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.)
A 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.
WAL also makes it possible to support on-line
backup and point-in-time recovery, as described in Section 23.3. 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.
