Re: Asking for assistance in determining storage requirements

On Thu, 2009-07-09 at 11:15 -0400, Chris Barnes wrote:

>
> We would like to get as much performance from our file systems
> as possible.

Then avoid RAID 5. Raid 10 is a pretty good option for most loads.

Actually, RAID 5 is quite decent for read-mostly large volume storage
where you really need to be disk-space efficient. However, if you spread
the RAID 5 out over enough disks for it to start getting fast reads, you
face a high risk of disk failure during RAID rebuild. For that reason,
consider using RAID 6 instead - over a large set of disks - so you're
better protected against disk failures during rebuild.

If you're doing much INSERTing / UPDATEing then RAID 5/6 are not for
you. RAID 10 is pretty much the default choice for write-heavy loads.

> The postgres database is on 5 drives configured as raid 5 with
> a global hot spare.

> We are curious about using SAN with fiber channel hba and if
> anyone else uses this technology.

There are certainly people on the list using PostgreSQL on a FC SAN. It
comes up in passing quite a bit.

It's really, REALLY important to make sure your SAN honours fsync()
though - at least to the point making sure the SAN hardware has the data
in battery-backed cache before returning from the fsync() call.
Otherwise you risk serious data loss. I'd be unpleasantly surprised if
any SAN shipped with SAN or FC HBA configuration that disregarded
fsync() but it _would_ make benchmark numbers look better, so it's not
safe to assume without testing.

>From general impressions gathered from the list ( I don't use such large
scale gear myself and can't speak personally ) it does seem like most
systems built for serious performance use direct-attached SAS arrays.
People also seem to separate out read-mostly/archival tables,
update-heavy tables, the WAL, temp table space, and disk sort space into
different RAID sets.

--
Craig Ringer