Re: [GIT PATCH 0/2] external-metadata recovery checkpointing for 2.6.33

[Dan Williams <dan.j.williams@intel.com>]

On Mon, Dec 14, 2009 at 9:19 PM, Dan Williams <dan.j.williams@intel.com> wrote:
> On second thought, if we get to activate_spare() it's already too
> late.  Moving this to mdadm at assembly time (prior to setting
> readonly) is a better approach.
>

Problem.  slot_store() in the array inactive case currently does:

                /* assume it is working */
                clear_bit(Faulty, &rdev->flags);
                clear_bit(WriteMostly, &rdev->flags);
                set_bit(In_sync, &rdev->flags);
                sysfs_notify_dirent(rdev->sysfs_state);

i.e. sets the disk insync even if we specified a recovery_start <
MaxSector.  If userspace can guarantee that the array stays inactive
then it can write to 'recovery_start' after 'slot' and catch attempts
to cold_add() out-of-sync disks on pre-2.6.33 kernels, but that gives
a window of invalid configuration.  The other fix is to remove the
set_bit(In_sync), and then for the pre-2.6.33 case userspace would
need to disallow adding out-of-sync disks and force them through the
hot_add() case.  This is how mdadm/mdmon currently operates, but that
is a surprising ABI quirk when switching to/from 2.6.33.  A third
option is to allow recovery_start_store to be modified while the array
is read only. Although not my favorite, because it requires tricky
mdmon logic to catch activate_spare() attempts before the monitor
thread starts touching the array, it has the benefit of not changing
any old behavior and no window of invalid configuration.  Thoughts??

--
Dan
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