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.