30.3. Asynchronous Commit
Asynchronous commit is an option that allows transactions to complete more quickly, at the cost that the most recent transactions may be lost if the database should crash. In many applications this is an acceptable trade-off.
As described in the previous section, transaction commit is normally synchronous: the server waits for the transaction's WAL records to be flushed to permanent storage before returning a success indication to the client. The client is therefore guaranteed that a transaction reported to be committed will be preserved, even in the event of a server crash immediately after. However, for short transactions this delay is a major component of the total transaction time. Selecting asynchronous commit mode means that the server returns success as soon as the transaction is logically completed, before the WAL records it generated have actually made their way to disk. This can provide a significant boost in throughput for small transactions.
Asynchronous commit introduces the risk of data loss. There is a short time window between the report of transaction completion to the client and the time that the transaction is truly committed (that is, it is guaranteed not to be lost if the server crashes). Thus asynchronous commit should not be used if the client will take external actions relying on the assumption that the transaction will be remembered. As an example, a bank would certainly not use asynchronous commit for a transaction recording an ATM's dispensing of cash. But in many scenarios, such as event logging, there is no need for a strong guarantee of this kind.
The risk that is taken by using asynchronous commit is of data loss, not data corruption. If the database should crash, it will recover by replaying WAL up to the last record that was flushed. The database will therefore be restored to a self-consistent state, but any transactions that were not yet flushed to disk will not be reflected in that state. The net effect is therefore loss of the last few transactions. Because the transactions are replayed in commit order, no inconsistency can be introduced — for example, if transaction B made changes relying on the effects of a previous transaction A, it is not possible for A's effects to be lost while B's effects are preserved.
The user can select the commit mode of each transaction, so that
it is possible to have both synchronous and asynchronous commit
transactions running concurrently. This allows flexible trade-offs
between performance and certainty of transaction durability.
The commit mode is controlled by the user-settable parameter
synchronous_commit, which can be changed in any of
the ways that a configuration parameter can be set. The mode used for
any one transaction depends on the value of
synchronous_commit when transaction commit begins.
Certain utility commands, for instance
DROP TABLE, are
forced to commit synchronously regardless of the setting of
synchronous_commit. This is to ensure consistency
between the server's file system and the logical state of the database.
The commands supporting two-phase commit, such as
TRANSACTION, are also always synchronous.
If the database crashes during the risk window between an
asynchronous commit and the writing of the transaction's
then changes made during that transaction will be lost.
The duration of the
risk window is limited because a background process (the “WAL
writer”) flushes unwritten WAL records to disk
every wal_writer_delay milliseconds.
The actual maximum duration of the risk window is three times
wal_writer_delay because the WAL writer is
designed to favor writing whole pages at a time during busy periods.
An immediate-mode shutdown is equivalent to a server crash, and will therefore cause loss of any unflushed asynchronous commits.
Asynchronous commit provides behavior different from setting
fsync = off.
fsync is a server-wide
setting that will alter the behavior of all transactions. It disables
all logic within PostgreSQL that attempts to synchronize
writes to different portions of the database, and therefore a system
crash (that is, a hardware or operating system crash, not a failure of
PostgreSQL itself) could result in arbitrarily bad
corruption of the database state. In many scenarios, asynchronous
commit provides most of the performance improvement that could be
obtained by turning off
fsync, but without the risk
of data corruption.
commit_delay also sounds very similar to
asynchronous commit, but it is actually a synchronous commit method
commit_delay is ignored during an
commit_delay causes a delay
just before a transaction flushes WAL to disk, in
the hope that a single flush executed by one such transaction can also
serve other transactions committing at about the same time. The
setting can be thought of as a way of increasing the time window in
which transactions can join a group about to participate in a single
flush, to amortize the cost of the flush among multiple transactions.