There are several WAL-related parameters that
affect database performance. This section explains their use.
Consult Section 3.4 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 undo 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 postmaster spawns a special backend 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 16MB 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 sequential future
segments). 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 and LogFlush.
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,
tuple 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 log output, 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 postgresql.conf WAL_BUFFERS parameter. The default number of WAL buffers is 8. Increasing this value will
correspondingly increase shared memory usage.
The COMMIT_DELAY parameter defines for how many
microseconds the backend will sleep after writing a commit
record to the log with LogInsert but before
performing a LogFlush. This delay allows other
backends 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 or if fewer than COMMIT_SIBLINGS
other backends are not currently in active transactions; this avoids
sleeping when it's unlikely that any other backend 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 will 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 LogInsert and
LogFlush WAL call being
logged to standard error. At present, it makes no difference what
the nonzero value is. This option may be replaced by a more
general mechanism in the future.
