We would like to add read performance to our RAID10 volume by adding
another drive (we don't care about space), so I did the following test
with poor results.
# mdadm --create /dev/md13 --level 10 --run --assume-clean -p n2
--raid-devices 2 /dev/loop{2..3}
mdadm: /dev/loop2 appears to be part of a raid array:
level=raid10 devices=3 ctime=Wed Nov 2 11:25:22 2016
mdadm: /dev/loop3 appears to be part of a raid array:
level=raid10 devices=3 ctime=Wed Nov 2 11:25:22 2016
mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md13 started.
# mdadm --detail /dev/md13
/dev/md13:
Version : 1.2
Creation Time : Wed Nov 2 11:47:48 2016
Raid Level : raid10
Array Size : 10477568 (9.99 GiB 10.73 GB)
Used Dev Size : 10477568 (9.99 GiB 10.73 GB)
Raid Devices : 2
Total Devices : 2
Persistence : Superblock is persistent
Update Time : Wed Nov 2 11:47:48 2016
State : clean
Active Devices : 2
Working Devices : 2
Failed Devices : 0
Spare Devices : 0
Layout : near=2
Chunk Size : 512K
Name : rleblanc-pc:13 (local to host rleblanc-pc)
UUID : 1eb66d7c:21308453:1e731c8b:1c43dd55
Events : 0
Number Major Minor RaidDevice State
0 7 2 0 active sync set-A /dev/loop2
1 7 3 1 active sync set-B /dev/loop3
# mdadm /dev/md13 -a /dev/loop4
mdadm: added /dev/loop4
# mdadm --detail /dev/md13
/dev/md13:
Version : 1.2
Creation Time : Wed Nov 2 11:47:48 2016
Raid Level : raid10
Array Size : 10477568 (9.99 GiB 10.73 GB)
Used Dev Size : 10477568 (9.99 GiB 10.73 GB)
Raid Devices : 2
Total Devices : 3
Persistence : Superblock is persistent
Update Time : Wed Nov 2 11:48:13 2016
State : clean
Active Devices : 2
Working Devices : 3
Failed Devices : 0
Spare Devices : 1
Layout : near=2
Chunk Size : 512K
Name : rleblanc-pc:13 (local to host rleblanc-pc)
UUID : 1eb66d7c:21308453:1e731c8b:1c43dd55
Events : 1
Number Major Minor RaidDevice State
0 7 2 0 active sync set-A /dev/loop2
1 7 3 1 active sync set-B /dev/loop3
2 7 4 - spare /dev/loop4
# mdadm --grow /dev/md13 -p n3 --raid-devices 3
mdadm: Cannot change number of copies when reshaping RAID10
I also tried to add the device, grow raid-devices, let it reshape,
then try to change the number of copies and it didn't like that
either. It would be nice to supply -p nX and --raid-devices X at the
same time to prevent the reshape and only copy the data over to the
new drive (or drop a drive out completely). I could see changing -p
separately or at a different rate of drives added/removed could be
difficult, but for lockstep changes, it seems that it would be rather
easy.
Any ideas?
Thanks,
----------------
Robert LeBlanc
PGP Fingerprint 79A2 9CA4 6CC4 45DD A904 C70E E654 3BB2 FA62 B9F1
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