zfs; adding vdev with same size and raid level but different layout

Paul Kraus 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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