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

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

[ ... File I/O to /dev/md3 ... ]

% mdadm -X /dev/sd[qr]2
        Filename : /dev/sdq2
           Magic : 6d746962
         Version : 4
            UUID : 073511ee:0b0c20e0:662ae8da:b53c7979
          Events : 8526                                              << ECq
  Events Cleared : 8498
           State : OK
       Chunksize : 64 MB
          Daemon : 5s flush period
      Write Mode : Normal
       Sync Size : 16768896 (15.99 GiB 17.17 GB)
          Bitmap : 256 bits (chunks), 5 dirty (2.0%)
        Filename : /dev/sdr2
           Magic : 6d746962
         Version : 4
            UUID : 073511ee:0b0c20e0:662ae8da:b53c7979
          Events : 8513                                              << ECr
  Events Cleared : 8498
           State : OK
       Chunksize : 64 MB
          Daemon : 5s flush period
      Write Mode : Normal
       Sync Size : 16768896 (15.99 GiB 17.17 GB)
          Bitmap : 256 bits (chunks), 5 dirty (2.0%)

[ Note that ECq > ECr, which makes sense since sdq was the remaining
  disk standing in the RAID. ]

% mdadm --add /dev/md3 /dev/sdr2
% mdadm --detail /dev/md3
/dev/md3:
        Version : 1.2
  Creation Time : Thu Nov 13 15:47:19 2014
     Raid Level : raid1
     Array Size : 16768896 (15.99 GiB 17.17 GB)
  Used Dev Size : 16768896 (15.99 GiB 17.17 GB)
   Raid Devices : 2
  Total Devices : 2
    Persistence : Superblock is persistent

  Intent Bitmap : Internal

    Update Time : Mon Dec  1 16:07:55 2014
          State : active, degraded, recovering 
 Active Devices : 1
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 1

 Rebuild Status : 0% complete

           Name : dhcp-linux-2192-2025:3
           UUID : 073511ee:0b0c20e0:662ae8da:b53c7979
         Events : 8528

    Number   Major   Minor   RaidDevice State
       0      65        2        0      active sync   /dev/sdq2
       1      65       18        1      spare rebuilding   /dev/sdr2

% head /sys/block/md3/md/dev-sd*/state
==> /sys/block/md3/md/dev-sdq2/state <==
in_sync

==> /sys/block/md3/md/dev-sdr2/state <==
spare

In this scenario, sdr was re-added to the RAID and with a lower events-
cleared count.  I assume that MD will only need to read the data
represented by the dirty bitmap bits from the "active sync" disk to the
"spare rebuilding" disk.  Is this not the case?

Regards,

-- Joe