Server Fault is a question and answer site for system and network administrators. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

What is a "hot spare" with regard to ZFS RAIDZ on FreeNAS?

I'm hoping that it means an extra, attached disk that can automatically take over for a failed disk in the RAID ... ?

If so, how do I set that up?

share|improve this question
up vote 6 down vote accepted

Yes, that is exactly what a hot spare is. The hot spare drive would occupy a drive bay and could be assigned to one or more data pools (global spare), and would automatically start a rebuilding process in the event of a failed disk.

This is in contrast to a cold spare drive, that would sit outside of the server/enclosure in order to be swapped manually when there's a failure.

share|improve this answer
What exactly do I need to do to set it up to take over? – Ze'ev Aug 18 '12 at 21:32
I used zpool add raid-5x3 spare ada0, and now zpool status lists ada0 as a spare. But is that it? No need to format, create a volume, etc? – Ze'ev Aug 18 '12 at 21:56
Nope. Not much else to do. I also saw the note about spares not working properly in FreeNAS. I would check on that just to make sure... However, spares have always functioned well in my Solaris and Nexenta ZFS environments. – ewwhite Aug 18 '12 at 21:58

the FreeNAS docs are the best place to get this information. Take a look:

share|improve this answer
I don't trust those docs, because so much is out of date. There are many references to obsolete errors & procedures. One thing it says there is: "Use this option with caution as there is a known bug in the current FreeBSD implementation. This will be fixed by zfsd which will be implemented once it is committed to FreeBSD." Is that still the case? – Ze'ev Aug 18 '12 at 21:54

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.