Bug#628668: mdadm: Impossible to clear the write-mostly flag in metadata

[Anders Boström]

Package: mdadm
Version: 3.1.4-1+8efb9d1
Severity: normal

When turning off write-mostly on a device in an active raid1 array,
using something like this:

echo -writemostly >/sys/block/md127/md/dev-md2/state

write-mostly is turned off:

flash:~ >mdadm -D /dev/md127 
/dev/md127:
        Version : 1.2
  Creation Time : Thu Oct 15 13:32:06 2009
     Raid Level : raid1
     Array Size : 312581688 (298.10 GiB 320.08 GB)
  Used Dev Size : 312581688 (298.10 GiB 320.08 GB)
   Raid Devices : 2
  Total Devices : 2
    Persistence : Superblock is persistent

    Update Time : Tue May 31 10:35:15 2011
          State : clean
 Active Devices : 2
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 0

           Name : flash:mirror1  (local to host flash)
           UUID : 066d46b2:5ca5113d:261cfcc0:626c2d62
         Events : 842

    Number   Major   Minor   RaidDevice State
       3       9        2        0      active sync   /dev/md/2
       2       8        3        1      active sync writemostly   /dev/sda3
flash:~ >

But the metadata isn't updated:

flash:~ >mdadm -E /dev/md/2
/dev/md/2:
          Magic : a92b4efc
        Version : 1.2
    Feature Map : 0x0
     Array UUID : 066d46b2:5ca5113d:261cfcc0:626c2d62
           Name : flash:mirror1  (local to host flash)
  Creation Time : Thu Oct 15 13:32:06 2009
     Raid Level : raid1
   Raid Devices : 2

 Avail Dev Size : 625163376 (298.10 GiB 320.08 GB)
     Array Size : 625163376 (298.10 GiB 320.08 GB)
    Data Offset : 144 sectors
   Super Offset : 8 sectors
          State : clean
    Device UUID : c995047e:7f56e673:9fc386be:dac782e1

      Flags : write-mostly
    Update Time : Tue May 31 10:29:08 2011
       Checksum : 8020a873 - correct
         Events : 842


   Device Role : Active device 0
   Array State : AA ('A' == active, '.' == missing)
flash:~ >

And after a reboot, md2 is flagged as writemostly again.

/ Anders