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SubjectRe: [PATCH] - filesystem corruption on soft RAID5 in 2.4.0+
We'll test and get back to you.

Hans

Neil Brown wrote:
>
> There have been assorted reports of filesystem corruption on raid5 in
> 2.4.0, and I have finally got a patch - see below.
> I don't know if it addresses everybody's problems, but it fixed a very
> really problem that is very reproducable.
>
> The problem is that parity can be calculated wrongly when doing a
> read-modify-write update cycle. If you have a fully functional, you
> wont notice this problem as the parity block is never used to return
> data. But if you have a degraded array, you will get corruption very
> quickly.
> So I think this will solve the reported corruption with ext2fs, as I
> think they were mostly on degradred arrays. I have no idea whether it
> will address the reiserfs problems as I don't think anybody reporting
> those problems described their array.
>
> In any case, please apply, and let me know of any further problems.
>
> --- ./drivers/md/raid5.c 2001/01/21 04:01:57 1.1
> +++ ./drivers/md/raid5.c 2001/01/21 20:36:05 1.2
> @@ -714,6 +714,11 @@
> break;
> }
> spin_unlock_irq(&conf->device_lock);
> + if (count>1) {
> + xor_block(count, bh_ptr);
> + count = 1;
> + }
> +
> for (i = disks; i--;)
> if (chosen[i]) {
> struct buffer_head *bh = sh->bh_cache[i];
>
> From my notes for this patch:
>
> For the read-modify-write cycle, we need to calculate the xor of a
> bunch of old blocks and bunch of new versions of those blocks. The
> old and new blocks occupy the same buffer space, and because xoring
> is delayed until we have lots of buffers, it could get delayed too
> much and parity doesn't get calculated until after data had been
> over-written.
>
> This patch flushes any pending xor's before copying over old buffers.
>
> Everybody running raid5 on 2.4.0 or 2.4.1-pre really should apply this
> patch, and then arrange the get parity checked and corrected on their
> array.
> There currently isn't a clean way to correct parity.
> One way would be to shut down to single user, remount all filesystems
> readonly, or un mount them, and the pull the plug.
> On reboot, raid will rebuild parity, but the filesystems should be
> clean.
> An alternate it so rerun mkraid giving exactly the write configuration.
> This doesn't require pulling the plug, but if you get the config file
> wrong, you could loose your data.
>
> NeilBrown
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