634
+a1254 2
+
+\change_inserted 0 1284015634
+a1258 2
+
+\change_inserted 0 1284422552
+a1267 2
+
+\change_inserted 0 1284422568
+a1271 2
+
+\change_inserted 0 1284422646
+a1276 2
+
+\change_inserted 0 1284422656
+a1280 2
+
+\change_inserted 0 1284423065
+a1305 2
+
+\change_inserted 0 1284423042
+a1310 2
+\change_unchanged
+
+a1457 2
+
+\change_inserted 0 1283336713
+a1463 2
+
+\change_unchanged
+d1482 2
+d1485 1
+a1485 51
+\change_deleted 0 1283307675
+There are three details which become important:
+\end_layout
+
+\begin_layout Enumerate
+
+\change_deleted 0 1283307675
+On encountering a full bucket, we use the next bucket.
+\end_layout
+
+\begin_layout Enumerate
+
+\change_deleted 0 1283307675
+Extra hash bits are stored with the offset, to reduce comparisons.
+\end_layout
+
+\begin_layout Enumerate
+
+\change_deleted 0 1283307675
+A marker entry is used on deleting an entry.
+\end_layout
+
+\begin_layout Standard
+
+\change_deleted 0 1283307675
+The doubling of the table must be done under a transaction; we will not
+ reduce it on deletion, so it will be an unusual case.
+ It will either be placed at the head (other entries will be moved out the
+ way so we can expand).
+ We could have a pointer in the header to the current hashtable location,
+ but that pointer would have to be read frequently to check for hashtable
+ moves.
+\end_layout
+
+\begin_layout Standard
+
+\change_deleted 0 1283307675
+The locking for this is slightly more complex than the chained case; we
+ currently have one lock per bucket, and that means we would need to expand
+ the lock if we overflow to the next bucket.
+ The frequency of such collisions will effect our locking heuristics: we
+ can always lock more buckets than we need.
+\end_layout
+
+\begin_layout Standard
+
+\change_deleted 0 1283307675
+One possible optimization is to only re-check the hash size on an insert
+ or a lookup miss.
+
+\change_inserted 0 1283307770
+a1492 2
+
+\change_inserted 0 1283336187
+a1500 2
+
+\change_inserted 0 1283336586
+a1510 2
+\change_unchanged
+
+d1636 3
+a1638 8
+Proposed Solution
+\change_deleted 0 1283336858
+
+\end_layout
+
+\begin_layout Standard
+The first step is to remove all the current heuristics, as they obviously
+ interact, then examine them once the lock contention is addressed.
+a1647 2
+\change_inserted 0 1283336910
+
+a1650 2
+
+\change_inserted 0 1283337052
+a1655 2
+\change_unchanged
+
+a1776 2
+\change_inserted 0 1283309850
+
+a1779 2
+
+\change_inserted 0 1283337216
+a1813 2
+
+\change_inserted 0 1284424151
+a1825 2
+\change_unchanged
+
+a1830 2
+\change_unchanged
+
+a2031 2
+
+\change_inserted 0 1283336739
+a2040 2
+\change_unchanged
+
+a2117 2
+\change_inserted 0 1283337133
+
+a2120 2
+
+\change_inserted 0 1283337139
+a2121 2
+\change_unchanged
+
+a2136 2
+
+\change_inserted 0 1283337235
+a2147 2
+\change_unchanged
+
+d2251 1
+a2251 7
+Proposed Solution
+\change_deleted 0 1284423472
+
+\end_layout
+
+\begin_layout Standard
+None.
+d2261 1
+a2261 1
+\change_inserted 0 1284423891
+d2263 1
+a2263 4
+\change_deleted 0 1284423891
+.
+
+\change_inserted 0 1284423901
+a2271 2
+\change_unchanged
+
+a2293 2
+\change_inserted 0 1284423495
+
+a2312 2
+
+\change_inserted 0 1284424201
+d2321 1
+a2321 3
+
+\change_unchanged
+We could solve a small part of the problem by providing read-only transactions.
