[illumos-Discuss] [illumos-Developer] ZFS spare disk usage issue

[Garrett D'Amore]

We've talked about this, and I will be putting together a fix for this
incorrect state handling. :-)

	- Garrett

On Fri, 2011-03-04 at 11:50 -0500, Eric Schrock wrote:
> This looks like a pretty simple bug.  The issue is that the state of
> the SPARE vdev is being reported as REMOVED instead of DEGRADED.  If
> it were the latter (as it should be), then everything would work just
> fine.  Please file a bug at bugs.illumos.org.
> 
> 
> On a side note, this continues to expose the overly simplistic vdev
> state model used by ZFS (one which I can take a bulk of the
> responsibility for).  Back before the days of ditto blocks and SPA3.0,
> it was sufficient to model state as a fairly binary proposition.  But
> this now has ramifications that don't necessarily make sense.  For
> example, one may be able open a pool even if a toplevel vdev is
> faulted.  And even when a spare has finished resilvering, it's left in
> the DEGRADED state, which has implications for allocation policies
> (though I remember discussions around changing this).  But the pool
> state is derived directly from the toplevel vdev state, so if you
> switch spares to be ONLINE, then 'zpool status' would think your pool
> is perfectly healthy.  In this case it's true from a data protection
> standpoint, but not necessarily from a "all is well in the world"
> standpoint, as you are down one spare, and that spare may not have the
> same RAS properties as other devices in your RAID-Z stripe (it may put
> 3 disks on the same controller in one stripe, for example).
> 
> 
> - Eric
> 
> On Fri, Mar 4, 2011 at 7:06 AM, Roy Sigurd Karlsbakk
> <roy at karlsbakk.net> wrote:
>         Hi all
>         
>         I just did a small test on RAIDz2 to check whether my
>         suspicion was right about ZFS not treating spares as
>         replicas/copies of drives, and I think I've found it true. The
>         short story: If two spares replaces two drives in raidz2,
>         losing a third drive, even with the spares active, makes the
>         pool unavailable. See full report on
>         
>         ODT: http://karlsbakk.net/ZFS/ZFS%20Spare%20disk%20usage.odt
>         PDF: http://karlsbakk.net/ZFS/ZFS%20Spare%20disk%20usage.pdf
>         
>         Vennlige hilsener / Best regards
>         
>         roy
>         --
>         Roy Sigurd Karlsbakk
>         (+47) 97542685
>         roy at karlsbakk.net
>         http://blogg.karlsbakk.net/
>         --
>         I all pedagogikk er det essensielt at pensum presenteres
>         intelligibelt. Det er et elementært imperativ for alle
>         pedagoger å unngå eksessiv anvendelse av idiomer med fremmed
>         opprinnelse. I de fleste tilfeller eksisterer adekvate og
>         relevante synonymer på norsk.
>         
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> 
> 
> -- 
> Eric Schrock
> Delphix
> 
> 
> 275 Middlefield Road, Suite 50
> Menlo Park, CA 94025
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> 
> 
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