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Re: [RFD] diskfilter stale RAID member detection vs. lazy scanning

From: Vladimir 'φ-coder/phcoder' Serbinenko
Subject: Re: [RFD] diskfilter stale RAID member detection vs. lazy scanning
Date: Thu, 16 Jul 2015 10:01:43 +0200
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On 16.07.2015 05:42, Andrei Borzenkov wrote:
> В Wed, 15 Jul 2015 20:05:56 +0200
> Vladimir 'φ-coder/phcoder' Serbinenko <address@hidden> пишет:
>> On 28.06.2015 20:06, Andrei Borzenkov wrote:
>>> I was looking at implementing detection of outdated RAID members.
>>> Unfortunately it appears to be fundamentally incompatible with lazy
>>> scanning as implemented currently by GRUB. We simply cannot stop
>>> scanning for other copies of metadata once "enough" was seen. Because
>>> any other disk may contain more actual copy which invalidates
>>> everything seen up to this point.
>>> So basically either we officially admit that GRUB is not able to detect
>>> stale members or we drop lazy scanning.
>>> Comments, ideas?
>> We don't need to see all disks to decide that there is no staleness. If
>> you have an array with N devices and you can lose at most K of them,
>> then you can check for staleness after you have seen max(K+1, N-K)
>> drives. Why?
> It's not the problem. The problem is what to do if you see disk with
> generation N+1 after you assembled array with generation N. This can
> mean that what we see is old copy and we should through it away and
> start collecting new one. If I read Linux MD code correctly, that is
> what it actually does. And this means we cannot stop scanning even
> after array is complete.
While it's true that it's possible that all the members we have seen are
stale, it shouldn't be common and it's not the biggest problem. Biggest
problem is inconsistency.
We can never guarantee of having seen all the disks as they may not be
eeven visible through firmware but it shouldn't stop us from fixing the
inconsistency problem.
> Extreme example is three-pieces mirror where each piece is actually
> perfectly valid and usable by itself so losing two of them still means
> we can continue to work with remaining one.
Mirrors get completely assembled in my patch.

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