On 09/30/11 20:02, John Robinson wrote:
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.
...
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
Very good, but...
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),
This is the problem. And if you want to replace a drive, it's probably
because it has bad sectors.
I think that during raid1 device-add (which automatically initiates
rebuild of raid1), the raid1 would go down completely as soon as it hits
bad sectors.
So you wouldn't be in a position to use rebuilds by /sys "afterwards",
because there is no "afterwards".
For this to work, somebody would need to implement the bad block list
also on the legacy array without metadata (--build), so that the raid1
would stay up even on bad reads during its rebuild. Actually I don't
know if Neil implemented that or not, I have not tested bad blocks list
yet, I am intuitively assuming that he improved only the last version of
the arrays, that is 1.x .
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.
the hot-replace is the most wanted feature as of now.
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