+a2505 2
+\change_inserted 0 1284423555
+
+a2508 2
+
+\change_inserted 0 1284423617
+a2512 2
+
+\change_inserted 0 1284423719
+a2519 2
+
+\change_inserted 0 1284423864
+a2530 2
+
+\change_inserted 0 1284423850
+a2540 2
+\change_unchanged
+
+@
+
+
1.9
log
@Extension mechanism.
Rusty Russell, IBM Corporation
-14-September-2010
+1-December-2010
Abstract
This allows future attributes to be added, even if this expands
the size of the union.
+2.1.2 Status
+
+Complete.
+
2.2 tdb_traverse Makes Impossible Guarantees
tdb_traverse (and tdb_firstkey/tdb_nextkey) predate transactions,
You can prevent changes by using a transaction or the locking
API.
+2.2.2 Status
+
+Complete. Delete-during-traverse will still delete every record,
+too (assuming no other changes).
+
2.3 Nesting of Transactions Is Fraught
TDB has alternated between allowing nested transactions and not
tdb_add_flags() and tdb_remove_flags(), so the API should not be
expanded for this relatively-obscure case.
+2.3.2 Status
+
+Incomplete; nesting flag is still defined as per tdb1.
+
2.4 Incorrect Hash Function is Not Detected
tdb_open_ex() allows the calling code to specify a different hash
0xdeadbeef), and tdb_open_ex() should check that the given hash
function produces the same answer, or fail the tdb_open call.
+2.4.2 Status
+
+Complete.
+
2.5 tdb_set_max_dead/TDB_VOLATILE Expose Implementation
In response to scalability issues with the free list ([TDB-Freelist-Is]
as fetch in order to allow some internal tuning, but initially
will become a no-op.
+2.5.2 Status
+
+Incomplete. TDB_VOLATILE still defined, but implementation should
+fail on unknown flags to be future-proof.
+
2.6 <TDB-Files-Cannot>TDB Files Cannot Be Opened Multiple Times
In The Same Process
benefit to adding a call to detect when a tdb_context is shared,
to allow other to create such an API.
+2.6.2 Status
+
+Incomplete.
+
2.7 TDB API Is Not POSIX Thread-safe
The TDB API uses an error code which can be queried after an
proposed for [Proposed-Solution-locking-hook] could be used to
enable pthread locking at runtime.
+2.7.2 Status
+
+Incomplete.
+
2.8 *_nonblock Functions And *_mark Functions Expose
Implementation
It also keeps the complexity out of the API, and in ctdbd where
it is needed.
+2.8.2 Status
+
+Incomplete.
+
2.9 tdb_chainlock Functions Expose Implementation
tdb_chainlock locks some number of records, including the record
otherwise EAGAIN.
]
+2.10.2 Status
+
+Incomplete.
+
2.11 The API Uses Gratuitous Typedefs, Capitals
typedefs are useful for providing source compatibility when types
It should simply take an extra argument, since we are prepared to
break the API/ABI.
+2.12.2 Status
+
+Complete.
+
2.13 Various Callback Functions Are Not Typesafe
The callback functions in tdb_set_logging_function (after [tdb_log_func-Doesnt-Take]
See CCAN's typesafe_cb module at
http://ccan.ozlabs.org/info/typesafe_cb.html
+2.13.2 Status
+
+Incomplete.
+
2.14 TDB_CLEAR_IF_FIRST Must Be Specified On All Opens,
tdb_reopen_all Problematic
Remove TDB_CLEAR_IF_FIRST. Other workarounds are possible, but
see [TDB_CLEAR_IF_FIRST-Imposes-Performance].
+2.14.2 Status
+
+Incomplete, TDB_CLEAR_IF_FIRST still defined, but not
+implemented.
+
2.15 Extending The Header Is Difficult
We have reserved (zeroed) words in the TDB header, which can be
detection if older code (which doesn't understand the feature)
writes to the database.
+2.15.2 Status
+
+Incomplete.
+
2.16 Record Headers Are Not Expandible
If we later want to add (say) checksums on keys and data, it
the tail, and thus if there is no tail or the first byte is 0, we
would know the extension is not present on that record.
+2.16.2 Status
+
+Incomplete.
