Re: Database File system requirements.

On Mon, 2003-01-06 at 16:49, Gerard Samuel wrote:
> >
> >Surely we'd need to know how many _rows_ there are in those tables? Also
> >how many indexes and index rows?
> >
> True. But not sure how to gather all that info for right now.
> To get a more accurate picture, I dropped the one database that is in
> Postgre, stopped and restarted Postgre.
> It seems to be using just over 62M.
> I recreated the database and file usage remains unchanged (looking via
> df -H), the database isn't really that large maybe about 1000 rows of
> data currently.

That seems normal. I think that PostgreSQL default WAL setup is to have
four write-ahead log files of 16MB each, which would be 64M, and your
data would kind of pale into insignificance beside that.

The WAL files are where PostgreSQL writes changes during a database
transaction before the transaction is committed to the database. Or
something like that... (someone who really understands the database
internals is probably cringing at my description :-)

This overhead is kind of one-off. On systems that do very large
transactions before COMMIT you will find the numbers of these files will
increase, but in general 4 x 16M will be enough for most things. On a
much larger database than the one on my laptop, the WAL files take up
130M even though the DB itself takes up 2.5G.

The WAL files are in the data/pg_xlog directory, separate to the
data/base directory for the actual database files, at least that's how
it's organised under Debian - I think it's the same under other setups
but YMMV.

> >
> >I believe that PostgreSQL does have greater overhead on disk than MySQL,
> >however. Just to have the database started will mean that you have a
> >number of write-ahead logs (typically 16MB each - on my laptop these
> >take up 114M) which are used internally by the database server for
> >transaction handling - MySQL doesn't need these, of course.
> >
> Well if its normal for PostgreSQL to need more disk space, looks like Im
> going to have to do a couple of disk to disk copying soon,
> and repartion my HD.

Hopefully not too dramatic. I've given up on multiple partitions for my
laptop the last few years, currently: 100M /boot, 512M of tmpfs on /tmp
and the remaining 59G on / and I have saved myself a lot of hassles.

Cheers,
Andrew.
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