25.4. Internals

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 Section 25.3).

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 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 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.