tdb2: document problems with moving or enlarging hash table.

[ccan] / ccan / tdb2 / doc / design.txt

TDB2: A Redesigning The Trivial DataBase

Rusty Russell, IBM Corporation

1-September-2010

Abstract

The Trivial DataBase on-disk format is 32 bits; with usage cases 
heading towards the 4G limit, that must change. This required 
breakage provides an opportunity to revisit TDB's other design 
decisions and reassess them.

1 Introduction

The Trivial DataBase was originally written by Andrew Tridgell as 
a simple key/data pair storage system with the same API as dbm, 
but allowing multiple readers and writers while being small 
enough (< 1000 lines of C) to include in SAMBA. The simple design 
created in 1999 has proven surprisingly robust and performant, 
used in Samba versions 3 and 4 as well as numerous other 
projects. Its useful life was greatly increased by the 
(backwards-compatible!) addition of transaction support in 2005.

The wider variety and greater demands of TDB-using code has lead 
to some organic growth of the API, as well as some compromises on 
the implementation. None of these, by themselves, are seen as 
show-stoppers, but the cumulative effect is to a loss of elegance 
over the initial, simple TDB implementation. Here is a table of 
the approximate number of lines of implementation code and number 
of API functions at the end of each year:


+-----------+----------------+--------------------------------+
| Year End  | API Functions  | Lines of C Code Implementation |
+-----------+----------------+--------------------------------+
+-----------+----------------+--------------------------------+
|   1999    |      13        |              1195              |
+-----------+----------------+--------------------------------+
|   2000    |      24        |              1725              |
+-----------+----------------+--------------------------------+
|   2001    |      32        |              2228              |
+-----------+----------------+--------------------------------+
|   2002    |      35        |              2481              |
+-----------+----------------+--------------------------------+
|   2003    |      35        |              2552              |
+-----------+----------------+--------------------------------+
|   2004    |      40        |              2584              |
+-----------+----------------+--------------------------------+
|   2005    |      38        |              2647              |
+-----------+----------------+--------------------------------+
|   2006    |      52        |              3754              |
+-----------+----------------+--------------------------------+
|   2007    |      66        |              4398              |
+-----------+----------------+--------------------------------+
|   2008    |      71        |              4768              |
+-----------+----------------+--------------------------------+
|   2009    |      73        |              5715              |
+-----------+----------------+--------------------------------+


This review is an attempt to catalog and address all the known 
issues with TDB and create solutions which address the problems 
without significantly increasing complexity; all involved are far 
too aware of the dangers of second system syndrome in rewriting a 
successful project like this.

2 API Issues

2.1 tdb_open_ex Is Not Expandable

The tdb_open() call was expanded to tdb_open_ex(), which added an 
optional hashing function and an optional logging function 
argument. Additional arguments to open would require the 
introduction of a tdb_open_ex2 call etc.

2.1.1 Proposed Solution

tdb_open() will take a linked-list of attributes:

enum tdb_attribute {

    TDB_ATTRIBUTE_LOG = 0,

    TDB_ATTRIBUTE_HASH = 1

};

struct tdb_attribute_base {

    enum tdb_attribute attr;

    union tdb_attribute *next;

};

struct tdb_attribute_log {

    struct tdb_attribute_base base; /* .attr = TDB_ATTRIBUTE_LOG 
*/

    tdb_log_func log_fn;

    void *log_private;

};

struct tdb_attribute_hash {

    struct tdb_attribute_base base; /* .attr = TDB_ATTRIBUTE_HASH 
*/

    tdb_hash_func hash_fn;

    void *hash_private;

};

union tdb_attribute {

    struct tdb_attribute_base base;

    struct tdb_attribute_log log;

    struct tdb_attribute_hash hash;

};

This allows future attributes to be added, even if this expands 
the size of the union.

2.2 tdb_traverse Makes Impossible Guarantees

tdb_traverse (and tdb_firstkey/tdb_nextkey) predate transactions, 
and it was thought that it was important to guarantee that all 
records which exist at the start and end of the traversal would 
be included, and no record would be included twice.

