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 have been flushed to permanent
storage. When 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).
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 tuples pointing to nonexistent table rows
index tuples 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.
UNDO operation is not implemented. This means that changes
made by aborted transactions will still occupy disk space and that
we still need a permanent pg_clog file to hold
the status of transactions, since we are not able to re-use
transaction identifiers. 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
savepoints 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 (eg, 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.
