linux-kernel - Re: RAID performance / tuning?

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Message-ID: <6c62e9d84a16902aebba9438b4e299d9.squirrel@localhost>
Date:	Thu, 2 Apr 2009 12:41:37 -0500 (CDT)
From:	david.hagood@...il.com
To:	"Tiago Freire" <tiago.freire@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: RAID performance / tuning?

> Why is it that software RAID on current systems still gets less
> performance than hardware counterparts?

Part of it can be crappy disk interfaces: I was running software raid with
2 SATA-SIL cards, and would frequently be disk-bound with the CPU still
largely idle.

The cards were incapable of talking to more than one drive at a time. They
didn't support command queuing on the drives.

As a result, the system would set up a stripe, queue up the writes, then
have to wait as each write for EACH DISK in the 7 disk array was carried
out.

On a good hardware RAID controller, the disks can be written in parallel,
and the controller will support command queuing - so disk writes can be
run in parallel, and the writes themselves can be better optimized by the
disks.

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