Re: [mdadm PATCH] bcache: add bcache superblock

On Fri, 11 May 2012, Dan Williams wrote:

This is a hybrid proposal for supporting bcache as a md device.
Somewhat similar to the v1.x metadata format, where array assembly is
handled in userspace, but managed in the kernel.  In the bcache case it
is an "external" metadata format, but then the expectation is that the
kernel "bcache" personality takes over runtime maintenance of the
metadata.

I am having some trouble with this, can you clarify (perhaps by example) 
how to create a pairing of a cache and backing device?

I tried creating directly:

  # mdadm --create /dev/md0 --level=11 --raid-devices=2 /dev/sdb /dev/sdc 
  mdadm: unknown level 11

But as the code in Assemble.c is patched (and not Create.c), I had a hunch 
to format the devices as before then assemble the array:

  # make-bcache -C /dev/sdb
  # make-bcache -B /dev/sdc
  # mdadm -A /dev/md0 /dev/sdb /dev/sdc 
  mdadm: Cannot assemble mbr metadata on /dev/sdb
  mdadm: /dev/sdb has no superblock - assembly aborted

For a hack, I changed Create.c to accept case 11 and leave the chosen 
chunk size. It got further, but then failed with:

  # mdadm --create /dev/md/bcache --level=11 --raid-devices=2 /dev/sdb /dev/sdc 
  mdadm: /dev/sdb appears to be part of a raid array:
      level=raid0 devices=0 ctime=Thu Jan  1 01:00:00 1970
  mdadm: partition table exists on /dev/sdb but will be lost or
         meaningless after creating array
  mdadm: /dev/sdc appears to be part of a raid array:
      level=raid0 devices=0 ctime=Thu Jan  1 01:00:00 1970
  mdadm: partition table exists on /dev/sdc but will be lost or
         meaningless after creating array
  mdadm: largest drive (/dev/sdc) exceeds size (117155216K) by more than 1%
  Continue creating array? y
  mdadm: Defaulting to version 1.2 metadata
  mdadm: RUN_ARRAY failed: Cannot allocate memory

Maybe I missed something basic here, but I'm afraid can't see what? If 
not, hopefully this information is useful.

I use bcache v13 patches and the MD conversion, and this patch to mdadm.

  $ lsmod | grep bcache
  md_bcache               3202  0 
  bcache                138187  1 md_bcache
  md_mod                 88671  5 md_bcache,raid456,raid1

  $ uname -a
  Linux stax 3.4.0-rc7-mh+ #190 SMP PREEMPT Sun May 13 22:36:17 BST 2012 i686 Intel(R) Pentium(R) D CPU 2.80GHz GenuineIntel GNU/Linux

Thanks

-- 
Mark