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[linux-lvm] Apparent performance degradation for each PV with striping



Donald Thompson writes:
> I notice during the dd operation that my system CPU state is 90% or
> more.  So I think I just answered my own question, I'm CPU bound.
> Moving on, is there any known ways to improve my performance off
> each PV with this type of hardware setup?
> ...
> Should I expect that I won't see the performance drop on individual PV's
> with striping on SCSI drives? I originally setup this system with no
> intentions of it being a high performance file server, until a few
> people I work with decided they wanted to use it for a database machine.
> So I'm not afraid to spend a couple grand to get some faster disks in it
> if thats the only thing
> thats gonna help me.

Hi Donald,

I think what you're seeing is to be expected from vanilla IDE.
Not only is it not Linux LVM's fault but Linux LVM can't fix it.
IDE controllers are not able to do the things more sophisicated
controllers and host adapters do to increase performance in a
multi-spindle environment.  Fortunately, there is a solution that's
fast, cheap, and reliable.

I suggest that, rather than replacing the drives with
expensive SCSI drives and an expensive SCSI host adapter,
you buy an Escalade Switch from http://www.3ware.com/ and
use your existing drives.

The Escalade is a hybrid controller of sorts.  It presents itself as a
SCSI host adapter to the host's PCI bus and as multiple (up to 8)
independent IDE controllers to the IDE drives.  It's essentially a
cross-bar switch that lets multiple IDE drives act independently of
one another.  They use some clever controller software to get a BETTER
than 2x boost in read-performance when you mirror drives.

It has the additional advantage of providing RAID for the attached drives.
It supports RAID 0, 1, 10, and 5, so you get all those benefits
without imposing ANY additional CPU load.  The controller is actually
quite a gem and is very reasonably priced.  I've been using them on
all of my systems where performance and/or reliability are critical.

The Escalade driver is supported in the standard 2.2.x and 2.4.x Linux kernels.

In short, let 3ware's hardware handle the striping/RAID and use Linux LVM
to manage the volume.  It's a powerful combination.

Larry

DISCLAIMER:  These statements reflect my personal opinions... bla bla bla.
I don't have any affiliation with 3ware or anyone else in this business.

CLAIMER:  3ware makes good stuff that works very well.
It solved my problems and it could solve yours.


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