On Mon, May 22 2017, Andreas Klauer wrote: > Hi, > > this is not a recovery question, no real data involved. Thanks for helping! > > Suppose you have a failing drive in RAID-5 but you wanted to move to > fewer drives anyway, so one way or another you're going to reduce > the number of drives in your RAID. > > Given a RAID 5 with 5 drives [UUUUU] > Reducing it by one drive results in [UUUU] + Spare > Okay. > > Given a degraded RAID 5 with 5 drives [_UUUU] > Reducing it by one drive results in [_UUU] + Spare > Still okay? Rebuild must be started manually. > > It seems reducing a degraded RAID is a bad idea, > since there is no redundancy for a very long time. > > So what you might end up doing is a three step process: > > -> [_UUUU] (Degraded) > > Step 1: Add another drive (redundancy first) > > -> [UUUUU] > ^ added drive > > Step 2: Reduce by one drive > > -> [UUUU] + Spare > > Step 3: --replace the previously added drive > (if the spare happened to be one of the drives you wanted to keep) > > -> [UUUU] > ^ former spare > > This way the process is redundant but it takes a very long time, > three separate reshape/rebuilds instead of just one. > > Steps to reproduce the [_UUUU] -> [_UUU] + Spare case: > (using linux 4.10, mdadm 4.0) > > # truncate -s 100M 1.img 2.img 3.img 4.img > # devices=$(for f in ?.img; do losetup --find --show "$f"; done) > # mdadm --create /dev/md42 --level=5 --raid-devices=5 missing $devices > md42 : active raid5 loop4[4] loop3[3] loop2[2] loop1[1] > 405504 blocks super 1.2 level 5, 512k chunk, algorithm 2 [5/4] [_UUUU] > # mdadm --grow /dev/md42 --array-size 304128 > # mdadm --grow /dev/md42 --backup-file=md42.backup --raid-devices=4 > md42 : active raid5 loop4[4](S) loop3[3] loop2[2] loop1[1] > 304128 blocks super 1.2 level 5, 512k chunk, algorithm 2 [4/3] [_UUU] > # not rebuilding until you re-add the spare > > Is it possible to do [_UUUU] -> [UUUU] in a single step? > I haven't found a way. Any ideas? I hoped that mdadm --grow /dev/md42 --backup-file=... --raid-device=4 --add /dev/loop4 would have worked, but it doesn't. What does work is: # start with a degraded array, device 0 missing mdadm --grow /dev/md42 --array-size=..... echo frozen > /sys/block/md42/md/sync_action mdadm /dev/md42 --add /dev/loop0 echo 0 > /sys/block/md42/md/dev-loop0/slot mdadm --grow /dev/md42 --backup-file=... --raid-devices=4 NeilBrown
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