[OpenIndiana-discuss] Replacing both disks in a mirror set

Richard Elling richard.elling at richardelling.com
Tue Oct 9 00:39:27 UTC 2012

On Oct 8, 2012, at 4:07 PM, Martin Bochnig <martin at martux.org> wrote:

> Marilio,
> at first a reminder: never ever detach a disk before you have a third
> disk that already completed resilvering.
> The term "detach" is misleading, because it detaches the disk from the
> pool. Afterwards you cannot access the disk's previous contents
> anymore. Your "detached" half of a mirror can neither be imported, nor
> mounted and also not even rescued (unlike a disk with a "zpool
> destroy"ed disk). If I ever mentally recover from a zfs encryption
> caused 2TB (or 3 years!) data loss, then I may offer an implementation
> with less ambigous naming to Illumos.
> "zpool detach" suggests, that you could still use this disk as a
> reserve backup copy of the pool you were detaching it from.

No it doesn't -- there is no documentation that suggests this usage.

> And that
> you could simply "zpool attach" it again, in case the other disk would
> die.

You are confusing zpool detach and zpool split commands.
 -- richard

> Unfortunately, this is not the case.
> Well, you can of course attach it again. Like any new or empty disk.
> But only if and only if you have enough replicas, and that's not what
> one wanted if one fell in this misunderstanding trap.
> And there are no warnings in the zpool/zfs man pages.
> What you want:
> zpool replace <poolname> <vdev to be replaced> <new vdev>
> But last weekend I lost 7 years of trust that I had in ZFS.
> Because Oracle Solaris 11/11 x86 with an encrypted and gzip-9
> compressed mirror cannot be accessed anymore after VirtualBox forced
> me to remove prower from the host machine.
> Since then a 1:1 mirror of 2TB disks cannot be mounted anymore. It
> always ends in a kernel panic due to a pf in
> aes:aes_decrypt_contiguous_blocks.
> The problem is, that scrub doesn't find an error, and so has nothing
> to auto-repair.
> Even zpool attach sucessfully completes resilver, but the newly
> resilvered disk contains the same error. Be aware that ZFS is not free
> of bugs.
> If it stays like that (I contacted some folks for help), then my trust
> in ZFS has destroyed, VAPORIZED 3 years of my work and life.
> So, back to your question: To be as cautious as possible, what I would
> do in your case:
> 0.)  zpool offline <poolname> <vdev you want to replace>
> 1.) Physically remove this disc (important, because I have seen cases,
> where zfs forgets that you offlined a vdev after a reboot)
> 2.) AFTER (!IMPORTANT!) you physically disconnected the disc to be
> replaced, "zpool detach it" or alternatively take "zpool replace
> <poolname> <oldvdev_that_you_disconnected_BEFOREinordertokeepitasbafailsafebackup!>
> <newvdev>
> 3.) Depending on if you did detach or replace in step 2.), "zpool
> attach <poolname> <Firstvdevofthispool> <newvdev>  or ommit this step,
> if you took "zpool replace" in step 2.)
> What I do from now on: For each 1:1 mirror that I have I will take a
> third disk, resilver it, offline and physically disconnect it, and
> store it at a secure place.
> Because if you have this much bad luck as I had last weekend, ZFS
> replicates the data corruption, too.
> And then you could have 1000 discs mirrored, they would all contain
> the corruption.
> For this reason, you are only on the safe side, if you physically
> disconnect a third copy!
> Good luck!
> %martin
> On 10/8/12, Maurilio Longo <maurilio.longo at libero.it> wrote:
>> Dan Swartzendruber wrote:
>>> I'm not understanding your problem.  If you add a 3rd temporary disk,
>>> wait
>>> for it to resilver, then replace c1t5d0, let the new disk resilver, then
>>> detach the temporary disk, you will never have less than 2 up to date
>>> disks
>>> in the mirror. What am I missing?
>> Dan,
>> you're right, I was trying to find a way to "move" the new disk in the
>> failing
>> disk bay instead of simply replacing the failing one :)
>> Thanks for the advice!
>> Maurilio.
>> --
>> __________
>> |  |  | |__| Maurilio Longo
>> |_|_|_|____|
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> -- 
> regards
> %martin bochnig
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Richard.Elling at RichardElling.com

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