[CentOS] RAID6 in production?

Joshua Baker-LePain jlb17 at duke.edu
Thu Oct 20 15:49:31 UTC 2005

On Thu, 20 Oct 2005 at 10:27am, Aleksandar Milivojevic wrote

> Quoting Joshua Baker-LePain <jlb17 at duke.edu>:
>> Wonderful.  I'm testing RAID5 now, for which I had high hopes.
> Well, my guess is that there'll be fix for this by the time next kernel 
> version
> is out.  Obviously it is an bug that should be fixed.

There weren't any RAID6 specific bugs I could find in bugzilla.  I'll 
probably file one once I test on a 2nd system and with the most recent 
kernel (all my testing has been with 2.6.9-11 as our very excellent 
campus-based distro hasn't pushed 4.2 yet).

>>> BTW, any particular reason not to use hardware RAID on 3ware?
>> That was the initial plan.  But I can't get anything even resembling decent 
>> speeds out of them.  These systems have 2 7500-8 boards in 'em (on separate 
>> PCI buses).  I had been running RH7.3 (!!) and XFS with the boards in 
>> hardware RAID5 and a software RAID0 stripe across those, and would get 
>> >100MB/s writes and >300MB/s reads.  With centos-4 and ext3, I was getting 
>> ~30MB/s writes and ~200MB/s reads.  The reads I'd be OK with, but the write 
>> speed is absurd.  I tweaked all I could think of, but nothing helped much.
> Yeah, mixing RAIDs like that sometimes can get you into trouble.

It's worked well in the past (obviously), and on my dual 9500 based 
system.  It's the same general setup, and gets >150MB/s writes and nearly 
400MB/s reads with centos-4 and ext3.

> I guess 3ware was not rebuilding anything while you were testing?  Have you
> tested the speed of ext3 under RH7.3 (to rule out file system differences
> influencing your particular test)?

No, the 3wares weren't rebuilding.  I can't seem to find any RH7.3 ext3 
benchmarking I did.  I *did* try the centosplus kernel and XFS with the 
mixed hardware/software RAID setup.  It got ~100MB/s writes but the reads 
went down to ~150MB/s.  I just don't trust the stack to handle XFS and NFS 
on a production server.

> Is the speed OK if you use hardware RAID-5 on one card on its own (not mixing 
> it
> with sofware RAID-0 like you had under RH-7.3)?  If it is, try this
> configuration instead.  Create two hardware RAID-5 volumes (one on each 
> card). Then use fdisk to mark them as Linux LVM (8e).  Use pvcreate to create 
> physical volumes on them, put them into volume group (for example create a 
> new
> one) and make a logical volume on them.  Example:

I haven't tested a single board yet -- it'll be interesting to see what 
that yields.  I hadn't thought of using LVM -- I may give that a shot. 

Joshua Baker-LePain
Department of Biomedical Engineering
Duke University

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