|PostgreSQL 7.4.30 Documentation|
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There are several WAL-related configuration parameters that affect database performance. This section explains their use. Consult Section 16.4 for details about setting configuration parameters.
Checkpoints are points in the sequence of transactions at which it is guaranteed that the data files have been updated with all information logged before the checkpoint. At checkpoint time, all dirty data pages are flushed to disk and a special checkpoint record is written to the log file. As result, in the event of a crash, the recoverer knows from what record in the log (known as the redo record) it should start the REDO operation, since any changes made to data files before that record are already on disk. After a checkpoint has been made, any log segments written before the redo records are no longer needed and can be recycled or removed. (When WAL-based BAR is implemented, the log segments would be archived before being recycled or removed.)
The server spawns a special process every so often to create the next checkpoint. A checkpoint is created every checkpoint_segments log segments, or every checkpoint_timeout seconds, whichever comes first. The default settings are 3 segments and 300 seconds respectively. It is also possible to force a checkpoint by using the SQL command CHECKPOINT.
Reducing checkpoint_segments and/or checkpoint_timeout causes checkpoints to be done more often. This allows faster after-crash recovery (since less work will need to be redone). However, one must balance this against the increased cost of flushing dirty data pages more often. In addition, to ensure data page consistency, the first modification of a data page after each checkpoint results in logging the entire page content. Thus a smaller checkpoint interval increases the volume of output to the log, partially negating the goal of using a smaller interval, and in any case causing more disk I/O.
There will be at least one 16 MB segment file, and will normally not be more than 2 * checkpoint_segments + 1 files. You can use this to estimate space requirements for WAL. Ordinarily, when old log segment files are no longer needed, they are recycled (renamed to become the next segments in the numbered sequence). If, due to a short-term peak of log output rate, there are more than 2 * checkpoint_segments + 1 segment files, the unneeded segment files will be deleted instead of recycled until the system gets back under this limit.
There are two commonly used WAL functions:
LogInsert is used to place a new record into
the WAL buffers in shared
memory. If there is no space for the new record,
LogInsert will have to write (move to kernel
cache) a few filled WAL
buffers. This is undesirable because
LogInsert is used on every database low level
modification (for example, row insertion) at a time when an
exclusive lock is held on affected data pages, so the operation
needs to be as fast as possible. What is worse, writing
WAL buffers may also force the
creation of a new log segment, which takes even more time.
Normally, WAL buffers should
be written and flushed by a
LogFlush request, which is made, for the most
part, at transaction commit time to ensure that transaction
records are flushed to permanent storage. On systems with high
LogFlush requests may
not occur often enough to prevent WAL buffers being written by
LogInsert. On such systems one should increase
the number of WAL buffers by
modifying the configuration parameter wal_buffers. The default number of WAL buffers is 8. Increasing this value will
correspondingly increase shared memory usage.
Checkpoints are fairly expensive because they force all dirty kernel buffers to disk using the operating system sync() call. Busy servers may fill checkpoint segment files too quickly, causing excessive checkpointing. If such forced checkpoints happen more frequently than checkpoint_warning seconds, a message, will be output to the server logs recommending increasing checkpoint_segments.
The commit_delay parameter defines
for how many microseconds the server process will sleep after
writing a commit record to the log with
LogInsert but before performing a
LogFlush. This delay allows other server
processes to add their commit records to the log so as to have
all of them flushed with a single log sync. No sleep will occur
if fsync is not enabled, nor if fewer
than commit_siblings other sessions are
currently in active transactions; this avoids sleeping when it's
unlikely that any other session will commit soon. Note that on
most platforms, the resolution of a sleep request is ten
milliseconds, so that any nonzero commit_delay setting between 1 and 10000
microseconds would have the same effect. Good values for these
parameters are not yet clear; experimentation is encouraged.
The wal_sync_method parameter determines how PostgreSQL will ask the kernel to force WAL updates out to disk. All the options should be the same as far as reliability goes, but it's quite platform-specific which one will be the fastest. Note that this parameter is irrelevant if fsync has been turned off.
Setting the wal_debug parameter to
any nonzero value will result in each
call being logged to the server log. At present, it makes no
difference what the nonzero value is. This option may be replaced
by a more general mechanism in the future.