Re: [PATCH] md: wake up personality thread after array state update

[Tomasz Majchrzak <tomasz.majchrzak@intel.com>]

On Wed, Oct 26, 2016 at 12:14:55PM -0700, Shaohua Li wrote:
> On Tue, Oct 25, 2016 at 05:07:08PM +0200, Tomasz Majchrzak wrote:
> > When raid1/raid10 array fails to write to one of the drives, the request
> > is added to bio_end_io_list and finished by personality thread. The
> > thread doesn't handle it as long as MD_CHANGE_PENDING flag is set. In
> > case of external metadata this flag is cleared, however the thread is
> > not woken up. It causes request to be blocked for few seconds (until
> > another action on the array wakes up the thread) or to get stuck
> > indefinitely.
> > 
> > Wake up personality thread once MD_CHANGE_PENDING has been cleared.
> > Moving 'restart_array' call after the flag is cleared it not a solution
> > because in read-write mode the call doesn't wake up the thread.
> 
> The patch looks good. However can you elaborate how userspace handles the case?
> I'd like to understand what the user interface should be to support external
> metadata array.

1. Kernel encounters new bad block that needs to be acknowledged.

	sysfs array state == "write-pending" (as MD_CHANGE_PENDING set)
	sysfs rdev state == "blocked" (as unacked_exists + external_bbl set)

2. mdmon wakes up as there is an update to sysfs array state and unacknowledged
bad blocks list.

3. mdmon checks the state of each disk. If any is 'blocked' and there is a
support for bad blocks in metadata, it reads unacknowledged bad block list and
records new bad blocks in metadata. If successful, it acknowledges bad blocks by
writing to sysfs bad block file. If all bad blocks have been acknowledged, it
schedules disk unblock.

As soon as kernel marks all bad blocks as acknowledged, it will clear
unacked_exists flag.

4. mdmon checks 'faulty' flag for each disk. If it is set, the disk is removed
from array and unblock is scheduled.

5. mdmon requests to unblock the array by writing '-blocked' to sysfs disk
state.

Requests awaiting for bad block confirmation are woken up in kernel.

6. mdmon writes 'active' to sysfs array state.

MD_CHANGE_PENDING flag is cleared by this step but personality thread is not
woken up. The patch resolves this problem.

I hope it answers your question.

Tomek