unexplainable low ide raid performance, slower than a single raid disk

From: Marc Lehmann
Date: Sat May 08 2004 - 07:54:48 EST


I've got a lvm-on-raid-on-ide setup, with 5 IDE disks, all on their own

However, neither read nor write speed is as high as one would expect.

Every single disk is capable of delivering roughly 50MB/s (measured with
hdparm -tT), except one, which only does 46MB/s (it's only 5400rpm):

Timing buffer-cache reads: 812 MB in 2.01 seconds = 404.65 MB/sec
Timing buffered disk reads: 158 MB in 3.03 seconds = 52.17 MB/sec

When I do dd if=/dev/somedisk of=/dev/null bs=1024x1024 on two disks, I
get ~62MB/s and three disks deliver about 68MB/s, which seems to be the
maximum this (slightly older) system can deliver.

The reconstruction speed I get on an idle system is ~13500kb/s, which,
with five disks, corresponds nicely with the 68MB/s my system is capable
of delivering.

So all this is relatively consistent.

However, the raid device itself is much slower, never ever reaching the
speed of even a _single_ disk:

Timing buffer-cache reads: 812 MB in 2.00 seconds = 405.25 MB/sec
Timing buffered disk reads: 128 MB in 3.02 seconds = 42.32 MB/sec

Wether hdparm or reading/writing with dd, the linera throughput on the
raid device is always much lower, and I have no idea why this is the case,
and I wonder wether this is a problem with my setup and wether it can be

Here is some more info about my system that might or might not be

* Dual-PIII 1Ghz
* kernels 2.6.4 and 2.6.5 consistently show this behaviour.
* one disk is on the secondary via vt82c686 controller,
* all other disks are on promise 20268 devices, all ATA 100 controllers.
* the raid was clean (non-degraded) during all my tests, and I have re-run
them a few times on an otherwise idle system and got about the same figures.
* 1 x SAMSUNG SV1604N, 4 x SAMSUNG SP1604N drives
* md0 : active raid5 hdk1[3] hdi1[2] hdg1[1] hde1[0] hdc1[4]
625163264 blocks level 5, 64k chunk, algorithm 2 [5/5] [UUUUU]

Thanks a lot for any insights or ideas on this problem.

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