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15.5 WAL Internals

WAL is automatically enabled; no action is required from the administrator except ensuring that the disk-space requirements for the WAL logs are met, and that any necessary tuning is done (see section 15.4 WAL Configuration).

WAL logs are stored in the directory ‘pg_xlog’ under the data directory, as a set of segment files, normally each 16 MB in size (but the size can be changed by altering the --with-wal-segsize configure option when building the server). Each segment is divided into pages, normally 8 kB each (this size can be changed via the --with-wal-blocksize configure option). 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 ‘000000010000000000000000’. The numbers do not wrap, but it will take a very, very long time to exhaust the available stock of numbers.

It is advantageous if the log is located on a different disk from the main database files. This can be achieved by moving the ‘pg_xlog’ directory 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 is to ensure that the log is written before database records are altered, but this can be subverted by disk drives 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 might 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, at the start of recovery, 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 (assuming full_page_writes is not disabled), all pages changed since the checkpoint will be restored to a consistent state.

To deal with the case where ‘pg_control’ is corrupt, we should support the possibility of scanning existing log segments in reverse order--newest to oldest--in order to find the latest checkpoint. This has not been implemented yet. ‘pg_control’ is small enough (less than one disk page) that it is not subject to partial-write problems, and as of this writing there have been no reports of database failures due solely to the inability to read ‘pg_control’ itself. So while it is theoretically a weak spot, ‘pg_control’ does not seem to be a problem in practice.