This adds complexity (see[Reliable-Traversal-Adds]) and does not 
work anyway for records which are altered (in particular, those 
which are expanded may be effectively deleted and re-added behind 
the traversal).

2.2.1 <traverse-Proposed-Solution>Proposed Solution

Abandon the guarantee. You will see every record if no changes 
occur during your traversal, otherwise you will see some subset. 
You can prevent changes by using a transaction or the locking 
API.

2.3 Nesting of Transactions Is Fraught

TDB has alternated between allowing nested transactions and not 
allowing them. Various paths in the Samba codebase assume that 
transactions will nest, and in a sense they can: the operation is 
only committed to disk when the outer transaction is committed. 
There are two problems, however:

1. Canceling the inner transaction will cause the outer 
  transaction commit to fail, and will not undo any operations 
  since the inner transaction began. This problem is soluble with 
  some additional internal code.

2. An inner transaction commit can be cancelled by the outer 
  transaction. This is desirable in the way which Samba's 
  database initialization code uses transactions, but could be a 
  surprise to any users expecting a successful transaction commit 
  to expose changes to others.

The current solution is to specify the behavior at tdb_open(), 
with the default currently that nested transactions are allowed. 
This flag can also be changed at runtime.

2.3.1 Proposed Solution

Given the usage patterns, it seems that the “least-surprise” 
behavior of disallowing nested transactions should become the 
default. Additionally, it seems the outer transaction is the only 
code which knows whether inner transactions should be allowed, so 
a flag to indicate this could be added to tdb_transaction_start. 
However, this behavior can be simulated with a wrapper which uses 
tdb_add_flags() and tdb_remove_flags(), so the API should not be 
expanded for this relatively-obscure case.

2.4 Incorrect Hash Function is Not Detected

tdb_open_ex() allows the calling code to specify a different hash 
function to use, but does not check that all other processes 
accessing this tdb are using the same hash function. The result 
is that records are missing from tdb_fetch().

2.4.1 Proposed Solution

The header should contain an example hash result (eg. the hash of 
0xdeadbeef), and tdb_open_ex() should check that the given hash 
function produces the same answer, or fail the tdb_open call.

2.5 tdb_set_max_dead/TDB_VOLATILE Expose Implementation

In response to scalability issues with the free list ([TDB-Freelist-Is]
) two API workarounds have been incorporated in TDB: 
tdb_set_max_dead() and the TDB_VOLATILE flag to tdb_open. The 
latter actually calls the former with an argument of “5”.

This code allows deleted records to accumulate without putting 
them in the free list. On delete we iterate through each chain 
and free them in a batch if there are more than max_dead entries. 
These are never otherwise recycled except as a side-effect of a 
tdb_repack.

2.5.1 Proposed Solution

With the scalability problems of the freelist solved, this API 
can be removed. The TDB_VOLATILE flag may still be useful as a 
hint that store and delete of records will be at least as common 
as fetch in order to allow some internal tuning, but initially 
will become a no-op.

2.6 <TDB-Files-Cannot>TDB Files Cannot Be Opened Multiple Times 
  In The Same Process

No process can open the same TDB twice; we check and disallow it. 
This is an unfortunate side-effect of fcntl locks, which operate 
on a per-file rather than per-file-descriptor basis, and do not 
nest. Thus, closing any file descriptor on a file clears all the 
locks obtained by this process, even if they were placed using a 
different file descriptor!

Note that even if this were solved, deadlock could occur if 
operations were nested: this is a more manageable programming 
error in most cases.

2.6.1 Proposed Solution

We could lobby POSIX to fix the perverse rules, or at least lobby 
Linux to violate them so that the most common implementation does 
not have this restriction. This would be a generally good idea 
for other fcntl lock users.

Samba uses a wrapper which hands out the same tdb_context to 
multiple callers if this happens, and does simple reference 
counting. We should do this inside the tdb library, which already 
emulates lock nesting internally; it would need to recognize when 
deadlock occurs within a single process. This would create a new 
failure mode for tdb operations (while we currently handle 
locking failures, they are impossible in normal use and a process 
encountering them can do little but give up).

