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
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