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Chapter 25. Write-Ahead Logging (WAL)

Table of Contents
25.1. Benefits of WAL
25.2. WAL Configuration
25.3. Internals

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 can be redone from the log records. (This is roll-forward recovery, also known as REDO.)

25.1. Benefits of WAL

The first major benefit of using WAL is a significantly reduced number of disk writes, because only the log file needs to be flushed to disk at the time of transaction commit, rather than every data file changed by the transaction. 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. This is especially true for servers handling many small transactions touching different parts of the data store.

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:

  1. index rows pointing to nonexistent table rows

  2. index rows lost in split operations

  3. 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.

Finally, WAL makes it possible to support on-line backup and point-in-time recovery, as described in Section 22.3. By archiving the WAL data we can support reverting to any time instant covered by the available WAL data: we simply install a prior physical backup of the database, and replay the WAL log just as far as the desired time. What's more, the physical backup doesn't have to be an instantaneous snapshot of the database state — if it is made over some period of time, then replaying the WAL log for that period will fix any internal inconsistencies.

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