I do not see benefit in an additional tdb_open flag to indicate 
whether re-opening is allowed, as though there may be some 
benefit to adding a call to detect when a tdb_context is shared, 
to allow other to create such an API.

2.7 TDB API Is Not POSIX Thread-safe

The TDB API uses an error code which can be queried after an 
operation to determine what went wrong. This programming model 
does not work with threads, unless specific additional guarantees 
are given by the implementation. In addition, even 
otherwise-independent threads cannot open the same TDB (as in [TDB-Files-Cannot]
).

2.7.1 Proposed Solution

Reachitecting the API to include a tdb_errcode pointer would be a 
great deal of churn; we are better to guarantee that the 
tdb_errcode is per-thread so the current programming model can be 
maintained.

This requires dynamic per-thread allocations, which is awkward 
with POSIX threads (pthread_key_create space is limited and we 
cannot simply allocate a key for every TDB).

Internal locking is required to make sure that fcntl locks do not 
overlap between threads, and also that the global list of tdbs is 
maintained.

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.

2.8 *_nonblock Functions And *_mark Functions Expose 
  Implementation

CTDB[footnote:
Clustered TDB, see http://ctdb.samba.org
] wishes to operate on TDB in a non-blocking manner. This is 
currently done as follows:

1. Call the _nonblock variant of an API function (eg. 
  tdb_lockall_nonblock). If this fails:

2. Fork a child process, and wait for it to call the normal 
  variant (eg. tdb_lockall).

3. If the child succeeds, call the _mark variant to indicate we 
  already have the locks (eg. tdb_lockall_mark).

4. Upon completion, tell the child to release the locks (eg. 
  tdb_unlockall).

5. Indicate to tdb that it should consider the locks removed (eg. 
  tdb_unlockall_mark).

There are several issues with this approach. Firstly, adding two 
new variants of each function clutters the API for an obscure 
use, and so not all functions have three variants. Secondly, it 
assumes that all paths of the functions ask for the same locks, 
otherwise the parent process will have to get a lock which the 
child doesn't have under some circumstances. I don't believe this 
is currently the case, but it constrains the implementation. 

2.8.1 <Proposed-Solution-locking-hook>Proposed Solution

Implement a hook for locking methods, so that the caller can 
control the calls to create and remove fcntl locks. In this 
scenario, ctdbd would operate as follows:

1. Call the normal API function, eg tdb_lockall().

2. When the lock callback comes in, check if the child has the 
  lock. Initially, this is always false. If so, return 0. 
  Otherwise, try to obtain it in non-blocking mode. If that 
  fails, return EWOULDBLOCK.

3. Release locks in the unlock callback as normal.

4. If tdb_lockall() fails, see if we recorded a lock failure; if 
  so, call the child to repeat the operation.

5. The child records what locks it obtains, and returns that 
  information to the parent.

6. When the child has succeeded, goto 1.

This is flexible enough to handle any potential locking scenario, 
even when lock requirements change. It can be optimized so that 
the parent does not release locks, just tells the child which 
locks it doesn't need to obtain.

It also keeps the complexity out of the API, and in ctdbd where 
it is needed.

2.9 tdb_chainlock Functions Expose Implementation

tdb_chainlock locks some number of records, including the record 
indicated by the given key. This gave atomicity guarantees; 
no-one can start a transaction, alter, read or delete that key 
while the lock is held.

It also makes the same guarantee for any other key in the chain, 
which is an internal implementation detail and potentially a 
cause for deadlock.

2.9.1 Proposed Solution

None. It would be nice to have an explicit single entry lock 
which effected no other keys. Unfortunately, this won't work for 
an entry which doesn't exist. Thus while chainlock may be 
implemented more efficiently for the existing case, it will still 
have overlap issues with the non-existing case. So it is best to 
keep the current (lack of) guarantee about which records will be 
effected to avoid constraining our implementation.

2.10 Signal Handling is Not Race-Free

The tdb_setalarm_sigptr() call allows the caller's signal handler 
to indicate that the tdb locking code should return with a 
failure, rather than trying again when a signal is received (and 
errno == EAGAIN). This is usually used to implement timeouts.

