There are several WAL-related configuration parameters that
affect database performance. This section explains their use.
Consult Section 30.4 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 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 a
result, in the event of a crash, the crash recovery procedure knows from
what point in the log (known as the redo record) it should start the
REDO operation, since any changes made to data files before that point
are already on disk. After a checkpoint has been made, any log segments
written before 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. 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 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.
There will be at least one WAL segment file, and will normally
not be more than 2 * checkpoint_segments + 1
files. Each segment file size by default is 64 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 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, 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 log output, 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. (It should be noted
that there is presently little evidence to suggest that increasing
wal_buffers beyond the default is worthwhile.)
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
EnterpriseDB 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.