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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?

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2 Answers 2

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.

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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: http://doc.freenas.org/index.php/Volumes

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I don't trust those docs, because so much is out of date.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

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