Unfortunately, this does not work in the case where the signal is 
received before the tdb code enters the fcntl() call to place the 
lock: the code will sleep within the fcntl() code, unaware that 
the signal wants it to exit. In the case of long timeouts, this 
does not happen in practice.

2.10.1 Proposed Solution

The locking hooks proposed in[Proposed-Solution-locking-hook] 
would allow the user to decide on whether to fail the lock 
acquisition on a signal. This allows the caller to choose their 
own compromise: they could narrow the race by checking 
immediately before the fcntl call.[footnote:
It may be possible to make this race-free in some implementations 
by having the signal handler alter the struct flock to make it 
invalid. This will cause the fcntl() lock call to fail with 
EINVAL if the signal occurs before the kernel is entered, 
otherwise EAGAIN.
]

2.11 The API Uses Gratuitous Typedefs, Capitals

typedefs are useful for providing source compatibility when types 
can differ across implementations, or arguably in the case of 
function pointer definitions which are hard for humans to parse. 
Otherwise it is simply obfuscation and pollutes the namespace.

Capitalization is usually reserved for compile-time constants and 
macros.

  TDB_CONTEXT There is no reason to use this over 'struct 
  tdb_context'; the definition isn't visible to the API user 
  anyway.

  TDB_DATA There is no reason to use this over struct TDB_DATA; 
  the struct needs to be understood by the API user.

  struct TDB_DATA This would normally be called 'struct 
  tdb_data'.

  enum TDB_ERROR Similarly, this would normally be enum 
  tdb_error.

2.11.1 Proposed Solution

None. Introducing lower case variants would please pedants like 
myself, but if it were done the existing ones should be kept. 
There is little point forcing a purely cosmetic change upon tdb 
users.

2.12 <tdb_log_func-Doesnt-Take>tdb_log_func Doesn't Take The 
  Private Pointer

For API compatibility reasons, the logging function needs to call 
tdb_get_logging_private() to retrieve the pointer registered by 
the tdb_open_ex for logging.

2.12.1 Proposed Solution

It should simply take an extra argument, since we are prepared to 
break the API/ABI.

2.13 Various Callback Functions Are Not Typesafe

The callback functions in tdb_set_logging_function (after [tdb_log_func-Doesnt-Take]
 is resolved), tdb_parse_record, tdb_traverse, tdb_traverse_read 
and tdb_check all take void * and must internally convert it to 
the argument type they were expecting.

If this type changes, the compiler will not produce warnings on 
the callers, since it only sees void *.

2.13.1 Proposed Solution

With careful use of macros, we can create callback functions 
which give a warning when used on gcc and the types of the 
callback and its private argument differ. Unsupported compilers 
will not give a warning, which is no worse than now. In addition, 
the callbacks become clearer, as they need not use void * for 
their parameter.

See CCAN's typesafe_cb module at 
http://ccan.ozlabs.org/info/typesafe_cb.html

2.14 TDB_CLEAR_IF_FIRST Must Be Specified On All Opens, 
  tdb_reopen_all Problematic

The TDB_CLEAR_IF_FIRST flag to tdb_open indicates that the TDB 
file should be cleared if the caller discovers it is the only 
process with the TDB open. However, if any caller does not 
specify TDB_CLEAR_IF_FIRST it will not be detected, so will have 
the TDB erased underneath them (usually resulting in a crash).

There is a similar issue on fork(); if the parent exits (or 
otherwise closes the tdb) before the child calls tdb_reopen_all() 
to establish the lock used to indicate the TDB is opened by 
someone, a TDB_CLEAR_IF_FIRST opener at that moment will believe 
it alone has opened the TDB and will erase it.

2.14.1 Proposed Solution

Remove TDB_CLEAR_IF_FIRST. Other workarounds are possible, but 
see [TDB_CLEAR_IF_FIRST-Imposes-Performance].

