On 4-Aug-09, at 9:28 AM, Roch Bourbonnais wrote:
Le 26 juil. 09 à 01:34, Toby Thain a écrit :
On 25-Jul-09, at 3:32 PM, Frank Middleton wrote:
On 07/25/09 02:50 PM, David Magda wrote:
Yes, it can be affected. If the snapshot's data structure /
record is
underneath the corrupted data in the tree then it won't be able
to be
reached.
Can you comment on if/how mirroring or raidz mitigates this, or tree
corruption in general? I have yet to lose a pool even on a machine
with fairly pathological problems, but it is mirrored (and
copies=2).
I was also wondering if you could explain why the ZIL can't
repair such damage.
Finally, a number of posters blamed VB for ignoring a flush, but
according to the evil tuning guide, without any application syncs,
ZFS may wait up to 5 seconds before issuing a synch,
^^ of course this can never cause inconsistency. The issue under
discussion is inconsistency - unexpected corruption of on-disk
structures.
and there
must be all kinds of failure modes even on bare hardware where
it never gets a chance to do one at shutdown. This is interesting
if you do ZFS over iscsi because of the possibility of someone
tripping over a patch cord or a router blowing a fuse. Doesn't
this mean /any/ hardware might have this problem, albeit with much
lower probability?
The problem is assumed *ordering*. In this respect VB ignoring
flushes and real hardware are not going to behave the same.
--Toby
I agree that noone should be ignoring cache flushes. However the
path to corruption must involve
some dropped acknowledged I/Os. The ueberblock I/O was issued to
stable storage but the blocks it pointed to, which had reached the
disk firmware earlier,
never make it to stable storage. I can see this scenerio when the
disk looses power
Or if the host O/S crashes. All this applies to virtual IDE devices
alone, of course. iSCSI is a different case entirely as presumably
flushes/barriers are processed normally.
but I don't see it with cutting power to the guest.
Right, in this case it's unlikely or nearly impossible.
--Toby
When managing a zpool on external storage, do people export the
pool before taking snapshots of the guest ?
-r
Thanks
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