Re: Safely swapping a disk in a RAID456

[John Robinson <john.robinson@yuiop.co.uk>]

On 30/09/2011 16:42, martin f krafft wrote:
> Dear list,
>
> I would like to swap out a drive in a RAID6 (4 drives), but ideally
> without letting the RAID degrade. I was thinking that it should be
> possible to declare the new disc a copy of the old one, let it sync,
> then remove the old one and let the new one take over, but MD does
> not seem to support that.

Not yet, but it's in the roadmap as a hot-replace. In the mean time, you 
can do something very similar by hand with almost no lost of redundancy, 
if your RAID456 has a bitmap.

> Next, I thought that I could add the new disc as a spare, grow the
> array to 5 discs, and somehow shrink it back to 4 again, but of
> course that does not work either, since 4→5 is a reshape, and there
> seems to be no way to control which disc to remove from the RAID6 on
> a shrink (making it a spare), without degrading the array in the
> process.

Doing a shrink while specifying which drive to remove is also on the 
roadmap, I believe. But doing huge reshapes like this are not the way 
forward for a hot-replace.

> Does anyone have any ideas? This should not be so hard…

I'd be glad to hear of any "real" RAID card that made it easier, or even 
possible.

Something along the lines of:

# briefly remove your disc from the array
mdadm --manage /dev/md_raid456 --remove /dev/the_disc_i_want_to_remove
# make a temp raid1 with only the one disc, using build so metadata is
# only in RAM and nothing is written to the disc
mdadm --build /dev/md_temp_raid1 --level 1 
/dev/the_disc_i_want_to_remove missing
# now re-add something which looks identical to your original disc,
# but is actually a single-sided mirror, back into the array
# the above can all be done very quickly so your raid456 only runs
# without a drive for seconds
mdadm --manage /dev/md_raid456 --re-add /dev/md_temp_raid1
# now get md to make a mirror (copy) to the new disc
mdadm --manage /dev/md_temp_raid1 --add /dev/the_new_disc
# wait for it to finish, or just wait for it yourself
mdadm --wait /dev/md_temp_raid1
# and switch back again: remove the temporary raid1
mdadm --manage /dev/md_raid456 --remove /dev/md_temp_raid1
# stop it so the new disc becomes available again
mdadm --stop /dev/md_temp_raid1
# and put the new disc which is now a complete copy of the old one
# back in to the array
mdadm --manage /dev/md_raid456 --re-add /dev/the_new_disc
# and you're done

This is all fine if your old disc has no faulty sectors. If it does 
have, you need more help from someone much more clued-up than me 
(because it is already possible to do partial rebuilds by manipulating 
/sys), or mdadm's roadmap feature of hot-replace, which will do the 
above and also automatically perform partial rebuilds from the rest of 
the array when the old disc has bad sectors.

Hope this helps.

Cheers,

John.

-- 
John Robinson, yuiop IT services
0131 557 9577 / 07771 784 058
46/12 Broughton Road, Edinburgh EH7 4EE
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