3 Performance And Scalability Issues

3.1 <TDB_CLEAR_IF_FIRST-Imposes-Performance>TDB_CLEAR_IF_FIRST 
  Imposes Performance Penalty

When TDB_CLEAR_IF_FIRST is specified, a 1-byte read lock is 
placed at offset 4 (aka. the ACTIVE_LOCK). While these locks 
never conflict in normal tdb usage, they do add substantial 
overhead for most fcntl lock implementations when the kernel 
scans to detect if a lock conflict exists. This is often a single 
linked list, making the time to acquire and release a fcntl lock 
O(N) where N is the number of processes with the TDB open, not 
the number actually doing work.

In a Samba server it is common to have huge numbers of clients 
sitting idle, and thus they have weaned themselves off the 
TDB_CLEAR_IF_FIRST flag.[footnote:
There is a flag to tdb_reopen_all() which is used for this 
optimization: if the parent process will outlive the child, the 
child does not need the ACTIVE_LOCK. This is a workaround for 
this very performance issue.
]

3.1.1 Proposed Solution

Remove the flag. It was a neat idea, but even trivial servers 
tend to know when they are initializing for the first time and 
can simply unlink the old tdb at that point.

3.2 TDB Files Have a 4G Limit

This seems to be becoming an issue (so much for “trivial”!), 
particularly for ldb.

3.2.1 Proposed Solution

A new, incompatible TDB format which uses 64 bit offsets 
internally rather than 32 bit as now. For simplicity of endian 
conversion (which TDB does on the fly if required), all values 
will be 64 bit on disk. In practice, some upper bits may be used 
for other purposes, but at least 56 bits will be available for 
file offsets.

tdb_open() will automatically detect the old version, and even 
create them if TDB_VERSION6 is specified to tdb_open.

32 bit processes will still be able to access TDBs larger than 4G 
(assuming that their off_t allows them to seek to 64 bits), they 
will gracefully fall back as they fail to mmap. This can happen 
already with large TDBs.

Old versions of tdb will fail to open the new TDB files (since 28 
August 2009, commit 398d0c29290: prior to that any unrecognized 
file format would be erased and initialized as a fresh tdb!)

3.3 TDB Records Have a 4G Limit

This has not been a reported problem, and the API uses size_t 
which can be 64 bit on 64 bit platforms. However, other limits 
may have made such an issue moot.

3.3.1 Proposed Solution

Record sizes will be 64 bit, with an error returned on 32 bit 
platforms which try to access such records (the current 
implementation would return TDB_ERR_OOM in a similar case). It 
seems unlikely that 32 bit keys will be a limitation, so the 
implementation may not support this (see [sub:Records-Incur-A]).

3.4 Hash Size Is Determined At TDB Creation Time

TDB contains a number of hash chains in the header; the number is 
specified at creation time, and defaults to 131. This is such a 
bottleneck on large databases (as each hash chain gets quite 
long), that LDB uses 10,000 for this hash. In general it is 
impossible to know what the 'right' answer is at database 
creation time.

3.4.1 <sub:Hash-Size-Solution>Proposed Solution

After comprehensive performance testing on various scalable hash 
variants[footnote:
http://rusty.ozlabs.org/?p=89 and http://rusty.ozlabs.org/?p=94 
This was annoying because I was previously convinced that an 
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. 
Particularly insidious are insertions done under tdb_chainlock.

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. This has 
implications for locking: we must lock the entire group in case 
we need to expand it, yet we don't know how deep the tree is at 
that point.

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. If we limit the size of 
the database to 64 exabytes, we can use the top 8 bits of the 
hash entry as well. These 11 bits would reduce false positives 
down to 1 in 2000 which is more than we need: we can use one of 
the bits to indicate that the extra hash 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.

3.5 <TDB-Freelist-Is>TDB Freelist Is Highly Contended

TDB uses a single linked list for the free list. Allocation 
occurs as follows, using heuristics which have evolved over time:

1. Get the free list lock for this whole operation.

2. Multiply length by 1.25, so we always over-allocate by 25%.

3. Set the slack multiplier to 1.

4. Examine the current freelist entry: if it is > length but < 
  the current best case, remember it as the best case.

5. Multiply the slack multiplier by 1.05.

6. If our best fit so far is less than length * slack multiplier, 
  return it. The slack will be turned into a new free record if 
  it's large enough.

