On Wed, 2013-01-09 at 20:00 -0600, Jonathan Brassow wrote:
> DM RAID: Fix RAID10's check for sufficient redundancy
>
> Before attempting to activate a RAID array, it is checked for sufficient
> redundancy. That is, we make sure that there are not too many failed
> devices - or devices specified for rebuild - to undermine our ability to
> activate the array. The current code performs this check twice - once to
> ensure there were not too many devices specified for rebuild by the user
> ('validate_rebuild_devices') and again after possibly experiencing a failure
> to read the superblock ('analyse_superblocks'). Neither of these checks are
> sufficient. The first check is done properly but with insufficient
> information about the possible failure state of the devices to make a good
> determination if the array can be activated. The second check is simply
> done wrong in the case of RAID10 because it doesn't account for the
> independence of the stripes (i.e. mirror sets). The solution is to use the
> properly written check ('validate_rebuild_devices'), but perform the check
> after the superblocks have been read and we know which devices have failed.
> This gives us one check instead of two and performs it in a location where
> it can be done right.
>
> Only RAID10 was affected and it was affected in the following ways:
> - the code did not properly catch the condition where a user specified
> a device for rebuild that already had a failed device in the same mirror
> set. (This condition would, however, be caught at a deeper level in MD.)
> - the code triggers a false positive and denies activation when devices in
> independent mirror sets have failed - counting the failures as though they
> were all in the same set.
>
> The most likely place this error was introduced (or this patch should have
> been included) is in commit 4ec1e369 - first introduced in v3.7-rc1.
>
> Signed-off-by: Jonathan Brassow <jbrassow@xxxxxxxxxx>
Neil,
This patch should apply cleanly on top of recent changes, but will not
apply cleanly on an older kernel (like 3.7) due to version # changes and
introduction of the new 'far' and 'offset' RAID10 algorithms. If you
think this fix should be pushed back into 3.7 rather than just applying
on the latest code, I will make a patch for 3.7 - although I'm not
certain how I'd handle the version number conflict.
Thanks,
brassow
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