[kwlug-disc] RAID, mdadm and Ubuntu

William Park opengeometry at yahoo.ca
Wed Apr 6 14:02:02 EDT 2011

Any reason why you're not using Raid5 ?  Because, if the power cord is pulled, 
or the computer hangs, or a hard disk dies, then you're screwed.  You then have 
to copy from your backup.

----- Original Message ----
> From: Richard Weait <richard at weait.com>
> To: KWLUG discussion <kwlug-disc at kwlug.org>
> Sent: Wed, April 6, 2011 11:26:15 AM
> Subject: [kwlug-disc] RAID, mdadm and Ubuntu
> 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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