zfs discuss - Feb 2007 - ZFS tie-breaking

Consider the following scenario involving various failures.

We have a zpool composed of a simple mirror of two devices D0 and D1 
(these may be local disks, slices, LUNs on a SAN, or whatever).  For the 
sake of this scenario, it''s probably most intuitive to think of them as
LUNs on a SAN.  Initially, all is well and both halves of the mirror are 
in sync; the data on D0 is fully consistent with that on D1.

Then there is a failure, such that D1 becomes disconnected.  ZFS 
continues to write on D0.  If D1 were to become reconnected, it would 
get resilvered normally and all would be well.

But suppose instead there is a crash, and when the system reboots it is 
connected only to D1, and D0 is not available.  Does ZFS have any way to 
know that the data on D1 (while self-consistent) is stale and should not 
be used?

The specific case of interest is not necessarily a single-server 
environment (although thinking of just one server simplifies the 
scenario without reducing it too far), but a cluster where ZFS is used 
as a fail-over file system and connectivity issues are more likely to 
arise.  SVM does have a means of detecting this scenario and refusing to 
mount the stale mirror.

Thanks.
-- 
	--Ed
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> Then there is a failure, such that D1 becomes disconnected.  ZFS 
> continues to write on D0.  If D1 were to become reconnected, it would 
> get resilvered normally and all would be well.
> 
> But suppose instead there is a crash, and when the system reboots it is 
> connected only to D1, and D0 is not available.  Does ZFS have any way to 
> know that the data on D1 (while self-consistent) is stale and should not 
> be used?
> 
> The specific case of interest is not necessarily a single-server 
> environment (although thinking of just one server simplifies the 
> scenario without reducing it too far), but a cluster where ZFS is used 
> as a fail-over file system and connectivity issues are more likely to 
> arise.  SVM does have a means of detecting this scenario and refusing to 
> mount the stale mirror.
Are you referring to SVMs requirement for a strict quorum or some other
mechanism?

I don''t know what requirements ZFS has on pool membership for import...

-- 
Darren Dunham                                           ddunham at taos.com
Senior Technical Consultant         TAOS            http://www.taos.com/
Got some Dr Pepper?                           San Francisco, CA bay area
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