Re: ZFS vs. UFS

On 07/24/2012 08:51 AM, Laszlo Nagy wrote:
> Under FreeBSD 9, what filesystem should I use for PostgreSQL? (Dell
> PowerEdge 2900, 24G mem, 10x2T SATA2 disk, Intel RAID controller.)

When Intel RAID controller is that? All of the ones on the motherboard
are pretty much useless if that's what you have. Those are slower than
software RAID and it's going to add driver issues you could otherwise
avoid. Better to connect the drives to the non-RAID ports or configure
the controller in JBOD mode first.

Using one of the better RAID controllers, one of Dell's good PERC models
for example, is one of the biggest hardware upgrades you could make to
this server. If your database is mostly read traffic, it won't matter
very much. Write-heavy loads really benefit from a good RAID
controller's write cache.

> * ZFS is journaled, and it is more independent of the hardware. So
> if the computer goes wrong, I can move the zfs array to a
> different server.
> * UFS is not journaled. Also I have to rely on the RAID card to
> build the RAID array. If there is a hw problem with it, then I
> won't be able to recover the data easily.
>

You should be able to get UFS working with a software mirror and
journaling using gstripe/gmirror or vinum. It doesn't matter that much
for PostgreSQL though. The data writes are journaled by the database,
and it tries to sync data to disk after updating metadata too. There
are plenty of PostgreSQL installs on FreeBSD/UFS that work fine.

ZFS needs more RAM and has higher CPU overhead than UFS does. It's a
heavier filesystem all around than UFS is. Your server is fast enough
that you should be able to afford it though, and the feature set is
nice. In addition to the RAID setup being simple to handle, having
checksums on your data is a good safety feature for PostgreSQL.

ZFS will heavily use server RAM for caching by default, much more so
than UFS. Make sure you check into that, and leave enough RAM for the
database to run too. (Doing *some* caching that way is good for
Postgres; you just don't want *all* the memory to be used for that)

Moving disks to another server is a very low probability fix for a
broken system. The disks are a likely place for the actual failure to
happen at in the first place. I like to think more in terms of "how can
I create a real-time replica of this data?" to protect databases, and
the standby server for that doesn't need to be an expensive system.
That said, there is no reason to set things up so that they only work
with that Intel RAID controller, given that it's not a very good piece
of hardware anyway.

--
Greg Smith 2ndQuadrant US greg(at)2ndQuadrant(dot)com Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.com