7. Otherwise, go onto the next freelist entry.

Deleting a record occurs as follows:

1. Lock the hash chain for this whole operation.

2. Walk the chain to find the record, keeping the prev pointer 
  offset.

3. If max_dead is non-zero:

  (a) Walk the hash chain again and count the dead records.

  (b) If it's more than max_dead, bulk free all the dead ones 
    (similar to steps 4 and below, but the lock is only obtained 
    once).

  (c) Simply mark this record as dead and return. 

4. Get the free list lock for the remainder of this operation.

5. <right-merging>Examine the following block to see if it is 
  free; if so, enlarge the current block and remove that block 
  from the free list. This was disabled, as removal from the free 
  list was O(entries-in-free-list).

6. Examine the preceeding block to see if it is free: for this 
  reason, each block has a 32-bit tailer which indicates its 
  length. If it is free, expand it to cover our new block and 
  return.

7. Otherwise, prepend ourselves to the free list.

Disabling right-merging (step [right-merging]) causes 
fragmentation; the other heuristics proved insufficient to 
address this, so the final answer to this was that when we expand 
the TDB file inside a transaction commit, we repack the entire 
tdb.

The single list lock limits our allocation rate; due to the other 
issues this is not currently seen as a bottleneck.

3.5.1 Proposed Solution

The first step is to remove all the current heuristics, as they 
obviously interact, then examine them once the lock contention is 
addressed.

The free list must be split to reduce contention. Assuming 
perfect free merging, we can at most have 1 free list entry for 
each entry. 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.

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 allocation 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.

There are various benefits in using per-size free lists (see [sub:TDB-Becomes-Fragmented]
) 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 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.

The basic algorithm is as follows. Freeing is simple:

1. Identify the correct zone.

2. Lock the corresponding list.

3. Re-check the zone (we didn't have a lock, sizes could have 
  changed): relock if necessary.

4. Place the freed entry in the list for that zone.

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.

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 
  return it.

5. Otherwise, coalesce entries in the list.If there was no entry 
  large enough, unlock the list and try the next zone.

6. If no zone satisfies, expand the file.

This optimizes rapid insert/delete of free list entries by not 
coalescing them all the time.. First-fit address ordering 
ordering seems to be fairly good for keeping fragmentation low 
(see [sub:TDB-Becomes-Fragmented]). Note that address ordering 
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.
]

3.6 <sub:TDB-Becomes-Fragmented>TDB Becomes Fragmented

Much of this is a result of allocation strategy[footnote:
The Memory Fragmentation Problem: Solved? Johnstone & Wilson 1995 
ftp://ftp.cs.utexas.edu/pub/garbage/malloc/ismm98.ps
] and deliberate hobbling of coalescing; internal fragmentation 
(aka overallocation) is deliberately set at 25%, and external 
fragmentation is only cured by the decision to repack the entire 
db when a transaction commit needs to enlarge the file.

3.6.1 Proposed Solution

The 25% overhead on allocation works in practice for ldb because 
indexes tend to expand by one record at a time. This internal 
fragmentation can be resolved by having an “expanded” bit in the 
header to note entries that have previously expanded, and 
allocating more space for them.

There are is a spectrum of possible solutions for external 
fragmentation: one is to use a fragmentation-avoiding allocation 
strategy such as best-fit address-order allocator. The other end 
of the spectrum would be to use a bump allocator (very fast and 
simple) and simply repack the file when we reach the end.

There are three problems with efficient fragmentation-avoiding 
allocators: they are non-trivial, they tend to use a single free 
list for each size, and there's no evidence that tdb allocation 
patterns will match those recorded for general allocators (though 
it seems likely).

Thus we don't spend too much effort on external fragmentation; we 
will be no worse than the current code if we need to repack on 
occasion. More effort is spent on reducing freelist contention, 
and reducing overhead.