+
2.17 TDB Does Not Use Talloc
Many users of TDB (particularly Samba) use the talloc allocator,
children of the tdb_context, and the caller would still have to
manage them (using talloc_free() or talloc_steal()).
+2.17.2 Status
+
+Deferred.
+
3 Performance And Scalability Issues
3.1 <TDB_CLEAR_IF_FIRST-Imposes-Performance>TDB_CLEAR_IF_FIRST
tend to know when they are initializing for the first time and
can simply unlink the old tdb at that point.
+3.1.2 Status
+
+Incomplete; TDB_CLEAR_IF_FIRST still defined, but does nothing.
+
3.2 TDB Files Have a 4G Limit
This seems to be becoming an issue (so much for “trivial”!),
August 2009, commit 398d0c29290: prior to that any unrecognized
file format would be erased and initialized as a fresh tdb!)
+3.2.2 Status
+
+Complete.
+
3.3 TDB Records Have a 4G Limit
This has not been a reported problem, and the API uses size_t
seems unlikely that 32 bit keys will be a limitation, so the
implementation may not support this (see [sub:Records-Incur-A]).
+3.3.2 Status
+
+Complete.
+
3.4 Hash Size Is Determined At TDB Creation Time
TDB contains a number of hash chains in the header; the number is
expanding tree of hashes would be very close to optimal.
], it became clear that it is hard to beat a straight linear hash
table which doubles in size when it reaches saturation.
-
-1.
-
-2.
-
-3.
-
-
-
-
-
- Unfortunately, altering the hash table introduces serious
-locking complications: the entire hash table needs to be locked
-to enlarge the hash table, and others might be holding locks.
+Unfortunately, altering the hash table introduces serious locking
+complications: the entire hash table needs to be locked to
+enlarge the hash table, and others might be holding locks.
Particularly insidious are insertions done under tdb_chainlock.
Thus an expanding layered hash will be used: an array of hash
hash group; simply use the next bits we need and mark them
invalid.
+3.4.2 Status
+
+Complete.
+
3.5 <TDB-Freelist-Is>TDB Freelist Is Highly Contended
TDB uses a single linked list for the free list. Allocation
case where all processes are allocating/freeing the same size.
Thus we almost certainly need to divide in other ways: the most
obvious is to divide the file into zones, and using a free list
-(or set of free lists) for each. This approximates address
+(or table of free lists) for each. This approximates address
ordering.
-Note that this means we need to split the free lists when we
-expand the file; this is probably acceptable when we double the
-hash table size, since that is such an expensive operation
-already. In the case of increasing the file size, there is an
-optimization we can use: if we use M in the formula above as the
-file size rounded up to the next power of 2, we only need
-reshuffle free lists when the file size crosses a power of 2
-boundary, and reshuffling the free lists is trivial: we simply
-merge every consecutive pair of free lists.
+Unfortunately it is difficult to know what heuristics should be
+used to determine zone sizes, and our transaction code relies on
+being able to create a “recovery area” by simply appending to the
+file (difficult if it would need to create a new zone header).
+Thus we use a linked-list of free tables; currently we only ever
+create one, but if there is more than one we choose one at random
+to use. In future we may use heuristics to add new free tables on
+contention. We only expand the file when all free tables are
+exhausted.
The basic algorithm is as follows. Freeing is simple:
-1. Identify the correct zone.
+1. Identify the correct free list.
2. Lock the corresponding list.
-3. Re-check the zone (we didn't have a lock, sizes could have
+3. Re-check the list (we didn't have a lock, sizes could have
changed): relock if necessary.
-4. Place the freed entry in the list for that zone.
+4. Place the freed entry in the list.
Allocation is a little more complicated, as we perform delayed
coalescing at this point:
-1. Pick a zone either the zone we last freed into, or based on a “
- random” number.
+1. Pick a free table; usually the previous one.
2. Lock the corresponding list.
-3. Re-check the zone: relock if necessary.
-
-4. If the top entry is -large enough, remove it from the list and
+3. If the top entry is -large enough, remove it from the list and
return it.
-5. Otherwise, coalesce entries in the list.If there was no entry
- large enough, unlock the list and try the next zone.
