On Tue, 2 Dec 2014 08:41:11 +1100
NeilBrown [off-list ref] wrote:

On Mon, 1 Dec 2014 16:19:47 -0500 Joe Lawrence [off-list ref]
wrote:

quoted

On Thu, 27 Nov 2014 07:41:58 +1100
NeilBrown [off-list ref] wrote:

quoted

On Mon, 24 Nov 2014 23:07:32 -0500 Joe Lawrence [off-list ref]
wrote:

quoted

Does anyone know how its possible to determine which side of an LVM RAID 1 
is the stale partner during RAID resync?

In ordinary MD RAID, I believe you can check 
/sys/block/md0/md/dev-XXX/state,

Why do you believe that?

During a resync (after an unclean shutdown) the devices are indistinguishable.
RAID1 reads all drives and if there is a difference it chooses one data block
to write to the others - always the one with the lowest index number.

So with md or LVM it is the same: first "first" is "copied" to the "second".

Hi Neil,

Here's a quick example of my thought-process, where md2 is an in-sync
RAID1 of sdq2 and sdr2 with an internal write bitmap:

% mdadm --fail /dev/md3 /dev/sdr2
% mdadm --remove /dev/md3 /dev/sdr2

You are referring to what I would call "recovery", not "resync"
(which is why I put "(after an unclean shutdown)" in my answer to make it
clear what circumstances I was talking about).

resync: fixing things after an unclean shutdown
recovery: restoring data after a device has been removed an another
          (or possibly the same) added.

I think

  dmsetup info 

should provide the info you want.
One of the fields is a sequence of letters 'D', 'a', 'A'.

		 * Status characters:
		 *  'D' = Dead/Failed device
		 *  'a' = Alive but not in-sync
		 *  'A' = Alive and in-sync

Does that provide the information you wanted?

Yes!  When I add a disk back to the array, I see the status characters
you mentioned during _recovery_:

% while [ true ]
do
  dmsetup status vg0-lvraid0
  sleep 10s
done
0 18857984 raid raid1 2 DA 18857984/18857984 idle 0
0 18857984 raid raid1 2 DA 18857984/18857984 idle 0
0 18857984 raid raid1 2 DA 18857984/18857984 idle 0
0 18857984 raid raid1 2 aA 0/18857984 recover 0
0 18857984 raid raid1 2 aA 0/18857984 recover 0
0 18857984 raid raid1 2 aA 256/18857984 recover 0
0 18857984 raid raid1 2 aA 8519680/18857984 recover 0
0 18857984 raid raid1 2 AA 18857984/18857984 idle 0

So now, determining which disk is which in the raid_set.  Can I use a
command like lvs to tie the n-th character status back to a device?

% lvs -a -o name,devices vg0
  LV                 Devices
  lvraid0            lvraid0_rimage_0(0),lvraid0_rimage_1(0)
  [lvraid0_rimage_0] /dev/sdr1(1)
  [lvraid0_rimage_1] /dev/sdt1(1)
  [lvraid0_rmeta_0]  /dev/sdr1(0)
  [lvraid0_rmeta_1]  /dev/sdt1(0)

Where the first character represents [lvraid0_rimage_0] and the second
[lvraid0_rimage_1].

Thanks,

-- Joe