Re: RAID-5 streaming read performance

[Dan Christensen <jdc@uwo.ca>]

Ming Zhang <mingz@ele.uri.edu> writes:

> On Mon, 2005-07-11 at 11:11 -0400, Dan Christensen wrote:
>> I was wondering what I should expect in terms of streaming read
>> performance when using (software) RAID-5 with four SATA drives.  I
>> thought I would get a noticeable improvement compared to reads from a
>> single device, but that's not the case.  I tested this by using dd to
>> read 300MB directly from disk partitions /dev/sda7, etc, and also using
>> dd to read 300MB directly from the raid device (/dev/md2 in this case).
>> I get around 57MB/s from each of the disk partitions that make up the
>> raid device, and about 58MB/s from the raid device.  On the other
>> hand, if I run parallel reads from the component partitions, I get
>> 25 to 30MB/s each, so the bus can clearly achieve more than 100MB/s.
>> 
>> [...]
>> 
>> System:
>> - Athlon 2500+
>> - kernel 2.6.12.2 (also tried 2.6.11.11)
>> - four SATA drives (3 160G, 1 200G); Samsung Spinpoint
>> - SiI3114 controller (latency_timer=32 by default; tried 128 too)
> 
> only 1 card? 4 port? try some other brand card and try to use several
> cards at the same time. i met some poor cards before.

Yes, one 4-port controller.  It's on the motherboard.

I thought that since I get good throughput doing parallel reads from
the four drives (see above) that would eliminate the controller as the
bottleneck.  Am I wrong?

Dan