Re: Looking for the cause of poor I/O performance

Gotcha.  Please excuse the loose use of terminology on my part.

But now I'm more convinced than ever that I should be getting better
performance than I am.  I'm getting 40MB/sec from each disk
individually, I've shown with hdparm that I can pull 40MB/sec from all
three disks simultaneously, but still my raid5 read performance (in a
three-disk array) is slightly less than 40MB/sec.

Any guesses what the issue could be?  Is there a switch for
read-ahead?

-Steve


On Mon, 06 Dec 2004 14:29:40 -0500, Guy wrote:

RAID5 can't do read balancing.  Any 1 piece of data is only on 1 drive.
However, RAID5 does do read ahead, my speed is about 3.5 times as fast as a
single disk.  A single disk: 18 M/sec, my RAID5 array, 65 M/sec.

Guy

-----Original Message-----
From: linux-raid-owner@vger.kernel.org
[mailto:linux-raid-owner@vger.kernel.org] On Behalf Of Steven Ihde
Sent: Monday, December 06, 2004 12:49 PM
To: linux-raid@vger.kernel.org
Subject: Re: Looking for the cause of poor I/O performance

On Sat, 04 Dec 2004 17:00:08 -0800, Steven Ihde wrote:
[snip]

quoted

A possible clue is that when tested individually but in parallel, hda
and hdc both halve their bandwidth:

/dev/hda:
 Timing cached reads:   1552 MB in  2.00 seconds = 774.57 MB/sec
 Timing buffered disk reads:   68 MB in  3.07 seconds =  22.15 MB/sec
/dev/hdc:
 Timing cached reads:   784 MB in  2.00 seconds = 391.86 MB/sec
 Timing buffered disk reads:   68 MB in  3.02 seconds =  22.54 MB/sec
/dev/sda:
 Timing cached reads:   836 MB in  2.00 seconds = 417.65 MB/sec
 Timing buffered disk reads:  120 MB in  3.00 seconds =  39.94 MB/sec

Could there be contention for some shared resource in the on-board
PATA chipset between hda and hdc?  Would moving one of them to a
separate IDE controller on a PCI card help?

Am I unreasonable to think that I should be getting better than 37
MB/sec on raid5 read performance, given that each disk alone seems
capable of 40 MB/sec?

To answer my own question... I moved one of the PATA drives to a PCI
PATA controller.  This did enable me to move 40MB/sec simultaneously
from all three drives.  Guess there's some issue with the built-in
PATA on the ICH5R southbridge.

However, this didn't help raid5 performance -- it was still about
35-39MB/sec.  I also have a raid1 array on the same physical disks,
and observed the same thing there (same read performance as a single
disk with hdparm -tT, about 40 MB/sec).  So:

2.6.8 includes the raid1 read balancing fix which was mentioned
previously on this list -- should this show up as substantially better
hdparm -tT numbers for raid1 or is it more complicated than that?

Does raid5 do read-balancing at all or am I just fantasizing?

Thanks,

Steve
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