Re: Making spare device into active

From: NeilBrown <neilb@suse.de>
To: "Patrik Horník" <patrik@dsl.sk>
Cc: linux-raid@vger.kernel.org
Subject: Re: Making spare device into active
Date: Sat, 9 Aug 2014 10:31:59 +1000	[thread overview]
Message-ID: <20140809103159.338bef61@notabene.brown> (raw)
In-Reply-To: <CAAOsTSmkyCqLxrG-q2PQxnbXiAC1zz=qw6+ekMUmY=N+ZoHZ-w@mail.gmail.com>

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On Fri, 8 Aug 2014 19:25:24 +0200 Patrik Horník <patrik@dsl.sk> wrote:

> Hello Neil,
> 
> I am experiencing the problem with one RAID6 array.
> 
> - I was running degraded array with 3 of 5 drives. When adding fourth
> HDD one of the drives reported read errors, later disconnected and
> then it was kicked out from array. (It was maybe doing of controller
> and not drive, not important.)
> 
> - The array has internal intent bitmap. After the drive reconnected
> I've tried it to --re-add to array with 2 of 5 drives. I am not sure
> if that should work? But it did not, recovery got interrupted just
> after start and drive was marked as spare.

No, that is not expected to work.  RAID6 survives 2 device failures, not 3.
Once three have failed, the array has failed.  You have to stop it, and maybe
put it back together.

> 
> - Right now I want to assemble array to get data out of it. Is it
> possible to change "device role" field in device's superblock so it
> can be assembled? I I have --examine and --detail output from before
> the problem and so I know at which position the kicked drive belongs.

Best option is to assemble with --force.
If that works then you might have a bit of data corruption, but most of the
array should be fine.

If it fails, you probably need to carefully re-create the array with all the
right bits in the right places.  Maybe sure to create it degraded so that it
doesn't automatically resync, otherwise if you did something wrong you could
suddenly lose all hope.

But before you do any of that, you should make sure your drives and
controller are actually working.  Completely.
If any drive has any bad blocks, then get a replacement drive and copy
everything (maybe using ddrescue) from the failing drive to a good drive.

There is no way to just change arbitrary fields in the superblock, so you
cannot simply "set the device role".

Good luck.

NeilBrown

> 
> - Changing device role field seems much safer way than recreating
> array with --assume-clean, because with recreating too much things can
> go wrong...
> 
> Thanks.
> 
> Patrik

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