linux-kernel - Re: Kernel 2.6.17 and RAID5 Grow Problem (critical section backup)

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Date:	Tue, 18 Jul 2006 10:16:27 +1000
From:	Neil Brown <neilb@...e.de>
To:	Petr Vyskocil <petr@...me.cz>
Cc:	linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Kernel 2.6.17 and RAID5 Grow Problem (critical section backup)

On Tuesday July 11, petr@...me.cz wrote:
> >>> Hm, what's superblock 0.91? It is not mentioned in mdadm.8.
> >>>
> >> Not sure, the block version perhaps?
> >>
> > Well yes of course, but what characteristics? The manual only lists
> >  0, 0.90, default
> >  1, 1.0, 1.1, 1.2
> > No 0.91 :(
> 
> 
> AFAICR superblock version gets raised by 0.01 for the duration of 
> reshape, so that non-reshape aware kernels do not try to assemble it 
> (and cause data corruption).

Exactly.  The following will be in the next mdadm - unless someone
wants to re-write it for me using shorter sentences :-)

NeilBrown



diff .prev/md.4 ./md.4
--- .prev/md.4	2006-06-20 10:01:17.000000000 +1000
+++ ./md.4	2006-07-18 10:14:47.000000000 +1000
@@ -74,6 +74,14 @@ UUID
 a 128 bit Universally Unique Identifier that identifies the array that
 this device is part of.
 
+When a version 0.90 array is being reshaped (e.g. adding extra devices
+to a RAID5), the version number is temporarily set to 0.91.  This
+ensures that if the reshape process is stopped in the middle (e.g. by
+a system crash) and the machine boots into an older kernel that does
+not support reshaping, then the array will not be assembled (which
+would cause data corruption) but will be left untouched until a kernel
+that can complete the reshape processes is used.
+
 .SS ARRAYS WITHOUT SUPERBLOCKS
 While it is usually best to create arrays with superblocks so that
 they can be assembled reliably, there are some circumstances where an
-
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