but aren't the next 'n' blocks of data on (about) n drives that can be
read concurrently (if the read is big enough)
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@xxxxxxxxxxxxxxx
[mailto:linux-raid-owner@xxxxxxxxxxxxxxx] On Behalf Of Steven Ihde
Sent: Monday, December 06, 2004 12:49 PM
To: linux-raid@xxxxxxxxxxxxxxx
Subject: Re: Looking for the cause of poor I/O performance
On Sat, 04 Dec 2004 17:00:08 -0800, Steven Ihde wrote:
[snip]
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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