[linux-lvm] lvm on software raid 5

[Ken Fuchs]

Rick van der Linde wrote:

>As I did something like this before I would like to know your experiences.
>I've (successfully) tried to install LVM on top of a RAID 5 volume
>consisting 4 disks of 80GB. However I switched back to simple ext3 volumes
>on RAID 5 due to performance problems. I've tried this on LVM 1.0.3 and
>the md-driver delivered with the 2.4.20 kernel. In this situation I
>experience a (LVM on RAID5) performance loss of approx 40-60% in comparison
>to the RAID5 performance.

That is a huge performance loss.  Usually the LVM extents will be
sequentially ordered within each logical volume providing good
performance.  Don't forget that the raw performance of the outer
cylinders of a drive can be double that of the inner cylinders.  Any
performance test setup that doesn't neutralize this variable has no
value period.  I'm not suggesting that this is the case, but only that
performance tests must be very carefully designed and executed.

I did some tests on LVM 1.0.8 (no RAID) using bonnie and noticed only a
few percent performance drop.

Sincerely,

Ken Fuchs <kfuchs at winternet.com>