Re: raid1 + writemostly

On Monday December 15, jnelson-linux-raid@jamponi.net wrote:

According to the manpage for mdadm, --write-mostly can only be used
add build, create, or add time.  How does one set write-mostly devices
*after* they've been added?

You cannot.
The easiest approach is to remove it and re-add it with the
write-mostly flag.

mdadm /dev/mdX --write-mostly /dev/someDevice
succeeds but does nothing.

According to Documentation/md.txt, writing writemostly into
/sys/block/mdX/md/dev-someDevice/state
should do it, and it sort of appears to:

turnip:~ # mdadm --detail /dev/md11
/dev/md11:

...

    Number   Major   Minor   RaidDevice State
       2      43        0        0      active sync   /dev/nbd0
       3       8        0        1      active sync   /dev/sda
turnip:~ # echo writemostly > /sys/block/md11/md/dev-nbd0/state
turnip:~ # mdadm --detail /dev/md11
/dev/md11:

..

    Number   Major   Minor   RaidDevice State
       2      43        0        0      active sync writemostly   /dev/nbd0
       3       8        0        1      active sync   /dev/sda
turnip:~ # mdadm --examine-bitmap /dev/nbd0
        Filename : /dev/nbd0
           Magic : 6d746962
         Version : 4
            UUID : cf24d099:9e174a79:2a2f6797:dcff1420
          Events : 3840
  Events Cleared : 3840
           State : OK
       Chunksize : 4 MB
          Daemon : 5s flush period
      Write Mode : Normal
       Sync Size : 78123988 (74.50 GiB 80.00 GB)
          Bitmap : 19074 bits (chunks), 0 dirty (0.0%)
turnip:~ #


Why doesn't --examine-bitmap show the right mode?

What were you expecting?  The bitmap never reports anything about
writemostly, only write-behind.

Also, removing the bitmap from the array does not zero out the bitmap
on the actual device. Shouldn't it do that?

No.  It simply records in the array metadata that there is no bitmap.
It might be sensible to get "--examine-bitmap" to report that the
bitmap is not active in some way.

NeilBrown

turnip:~ # mdadm --grow /dev/md11 --bitmap=none
turnip:~ # mdadm --examine-bitmap /dev/nbd0
        Filename : /dev/nbd0
           Magic : 6d746962
         Version : 4
            UUID : cf24d099:9e174a79:2a2f6797:dcff1420
          Events : 3841
  Events Cleared : 3841
           State : OK
       Chunksize : 4 MB
          Daemon : 5s flush period
      Write Mode : Normal
       Sync Size : 78123988 (74.50 GiB 80.00 GB)
          Bitmap : 19074 bits (chunks), 0 dirty (0.0%)
turnip:~ #

This is with mdadm 2.6.4 on 2.6.25.18-0.2-default
I can try mdadm 3.0 if you like.

-- 
Jon
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