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SubjectRe: [RFC][PATCH] md: avoid fullsync if a faulty member missed a dirty transition
On Thu, May 8, 2008 at 9:40 PM, Neil Brown <neilb@suse.de> wrote:
>
> On Thursday May 8, snitzer@gmail.com wrote:
> > On Thu, May 8, 2008 at 2:13 AM, Neil Brown <neilb@suse.de> wrote:
> > > On Tuesday May 6, snitzer@gmail.com wrote:
> > > >
> > > > It looks like bitmap_update_sb()'s incrementing of events_cleared (on
> > > > behalf of the local member) could be racing with the fact that the NBD
> > > > member becomes faulty (whereby making the array degraded). This
> > > > allows the events_cleared to reflect a clean->dirty transition last
> > > > occurred before the array became degraded. My reasoning is: If it was
> > > > a clean->dirty transition the bitmap still has the associated dirty
> > > > bit set in the local member's bitmap, so using the bitmap to resync is
> > > > valid.
> > > >
> > > > thanks,
> > > > Mike
> > >
> > > Thanks for persisting. I think I understand what is going on now.
> > >
> > > How about this patch? It is similar to your, but instead of depending
> > > on the odd/even state of the event counter, it directly checks the
> > > clean/dirty state of the array.
> >
> > Hi Neil,
> >
> > Your revised patch works great and is obviously cleaner.
>
> But I'm still not happy with it :-(
> I suspect there might be other cases where it will still do the wrong
> thing.
> The real problem is that we are updating events_cleared to early. We
> are setting to the new event counter before that is even written out.
>
> So I've come up with this patch, which I think more clearly
> encapsulated what events_cleared means. It is now set to the current
> 'events' counter immediately before we clear any bit.
>
> If you could test it, I'd really appreciate it.

Unfortunately my testing with this patch results in a full resync.

Here is the state of the array after shutdown:
# mdadm -X /dev/nbd0 /dev/sdq
Filename : /dev/nbd0
Magic : 6d746962
Version : 4
UUID : 7140cc3c:8681416c:12c5668a:984ca55d
Events : 896
Events Cleared : 897
State : OK
Chunksize : 128 KB
Daemon : 5s flush period
Write Mode : Normal
Sync Size : 52428736 (50.00 GiB 53.69 GB)
Bitmap : 409600 bits (chunks), 1 dirty (0.0%)

Filename : /dev/sdq
Magic : 6d746962
Version : 4
UUID : 7140cc3c:8681416c:12c5668a:984ca55d
Events : 898
Events Cleared : 897
State : OK
Chunksize : 128 KB
Daemon : 5s flush period
Write Mode : Normal
Sync Size : 52428736 (50.00 GiB 53.69 GB)
Bitmap : 409600 bits (chunks), 0 dirty (0.0%)

# mdadm --examine /dev/nbd0 /dev/sdq
/dev/nbd0:
Magic : a92b4efc
Version : 00.90.00
UUID : 7140cc3c:8681416c:12c5668a:984ca55d
Creation Time : Thu May 8 06:55:32 2008
Raid Level : raid1
Used Dev Size : 52428736 (50.00 GiB 53.69 GB)
Array Size : 52428736 (50.00 GiB 53.69 GB)
Raid Devices : 2
Total Devices : 2
Preferred Minor : 0

Update Time : Thu May 8 18:07:47 2008
State : clean
Internal Bitmap : present
Active Devices : 2
Working Devices : 2
Failed Devices : 0
Spare Devices : 0
Checksum : df65cb35 - correct
Events : 0.896


Number Major Minor RaidDevice State
this 1 43 0 1 active sync write-mostly /dev/nbd0

0 0 65 0 0 active sync /dev/sdq
1 1 43 0 1 active sync write-mostly /dev/nbd0


/dev/sdq:
Magic : a92b4efc
Version : 00.90.00
UUID : 7140cc3c:8681416c:12c5668a:984ca55d
Creation Time : Thu May 8 06:55:32 2008
Raid Level : raid1
Used Dev Size : 52428736 (50.00 GiB 53.69 GB)
Array Size : 52428736 (50.00 GiB 53.69 GB)
Raid Devices : 2
Total Devices : 2
Preferred Minor : 0

Update Time : Thu May 8 18:07:49 2008
State : clean
Internal Bitmap : present
Active Devices : 1
Working Devices : 1
Failed Devices : 1
Spare Devices : 0
Checksum : df65c956 - correct
Events : 0.898


Number Major Minor RaidDevice State
this 0 65 0 0 active sync /dev/sdq

0 0 65 0 0 active sync /dev/sdq
1 1 0 0 1 faulty removed

Was I supposed to use this latest patch in combination with your
previous patch (to validate_super)? Because you'll note that with
your most recent patch nbd0's events (ev1) is still one less than
sdq's events_cleared. As such the validate_super's "ev1 <
mddev->bitmap->events_cleared" check triggers a full rebuild.

The kernel log shows:
md: md0 stopped.
md: bind<nbd0>
md: bind<sdq>
md: kicking non-fresh nbd0 from array!
md: unbind<nbd0>
md: export_rdev(nbd0)
raid1: raid set md0 active with 1 out of 2 mirrors
md0: bitmap initialized from disk: read 13/13 pages, set 0 bits, status: 0
created bitmap (200 pages) for device md0
Nope!!! ev1 (896) < mddev->bitmap->events_cleared (897)
md: bind<nbd0>
RAID1 conf printout:
--- wd:1 rd:2
disk 0, wo:0, o:1, dev:sdq
disk 1, wo:1, o:1, dev:nbd0
md: recovery of RAID array md0

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