[kwlug-disc] RAID, mdadm and Ubuntu

Richard Weait richard at weait.com
Wed Apr 6 11:26:15 EDT 2011

RAID is nice.

I just added another drive to my RAID0 array and found a nice bulk
speed increase.  Using mdadm to stop and then remove the previous
array, then to create the new larger array all went as expected.

Rebooting, however introduced a little wobble.  Ubuntu couldn't find
the new array.  Others have pointed out before, that updating
/etc/mdadm/mdadm.conf is a good idea.  I've made a note of that.

One might also want to be aware that Ubuntu will do the best it can to
create a raid array if you've not updated the mdadm.conf.  So here is
something to watch for:

If your raid array fails to mount, and you can't get mdadm to --create
your new array, due to a

mdadm: Cannot open /dev/sdd1: Device or resource busy

... where sdd1 is your new drive.  Have a look at ls -al /dev/md*

Ubuntu may have helpfully created a RAID array of only your new drive,
called it /dev/md_d0, activated it and left it unmounted.  Thus, it
won't be available to add to your intended new array.  mdadm --stop
that array, --create your new one, then, this time, remember to update
mdadm.conf.  You'll be off to the races.

My approximate disk-performance increase from two RAID0 disks to three
RAID0 disks?

Action 2-disk 3-disk
Read 106MB/s 350MB/s
Write 123 MB/s 253 MB/s

That's not a typo, but it is mis-leading.  The 2-disk read was slower
than 2-disk write, which is unexpected.  Other activity on the machine
during that "benchmark" is then likely culprit.

Sorry if I derailed the political threads.  Carry on.

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