zfs; adding vdev with same size and raid level but different layout
paul at kraus-haus.org
Thu Sep 25 20:41:00 UTC 2014
On 9/25/14 16:09, Stefan Johansson wrote:
> I’m running FreeBSD 9.3-RELEASE with a zpool in raidz1 with 5 2TB disks.
> Now I’m planning to expand the pool with a vdev with the same effective size (8TB).
> Would it be a bad idea to add 3 4TB disks in raidz1 to the pool instead of another 5 2TB disks?
> What could be the disadvantage with such a setup?
The major disadvantage would be performance difference between the two
vdevs, but since most RAIDz1 performance limits are due to the speed of
one drive in the vdev, that should not be a major difference.
Remember to have a large enough hot spare (or even a cold spare on the
shelf) so that _when_ you lose a device you are not waiting a long time
to find out or replace it. With large zpools you are more vulnerable to
multiple device failures due to the longer resilver times. Unlike
hardware RAID, ZFS resilver times are NOT based on raw sequential
performance of the devices, but are limited by host CPU and random I/O
limits of the devices since a resilver is more like a walk of the zpool
in time replaying transactions.
You can even mix up vdev types within a zpool (RAIDz1, 2-way mirror,
RAIDz2, etc.). The biggest disadvantage that has been discussed over on
the ZFS list is that you really can't predict the performance of such a
"hybrid" zpool at all since you never _know_ which vdevs will be
handling the write/read.
Paul Kraus paul at kraus-haus.org
Co-Chair Albacon 2014.5 http://www.albacon.org/2014/
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