Re: What is WAL used for?

WAL is write-ahead logging. Basically, before the database actually
performs an operation, it writes in a log what it's about to do. Then, it
goes and does it. This ensures data consistency. Let's say that the
computer was powered off suddenly. There are several points that could
happen:

1) before a write - in this case the database would be fine with or
without write-ahead logging.

2) during a write - without write-ahead logging, if the machine is powered
off during a write, the database has no way of knowing what remained to be
written, or what was being written. WIth Postgres, this is furthere
broken down into two possibilities:

* The power-off occurred while it was writing to the log - in this
case, the log is rolled back. The database is unaffected because the data
was never written to the database proper.

* The power-off occurred after writing to the log, while writing to
disk - in this case, Postgres can simply read from the log what was
supposed to be written, and complete the write.

3) after a write - again, this does not affect Postgres either with or
without WAL.

In addition, WAL increases PostgreSQL's efficiency, because it can delay
random-access writes to disk, and just do sequential writes to the log for
a long time. This reduces the amount of head-seek the dissk are doing.
If you store your WAL files on a different disk, you get even more speed
advantages.

Jon

On Tue, 25 Nov 2003, Relaxin wrote:

> I'm just trying to figure out the terminology that is used on this board and
> wanted to know what is WAL and what roll does it play in Postgresql?
>
> Thanks
>
>
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