Re: Unable to grow raid10 (error -22)

NeilBrown wrote:

On Wed, 25 Apr 2012 10:04:19 +0200 Peter Rabbitson [off-list ref]
wrote:

quoted

I am facing a problem which I believe is new (I distinctly remember
doing the same on 2.6.somethingold couple years ago). When I try to
grow an array where all memembers have been replaced with large ones
I get an inexplicable.

   kernel: md: couldn't update array info. -22

Nothing else. I am using kernel 3.2.0 and mdadm 3.2.3, both from
debian:
http://packages.debian.org/sid/linux-image-3.2.0-2-amd64 (3.2.15-1)
http://packages.debian.org/sid/mdadm (3.2.3-2)

Here is my full test setup replicating the problem (was replicated
on a different machine). Let me know if I can assist further in
figuring this out:

root@Thesaurus:~/raidfail# for n in $(seq 4) ; do dd if=/dev/zero of=member_$n.img bs=1M count=256 ; done

root@Thesaurus:~/raidfail# for n in $(seq 4) ; do losetup /dev/loop$n member_$n.img ; done

root@Thesaurus:~/raidfail# losetup -a
/dev/loop1: [fd01]:1081442 (/root/raidfail/member_1.img)
/dev/loop2: [fd01]:1081481 (/root/raidfail/member_2.img)
/dev/loop3: [fd01]:1081483 (/root/raidfail/member_3.img)
/dev/loop4: [fd01]:1081515 (/root/raidfail/member_4.img)

root@Thesaurus:~/raidfail# mdadm -C /dev/md1010 -l 10 -p f3 -n 4 -c 256 -e 1.2 -z 128M /dev/loop[1234]
mdadm: largest drive (/dev/loop1) exceeds size (131072K) by more than 1%
Continue creating array? y
mdadm: array /dev/md1010 started.

root@Thesaurus:~/raidfail# mdadm -w /dev/md1010

root@Thesaurus:~/raidfail# cat /proc/mdstat
Personalities : [raid1] [raid6] [raid5] [raid4] [raid10]
md1010 : active raid10 loop4[3] loop3[2] loop2[1] loop1[0]
       174080 blocks super 1.2 256K chunks 3 far-copies [4/4] [UUUU]

                                             ^^^^^^^^^^^^

You cannot resize a 'far' RAID10.
In fact in 3.2 you cannot resize *any* RAID10.  That is only available in
3.4-rc, and even then only for 'near' and 'offset' layouts.

Ugh... >.< I guess I got confused by some other raid level then.

What are the reasons for this limitation?