Re: What is WAL used for?

Holger Hoffstaette wrote:
> No, you need to take the working of your filesystem into account. As soon
> as data journaling comes into play, it is normal and in fact unavoidable
> that performance drops, because everything is written effectively twice -
> once into the log, once into the file, and to do so the drive has to move.
> WAL with ext3's data journaling is quite unnecessary because the WAL
> sort of IS the database's journal.

Logically seems right but in practice may be untrue. I've found for my
apps, data=journal performs better. When I was picking filesystems, I
did a whole bunch of Googling and there were quite a few people who also
said data=journal performed faster for their Postgres or DB config.
Here's one explanation I found:

"If the database is seeking all over the filesystem and then running
fsync(), then ext3 in data=journal mode can make a huge difference,
because all the dirty data is written out *linearly* to the journal, for
later aysnchronous writeback. This can offer 10x speedups or more."