3.7 <sub:Records-Incur-A>Records Incur A 28-Byte Overhead

Each TDB record has a header as follows:

struct tdb_record {

        tdb_off_t next; /* offset of the next record in the list 
*/

        tdb_len_t rec_len; /* total byte length of record */

        tdb_len_t key_len; /* byte length of key */

        tdb_len_t data_len; /* byte length of data */

        uint32_t full_hash; /* the full 32 bit hash of the key */

        uint32_t magic;   /* try to catch errors */

        /* the following union is implied:

                union {

                        char record[rec_len];

                        struct {

                                char key[key_len];

                                char data[data_len];

                        }

                        uint32_t totalsize; (tailer)

                }

        */

};

Naively, this would double to a 56-byte overhead on a 64 bit 
implementation.

3.7.1 Proposed Solution

We can use various techniques to reduce this for an allocated 
block:

1. The 'next' pointer is not required, as we are using a flat 
  hash table.

2. 'rec_len' can instead be expressed as an addition to key_len 
  and data_len (it accounts for wasted or overallocated length in 
  the record). Since the record length is always a multiple of 8, 
  we can conveniently fit it in 32 bits (representing up to 35 
  bits).

3. 'key_len' and 'data_len' can be reduced. I'm unwilling to 
  restrict 'data_len' to 32 bits, but instead we can combine the 
  two into one 64-bit field and using a 5 bit value which 
  indicates at what bit to divide the two. Keys are unlikely to 
  scale as fast as data, so I'm assuming a maximum key size of 32 
  bits.

4. 'full_hash' is used to avoid a memcmp on the “miss” case, but 
  this is diminishing returns after a handful of bits (at 10 
  bits, 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. Note that it's 
  not clear that these bits will be a win, given the extra bits 
  in the hash table itself (see [sub:Hash-Size-Solution]).

5. 'magic' does not need to be enlarged: it currently reflects 
  one of 5 values (used, free, dead, recovery, and 
  unused_recovery). It is useful for quick sanity checking 
  however, and should not be eliminated.

6. 'tailer' is only used to coalesce free blocks (so a block to 
  the right can find the header to check if this block is free). 
  This can be replaced by a single 'free' bit in the header of 
  the following block (and the tailer only exists in free 
  blocks).[footnote:
This technique from Thomas Standish. Data Structure Techniques. 
Addison-Wesley, Reading, Massachusetts, 1980.
] The current proposed coalescing algorithm doesn't need this, 
  however.

This produces a 16 byte used header like this:

struct tdb_used_record {

        uint32_t magic : 16,

                 prev_is_free: 1,

                 key_data_divide: 5,

                 top_hash: 10;

        uint32_t extra_octets;

        uint64_t key_and_data_len;

};

And a free record like this:

struct tdb_free_record {

        uint32_t free_magic;

        uint64_t total_length;

        uint64_t prev, next;

        ...

        uint64_t tailer;

};

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].

3.8 Transaction Commit Requires 4 fdatasync

The current transaction algorithm is:

1. write_recovery_data();

2. sync();

3. write_recovery_header();

4. sync();

5. overwrite_with_new_data();

6. sync();

7. remove_recovery_header();

8. sync(); 

On current ext3, each sync flushes all data to disk, so the next 
3 syncs are relatively expensive. But this could become a 
performance bottleneck on other filesystems such as ext4.

3.8.1 Proposed Solution

Neil Brown points out that this is overzealous, and only one sync 
is needed:

1. Bundle the recovery data, a transaction counter and a strong 
  checksum of the new data.

2. Strong checksum that whole bundle.

3. Store the bundle in the database.

4. Overwrite the oldest of the two recovery pointers in the 
  header (identified using the transaction counter) with the 
  offset of this bundle.

5. sync.

6. Write the new data to the file.

Checking for recovery means identifying the latest bundle with a 
valid checksum and using the new data checksum to ensure that it 
has been applied. This is more expensive than the current check, 
but need only be done at open. For running databases, a separate 
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

3.9.1 Proposed Solution

None. At some point you say “use a real database”.

