Linux on-line Software RAID reshaping

Thu, 05/04/2007 - 5:43pm — tumbleweed

I first read about on-line Software RAID reshaping a year ago on LWN. Today I tried it on a live system (that's too big to be backed-up first :-))

I added 2 250GB drives to my existing RAID5 array of 4, making for a 1.2TiB array. The reshape took a while...:

md0 : active raid5 sdb1[4] sda1[5] sdf1[2] sde1[3] sdd1[1] sdc1[0]
732587712 blocks super 0.91 level 5, 64k chunk, algorithm 2 [6/6] [UUUUUU]
[>....................] reshape = 1.9% (4800784/244195904) finish=397.4min speed=10034K/sec

But when it was done:

$ df -h
/dev/md0 1.2T 657G 509G 57% /mnt/storage

This is one of the reasons why I love software RAID, while you have a kak load more I/O through the PCI(E) bus than you would with hardware RAID, you get the flexibility of the highest-end hardware controllers on a normal PC motherboard.

And of course, should things go pear-shaped, I don't need to find an identical controller, I just have to find a box with 6 SATA sockets.

HOWTO reshape

Lets say you have 4 SATA drives, /dev/sda to /dev/sdd, and you are adding a new one /dev/sde.

Check that everything is happy:

$ cat /proc/mdstat
md0 : active raid5 sdd1[3] sdc1[1] sdb1[2] sda1[0]
937705728 blocks level 5, 128k chunk, algorithm 2 [4/4] [UUUU]

Partition the new drive (clone sdas partition table onto sde):

# sfdisk -d /dev/sda | sfdisk /dev/sde

Add the new drive:

# mdadm -a /dev/md0 /dev/sde1

Grow the RAID:

# mdadm --grow -n5 /dev/md0

Watch the progress:

$ watch cat /proc/mdstat

Grow the filesystem:

# resize_reiserfs /dev/md0

See the extra space:

$ df -h

linux
raid

Comments

Mon, 23/04/2007 - 8:23pm — Sean Puttergill (not verified)

Out of curiosity, is the array

Out of curiosity, is the array available for I/O while the reshape occurs? I.e. is it a truly "online" reshape?

Mon, 23/04/2007 - 9:23pm — tumbleweed

Yes, totally online and availa

Yes, totally online and available. Although obviously it's faster with less activity.

Tue, 24/04/2007 - 6:09pm — Sean Puttergill (not verified)

Thanks.

Wed, 19/09/2007 - 2:58pm — Paul Timmins (not verified)

Did you add both of the additi

Did you add both of the additional drives at the same time?

Wed, 19/09/2007 - 4:16pm — tumbleweed

Yes, I did.

Wed, 18/11/2009 - 1:17pm — TomH (not verified)

I am using Linux for only

I am using Linux for only half of a year now, and you know it is really much more difficult than Windows. It has more functions but you must find those functions and know how to use them.. Software RAID reshaping was a real trouble for me until I have found this article on your blog. Now everything is easier for me to understand. Thanks one more time for this entry and for all your blog. I will be looking forward to other great entries from you in the nearest future.

Sincerely,

Tom Hunson from tiff to tiff to jpeg convert

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