WAL transaction log WAL Write-Ahead Logging (WAL) is a standard approach to transaction logging. Its detailed description may be found in most (if not all) books about transaction processing. Briefly, WAL's central concept is that changes to data files (where tables and indexes reside) must be written only after those changes have been logged, that is, when log records describing the changes have been flushed to permanent storage. If we follow this procedure, we do not need to flush data pages to disk on every transaction commit, because we know that in the event of a crash we will be able to recover the database using the log: any changes that have not been applied to the data pages will first be redone from the log records (this is roll-forward recovery, also known as REDO) and then changes made by uncommitted transactions will be removed from the data pages (roll-backward recovery, UNDO). fsync The first obvious benefit of using WAL is a significantly reduced number of disk writes, since only the log file needs to be flushed to disk at the time of transaction commit; in multiuser environments, commits of many transactions may be accomplished with a single fsync() of the log file. Furthermore, the log file is written sequentially, and so the cost of syncing the log is much less than the cost of flushing the data pages. The next benefit is consistency of the data pages. The truth is that, before WAL, PostgreSQL was never able to guarantee consistency in the case of a crash. Before WAL, any crash during writing could result in: index rows pointing to nonexistent table rows index rows lost in split operations totally corrupted table or index page content, because of partially written data pages Problems with indexes (problems 1 and 2) could possibly have been fixed by additional fsync() calls, but it is not obvious how to handle the last case without WAL; WAL saves the entire data page content in the log if that is required to ensure page consistency for after-crash recovery. The UNDO operation is not implemented. This means that changes made by aborted transactions will still occupy disk space and that a permanent pg_clog file to hold the status of transactions is still needed, since transaction identifiers cannot be reused. Once UNDO is implemented, pg_clog will no longer be required to be permanent; it will be possible to remove pg_clog at shutdown. (However, the urgency of this concern has decreased greatly with the adoption of a segmented storage method for pg_clog: it is no longer necessary to keep old pg_clog entries around forever.) With UNDO, it will also be possible to implement savepointssavepoint to allow partial rollback of invalid transaction operations (parser errors caused by mistyping commands, insertion of duplicate primary/unique keys and so on) with the ability to continue or commit valid operations made by the transaction before the error. At present, any error will invalidate the whole transaction and require a transaction abort. WAL offers the opportunity for a new method for database on-line backup and restore (BAR). To use this method, one would have to make periodic saves of data files to another disk, a tape or another host and also archive the WAL log files. The database file copy and the archived log files could be used to restore just as if one were restoring after a crash. Each time a new database file copy was made the old log files could be removed. Implementing this facility will require the logging of data file and index creation and deletion; it will also require development of a method for copying the data files (operating system copy commands are not suitable). A difficulty standing in the way of realizing these benefits is that they require saving WAL entries for considerable periods of time (e.g., as long as the longest possible transaction if transaction UNDO is wanted). The present WAL format is extremely bulky since it includes many disk page snapshots. This is not a serious concern at present, since the entries only need to be kept for one or two checkpoint intervals; but to achieve these future benefits some sort of compressed WAL format will be needed. There are several WAL-related configuration parameters that affect database performance. This section explains their use. Consult for details about setting configuration parameters. Checkpointscheckpoint 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 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 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. Enabling the wal_debug configuration parameter will result in each LogInsert and LogFlush WAL call being logged to the server log. This option may be replaced by a more general mechanism in the future. WAL is automatically enabled; no action is required from the administrator except ensuring that the additional disk-space requirements of the WAL logs are met, and that any necessary tuning is done (see ). WAL logs are stored in the directory pg_xlog under the data directory, as a set of segment files, each 16 MB in size. Each segment is divided into 8 kB pages. The log record headers are described in access/xlog.h; the record content is dependent on the type of event that is being logged. Segment files are given ever-increasing numbers as names, starting at 0000000000000000. The numbers do not wrap, at present, but it should take a very long time to exhaust the available stock of numbers. The WAL buffers and control structure are in shared memory and are handled by the server child processes; they are protected by lightweight locks. The demand on shared memory is dependent on the number of buffers. The default size of the WAL buffers is 8 buffers of 8 kB each, or 64 kB total. It is of advantage if the log is located on another disk than the main database files. This may be achieved by moving the directory pg_xlog to another location (while the server is shut down, of course) and creating a symbolic link from the original location in the main data directory to the new location. The aim of WAL, to ensure that the log is written before database records are altered, may be subverted by disk drivesdisk drive that falsely report a successful write to the kernel, when, in fact, they have only cached the data and not yet stored it on the disk. A power failure in such a situation may still lead to irrecoverable data corruption. Administrators should try to ensure that disks holding PostgreSQL's WAL log files do not make such false reports. After a checkpoint has been made and the log flushed, the checkpoint's position is saved in the file pg_control. Therefore, when recovery is to be done, the server first reads pg_control and then the checkpoint record; then it performs the REDO operation by scanning forward from the log position indicated in the checkpoint record. Because the entire content of data pages is saved in the log on the first page modification after a checkpoint, all pages changed since the checkpoint will be restored to a consistent state. Using pg_control to get the checkpoint position speeds up the recovery process, but to handle possible corruption of pg_control, we should actually implement the reading of existing log segments in reverse order -- newest to oldest -- in order to find the last checkpoint. This has not been implemented, yet.