[CentOS] RAID6 in production?

Aleksandar Milivojevic alex at milivojevic.org
Thu Oct 20 15:27:59 UTC 2005

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 
is out.  Obviously it is an bug that should be fixed.

>> 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.

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)?

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 LVM
physical volumes on them, put them into volume group (for example create a new
one) and make a logical volume on them.  Example:

# pvcreate /dev/sda1
# pvcreate /dev/sdb1
# vgcreate -s 32m foobar /dev/sda1 /dev/sdb1
# vgdisplay foo
# lvcreate -l nnn -n bar foo
# mkfs.ext3 /dev/foo/bar
# mount -t ext3 /dev/foo/bar /mnt

Replace sda1 and sdb1 with actual partitions on RAID-5 volumes.  Replace "nnn"
in lvcreate with the number of free physical extents (PE) as shown by 

You can also test if striping will make any difference.  For example, 
you might
also replace the above lvcreate command with:

# lvcreate -l nnn -i 2 -n bar foo /dev/sda1 /dev/sdb1

You can also give it -I (capital i) option to specify the stripe size, if you
don't want to use default value.  See "man lvcreate" for more info.

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