But as a thought experiment, if we implemented transactions to 
only overwrite free entries (this is tricky: there must not be a 
header in each entry which indicates whether it is free, but use 
of presence in metadata elsewhere), and a pointer to the hash 
table, we could create an entirely new commit without destroying 
existing data. Then it would be easy to implement snapshots in a 
similar way.

This would not allow arbitrary changes to the database, such as 
tdb_repack does, and would require more space (since we have to 
preserve the current and future entries at once). If we used hash 
trees rather than one big hash table, we might only have to 
rewrite some sections of the hash, too.

We could then implement snapshots using a similar method, using 
multiple different hash tables/free tables.

3.10 Transactions Cannot Operate in Parallel

This would be useless for ldb, as it hits the index records with 
just about every update. It would add significant complexity in 
resolving clashes, and cause the all transaction callers to write 
their code to loop in the case where the transactions spuriously 
failed.

3.10.1 Proposed Solution

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.

3.11 Default Hash Function Is Suboptimal

The Knuth-inspired multiplicative hash used by tdb is fairly slow 
(especially if we expand it to 64 bits), and works best when the 
hash bucket size is a prime number (which also means a slow 
modulus). In addition, it is highly predictable which could 
potentially lead to a Denial of Service attack in some TDB uses.

3.11.1 Proposed Solution

The Jenkins lookup3 hash[footnote:
http://burtleburtle.net/bob/c/lookup3.c
] is a fast and superbly-mixing hash. It's used by the Linux 
kernel and almost everything else. This has the particular 
properties that it takes an initial seed, and produces two 32 bit 
hash numbers, which we can combine into a 64-bit hash.

The seed should be created at tdb-creation time from some random 
source, and placed in the header. This is far from foolproof, but 
adds a little bit of protection against hash bombing.

3.12 <Reliable-Traversal-Adds>Reliable Traversal Adds Complexity

We lock a record during traversal iteration, and try to grab that 
lock in the delete code. If that grab on delete fails, we simply 
mark it deleted and continue onwards; traversal checks for this 
condition and does the delete when it moves off the record.

If traversal terminates, the dead record may be left 
indefinitely.

3.12.1 Proposed Solution

Remove reliability guarantees; see [traverse-Proposed-Solution].

3.13 Fcntl Locking Adds Overhead

Placing a fcntl lock means a system call, as does removing one. 
This is actually one reason why transactions can be faster 
(everything is locked once at transaction start). In the 
uncontended case, this overhead can theoretically be eliminated.

3.13.1 Proposed Solution

None.

We tried this before with spinlock support, in the early days of 
TDB, and it didn't make much difference except in manufactured 
benchmarks.

We could use spinlocks (with futex kernel support under Linux), 
but it means that we lose automatic cleanup when a process dies 
with a lock. There is a method of auto-cleanup under Linux, but 
it's not supported by other operating systems. We could 
reintroduce a clear-if-first-style lock and sweep for dead 
futexes on open, but that wouldn't help the normal case of one 
concurrent opener dying. Increasingly elaborate repair schemes 
could be considered, but they require an ABI change (everyone 
must use them) anyway, so there's no need to do this at the same 
time as everything else.

3.14 Some Transactions Don't Require Durability

Volker points out that gencache uses a CLEAR_IF_FIRST tdb for 
normal (fast) usage, and occasionally empties the results into a 
transactional TDB. This kind of usage prioritizes performance 
over durability: as long as we are consistent, data can be lost.

This would be more neatly implemented inside tdb: a “soft” 
transaction commit (ie. syncless) which meant that data may be 
reverted on a crash.

3.14.1 Proposed Solution

None.

Unfortunately any transaction scheme which overwrites old data 
requires a sync before that overwrite to avoid the possibility of 
corruption.

It seems possible to use a scheme similar to that described in [sub:TDB-Does-Not]
,where transactions are committed without overwriting existing 
data, and an array of top-level pointers were available in the 
header. If the transaction is “soft” then we would not need a 
sync at all: existing processes would pick up the new hash table 
and free list and work with that.

At some later point, a sync would allow recovery of the old data 
into the 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.