|PostgreSQL 8.1.23 Documentation|
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There are several WAL-related configuration parameters that affect database performance. This section explains their use. Consult Chapter 17 for general information about setting server 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 written 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. In the event of a crash, the crash recovery procedure looks at the latest checkpoint record to determine the point in the log (known as the redo record) from which it should start the REDO operation. Any changes made to data files before that point are known to be already on disk. Hence, after a checkpoint has been made, any log segments preceding the one containing the redo record are no longer needed and can be recycled or removed. (When WAL archiving is being done, the log segments must be archived before being recycled or removed.)
The server's background writer process will automatically perform a checkpoint every so often. 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. If full_page_writes is set (as is the default), there is another factor to consider. To ensure data page consistency, the first modification of a data page after each checkpoint results in logging the entire page content. In that case, a smaller checkpoint interval increases the volume of output to the WAL log, partially negating the goal of using a smaller interval, and in any case causing more disk I/O.
Checkpoints are fairly expensive, first because they require writing out all currently dirty buffers, and second because they result in extra subsequent WAL traffic as discussed above. It is therefore wise to set the checkpointing parameters high enough that checkpoints don't happen too often. As a simple sanity check on your checkpointing parameters, you can set the checkpoint_warning parameter. If checkpoints happen closer together than checkpoint_warning seconds, a message will be output to the server log recommending increasing checkpoint_segments. Occasional appearance of such a message is not cause for alarm, but if it appears often then the checkpoint control parameters should be increased. Bulk operations such as large COPY transfers may cause a number of such warnings to appear if you have not set checkpoint_segments high enough.
There will be at least one WAL segment file, and will normally not be more than 2 * checkpoint_segments + 1 files. Each segment file is normally 16 MB (though this size can be altered when building the server). 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 internal 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
LogInsert from having to do writes. 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. When full_page_writes
is set and the system is very busy, setting this value higher
will help smooth response times during the period immediately
following each checkpoint.
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.
Enabling the wal_debug
configuration parameter (provided that PostgreSQL has been compiled with support
for it) will result in each
call being logged to the server log. This option may be replaced
by a more general mechanism in the future.
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