ZFS performance gains real or imaginary?
James R. Van Artsdalen
james-freebsd-fs2 at jrv.org
Fri Dec 19 01:09:11 PST 2008
Matt Simerson wrote:
> I haven't benchmarked it with -HEAD but with FreeBSD 7, using a ZFS
> mirror across two 12-disk hardware RAID arrays (Areca 1231ML) was
> significantly (not quite double) faster than using JBOD and raidz. I
> tested a few variations (four disk pools, six disk zpools, 8 disk
> zpools, etc).
A backup server is a *highly* specialized type of server. It's likely
that data is only rarely updated, meaning that there are very few
partial parity-stripe writes for the Areca to deal with. A database
server receiving many updates would have an entirely different pattern
of write I/O, possibly forcing many partial stripe updates. Since ZFS
(almost?) never does partial stripe writes in a RAIDZ the performance
comparison between ZFS with JBOD and your hardware setup might change
considerably with a database workload. Not to mention the dominance of
sequential I/O in a backup server, etc.
For a backup server ZFS has other advantages. A client's backup server
recently ran low on space so I took over another 4x1GB enclosure and
added it to the pool with no downtime: there were a couple of large file
writes to that pool running when I arrived that were still going when I
There's also the issue of cost: once SATA port multiplier support works
in FreeBSD it will be very practical to build cheap ~15TB servers for a
small business using ZFS.
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