Problem:
The mdadm --grow command fails when trying to add disk to a RAID6.
The man page says it can do this.
"GROW MODE
The GROW mode is used for changing the size or shape of an
active array. For this to work,
the kernel must support the necessary change. Various types
of growth are being added dur-
ing 2.6 development, including restructuring a raid5 array to
have more active devices.
Currently the only support available is to
· change the "size" attribute for RAID1, RAID5 and RAID6.
· increase the "raid-disks" attribute of RAID1, RAID5, and
RAID6.
· add a write-intent bitmap to any array which supports
these bitmaps, or remove a write-
intent bitmap from such an array.
"
So far I have replicated this problem on RHEL5 and Ubuntu 7.04
running the latest official updates and patches. I have even tried it
with the most latest version of mdadm 2.6.3 under RHEL5. RHEL5 uses
version 2.5.4.
How to replicate the problem:
You can either use real physical disks or use the loopback device to
create fake disks.
Here are the steps using the loopback method as root.
cd /tmp
dd if=/dev/zero of=rd1 bs=10240 count=10240
cp rd1 rd2;cp rd1 rd3;cp rd1 rd4;cp rd1 rd5
losetup /dev/loop1 rd1;losetup /dev/loop2 rd2;losetup /dev/loop3
rd3;losetup /dev/loop4 rd4;losetup /dev/loop5 rd5
mdadm --create --verbose /dev/md0 --level=6 --raid-devices=4 /dev/
loop1 /dev/loop2 /dev/loop3 /dev/loop4
At this point wait a minute while the raid is being built.
mdadm --add /dev/md0 /dev/loop5
mdadm --grow /dev/md0 --raid-devices=5
You should get the following error
mdadm: Need to backup 384K of critical section..
mdadm: Cannot set device size/shape for /dev/md0: Invalid argument
How to clean up
mdadm --stop /dev/md0
mdadm --remove /dev/md0
losetup -d /dev/loop1;losetup -d /dev/loop2;losetup -d /dev/
loop3;losetup -d /dev/loop4;losetup -d /dev/loop5
rm rd1 rd2 rd3 rd4 rd5
David.
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