"Jeff Larsen" <jlar310@gmail.com> writes:
> Informix keeps transaction logs in a dedicated, pre-allocated disk
> area that, until very recent versions, could not grow dynamically. It
> is the DBA's responsibility to continually backup these transaction
> logs so the space may be recycled. As such, Informix is limited in the
> size of a transaction that it can roll back, because it eventually has
> to re-use existing transaction log space.
Yeah, Oracle has that problem too.
Postgres keeps the old row versions in the main data area, so the disk
space cost of a long transaction is paid out of your main data store,
not any dedicated area. Once the transaction is committed or rolled
back, a subsequent VACUUM will eventually make the now-redundant space
available for re-use.
Our way has its pluses and minuses compared to the other, but it's
definitely quite different. Instead of worrying about transaction
log size, you worry about how often to VACUUM.
> Since the WAL is not strictly pre-allocated space, can it just keep
> going until the WAL files fill up the free disk space? What would be
> the consequences of such an incident (filling up disk space with WAL
> files)? Is the WAL even relevant to rollbacks?
It's not; the amount of WAL space needed is determined only by the
checkpoint spacing. (You can have transactions that run much longer
than the checkpoint interval.)
If you do run out of space for WAL, the database PANICs and shuts
down (but without losing any committed transactions, so you can restart
once you've cleared off some space). One advantage to keeping WAL and
main data area on separate partitions is that then bloat of the main
data area cannot lead to a PANIC of this type, though out-of-space
in the data area is still going to lead to failures of inserts and
updates.
regards, tom lane