+4. Otherwise, coalesce entries in the list.If there was no entry
+ large enough, unlock the list and try the next largest list
+
+5. If no list has an entry which meets our needs, try the next
+ free table.
6. If no zone satisfies, expand the file.
does not need a tailer to coalesce, though if we needed one we
could have one cheaply: see [sub:Records-Incur-A].
-I anticipate that the number of entries in each free zone would
-be small, but it might be worth using one free entry to hold
-pointers to the others for cache efficiency.
-
-<freelist-in-zone>If we want to avoid locking complexity
-(enlarging the free lists when we enlarge the file) we could
-place the array of free lists at the beginning of each zone. This
-means existing array lists never move, but means that a record
-cannot be larger than a zone. That in turn implies that zones
-should be variable sized (say, power of 2), which makes the
-question “what zone is this record in?” much harder (and “pick a
-random zone”, but that's less common). It could be done with as
-few as 4 bits from the record header.[footnote:
-Using 2^{16+N*3}means 0 gives a minimal 65536-byte zone, 15 gives
-the maximal 2^{61} byte zone. Zones range in factor of 8 steps.
-Given the zone size for the zone the current record is in, we can
-determine the start of the zone.
-]
+Each free entry has the free table number in the header: less
+than 255. It also contains a doubly-linked list for easy
+deletion.
3.6 <sub:TDB-Becomes-Fragmented>TDB Becomes Fragmented
struct tdb_used_record {
- uint32_t magic : 16,
+ uint32_t used_magic : 16,
+
- prev_is_free: 1,
key_data_divide: 5,
- top_hash: 10;
+ top_hash: 11;
uint32_t extra_octets;
struct tdb_free_record {
- uint32_t free_magic;
+ uint64_t free_magic: 8,
+
+ prev : 56;
- uint64_t total_length;
- uint64_t prev, next;
- ...
+ uint64_t free_table: 8,
- uint64_t tailer;
+ total_length : 56
+
+ uint64_t next;;
};
-We might want to take some bits from the used record's top_hash
-(and the free record which has 32 bits of padding to spare
-anyway) if we use variable sized zones. See [freelist-in-zone].
+Note that by limiting valid offsets to 56 bits, we can pack
+everything we need into 3 64-byte words, meaning our minimum
+record size is 8 bytes.
+
+3.7.2 Status
+
+Complete.
3.8 Transaction Commit Requires 4 fdatasync
header field can be used to indicate a transaction in progress;
we need only check for recovery if this is set.
-3.9 <sub:TDB-Does-Not>TDB Does Not Have Snapshot Support
+3.8.2 Status
-3.9.1 Proposed Solution
+Deferred.
-None. At some point you say “use a real database” (but see [replay-attribute]
-).
+3.9 <sub:TDB-Does-Not>TDB Does Not Have Snapshot Support
+
+3.9.1 Proposed SolutionNone. At some point you say “use a real
+ database” (but see [replay-attribute]).
But as a thought experiment, if we implemented transactions to
only overwrite free entries (this is tricky: there must not be a
We could then implement snapshots using a similar method, using
multiple different hash tables/free tables.
+3.9.2 Status
+
+Deferred.
+
3.10 Transactions Cannot Operate in Parallel
This would be useless for ldb, as it hits the index records with
until all r/o transactions are done. This would require a new
RO_TRANSACTION_LOCK, which would be upgraded on commit.
+3.10.2 Status
+
+Deferred.
+
3.11 Default Hash Function Is Suboptimal
The Knuth-inspired multiplicative hash used by tdb is fairly slow
source, and placed in the header. This is far from foolproof, but
adds a little bit of protection against hash bombing.
+3.11.2 Status
+
+Complete.
+
3.12 <Reliable-Traversal-Adds>Reliable Traversal Adds Complexity
We lock a record during traversal iteration, and try to grab that
Remove reliability guarantees; see [traverse-Proposed-Solution].
+3.12.2 Status
+
+Complete.
+
3.13 Fcntl Locking Adds Overhead
Placing a fcntl lock means a system call, as does removing one.
could become the basis for this and future parallel transactions
and snapshot support.
+3.15.2 Status
+
+Deferred.
+