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?
Regards
Andreas Klauer
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