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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: Wed, 15 Jul 2015 20:05:56 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Icedove/31.7.0

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 I was looking at implementing detection of outdated RAID members.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?

Let those disks have generation numbers g_0,...,g_{N-1}. Our goal is to
find the largest number G s.t. number of indices with
g_i >= G is at least N-K.
In most common case when you have seen K+1 disks all of them will have
the same generation number
Then we know that
Suppose not then all of 0,...,K are stale and we have lost K+1 drives
which contradicts our goal.
On the other hand when we have seen N-K devices we know that
as with G=min(g_0,...,g_{N-K-1}) we already have N-K disks.

In cases other than mirror usually K+1<=N-K and so we don't even need to
scan for more disks to detect staleness.
The code will be slightly tricky as it has to handle tolerating
staleness if there are too little disks but it's totally feasible. Let
me figure out the rest of math and write a prototype.
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