[kwlug-disc] MDADM and RAID

Chris Irwin chris at chrisirwin.ca
Tue Mar 2 01:27:38 EST 2010

Is anybody running RAID5 with mdadm and getting extremely horrible write
performance? I understand there is an overhead, but I'm looking at 18
hours to copy ~500GB thus far. Did I leave out some --actually-usable
option somewhere?

The long story: I'm changing my RAID configuration on my (at home)
server. Yes, I have a backup.

I was originally running Raid10 -- the internal mdadm version, not
manually stacking arrays. My issues revolved around not knowing which
disks were configured how. I know that in theory I could lose two (very
specific) disks, but I couldn't tell you which two. I don't know if it
was a mirrored stripe or a striped mirror. I didn't like not knowing how
the data was sitting on disk. Getting only 50% of my storage capacity
wasn't the best either, but I'm willing to sacrifice for not having to
reach for my (offsite) backups.

Basically my options as I see them are:
- two plain raid1 arrays. This would give me simplicity, but still only
50% capacity.
- one raid5 array. This gives me 75% storage (1.5TB! WOO), keeps
simplicity in my disk layout (only one pv) and gives me something to
brag about at meetings ("ppft! I run raid5 at home!").

I migrated all my logical volumes to an esata disk. It took about two
hours if I recall. I then stopped the array, and created a new one with
the following command:
	mdadm --create --verbose /dev/md1 \
	--level=5 --raid-devices=4 --spare-devices=0 \
	/dev/sda2 /dev/sdb2 /dev/sdc2 /dev/sdd2

No problems, created and added my pv, and started migrating volumes
back. I'm now at about 18 hours, and I'm still only 92% complete. I
ensured that the device was finished syncing itself before I started
doing anything with it.

So what gives? Does mdadm just suck with raid5? Did I do it wrong? Are
there any reasonably cheap raid cards that are worthwhile that I can
just throw at the problem?

Chris Irwin
e:  chris at chrisirwin.ca
w: http://chrisirwin.ca
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