How To Resize RAID Partitions (Shrink & Grow) (Software RAID)
Version 1.0
Author: Falko Timme
This article describes how you can shrink and grow existing software RAID partitions. I have tested this with non-LVM RAID1 partitions that use ext3 as the file system. I will describe this procedure for an intact RAID array and also a degraded RAID array.
If you use LVM on your RAID partitions, the procedure will be different, so do not use this tutorial in this case!
I do not issue any guarantee that this will work for you!
1 Preliminary Note
A few days ago I found out that one of my servers had a degraded RAID1 array (/dev/md2, made up of /dev/sda3 and /dev/sdb3; /dev/sda3 had failed, /dev/sdb3 was still active):
server1:~# cat /proc/mdstat
Personalities : [raid1]
md2 : active raid1 sdb3[1]
4594496 blocks [2/1] [_U]
md1 : active raid1 sda2[0] sdb2[1]
497920 blocks [2/2] [UU]
md0 : active raid1 sda1[0] sdb1[1]
144448 blocks [2/2] [UU]
unused devices: <none>
server1:~#
I tried to fix it (using this tutorial), but unfortunately at the end of the sync process (with 99.9% complete), the sync stopped and started over again. As I found out, this happened because there were some defect sectors at the end of the (working) partition /dev/sdb3 - this was in /var/log/kern.log:
Nov 22 18:51:06 server1 kernel: sdb: Current: sense key: Aborted Command
Nov 22 18:51:06 server1 kernel: end_request: I/O error, dev sdb, sector 1465142856
So this was the worst case that could happen - /dev/sda dead and /dev/sdb about to die. To fix this, I imagined I could shrink /dev/md2 so that it leaves out the broken sectors at the end of /dev/sdb3, then add the new /dev/sda3 (from the replaced hard drive) to /dev/md2, let the sync finish, remove /dev/sdb3 from the array and replace /dev/sdb with a new hard drive, add the new /dev/sdb3 to /dev/md2, and grow /dev/md2 again.
This is one of the use cases for the following procedures (I will describe the process for an intact array and a degraded array).
Please note that /dev/md2 is my system partition (mount point /), so I had to use a rescue system (e.g. Knoppix Live-CD) to resize the array. If the array you want to resize is not your system partition, you probably don't need to boot into a rescue system; but in either case, make sure that the array is unmounted!
2 Intact Array
I will describe how to resize the array /dev/md2, made up of /dev/sda3 and /dev/sdb3.
2.1 Shrinking An Intact Array
Boot into your rescue system and activate all needed modules:
modprobe md
modprobe linear
modprobe multipath
modprobe raid0
modprobe raid1
modprobe raid5
modprobe raid6
modprobe raid10
Then activate your RAID arrays:
cp /etc/mdadm/mdadm.conf /etc/mdadm/mdadm.conf_orig
mdadm --examine --scan >> /etc/mdadm/mdadm.conf
mdadm -A --scan
Run
e2fsck -f /dev/md2
to check the file system.
/dev/md2 has a size of 40GB; I want to shrink it to 30GB. First we have to shrink the file system with resize2fs; to make sure that the file system fits into the 30GB, we make it a little bit smaller (25GB) so we have a little security margin, shrink /dev/md2 to 30GB, and the resize the file system (again with resize2fs) to the max. possible value:
resize2fs /dev/md2 25G
Now we shrink /dev/md2 to 30GB. The --size value must be in KiBytes (30 x 1024 x 1024 = 31457280); make sure it can be divided by 64:
mdadm --grow /dev/md2 --size=31457280
Next we grow the file system to the largest possible value (if you don't specify a size, resize2fs will use the largest possible value)...
resize2fs /dev/md2
... and run a file system check again:
e2fsck -f /dev/md2
That's it - you can now boot into the normal system again.
2.2 Growing An Intact Array
Boot into your rescue system and activate all needed modules:
modprobe md
modprobe linear
modprobe multipath
modprobe raid0
modprobe raid1
modprobe raid5
modprobe raid6
modprobe raid10
Then activate your RAID arrays:
cp /etc/mdadm/mdadm.conf /etc/mdadm/mdadm.conf_orig
mdadm --examine --scan >> /etc/mdadm/mdadm.conf
mdadm -A --scan
Now we can grow /dev/md2 as follows:
mdadm --grow /dev/md2 --size=max
--size=max means the largest possible value. You can as well specify a size in KiBytes (see previous chapter).
Then we run a file system check...
e2fsck -f /dev/md2
..., resize the file system...
resize2fs /dev/md2
... and check the file system again:
e2fsck -f /dev/md2
Afterwards you can boot back into your normal system.
