Re: io performance...

From: Phillip Susi
Date: Tue Jan 17 2006 - 12:05:10 EST


Did you direct the program to use O_DIRECT? If not then I believe the problem you are seeing is that the generic block layer is not performing large enough readahead to keep all the disks in the array reading at once, because the stripe width is rather large. What stripe factor did you format the array using?


I have a sata fakeraid at home of two drives using a stripe factor of 64 KB. If I don't issue O_DIRECT IO requests of at least 128 KB ( the stripe width ), then throughput drops significantly. If I issue multiple async requests of smaller size that totals at least 128 KB, throughput also remains high. If you only issue a single 32 KB request at a time, then two requests must go to one drive and be completed before the other drive gets any requests, so it remains idle a lot of the time.

Max Waterman wrote:
Hi,

I've been referred to this list from the linux-raid list.

I've been playing with a RAID system, trying to obtain best bandwidth
from it.

I've noticed that I consistently get better (read) numbers from kernel 2.6.8
than from later kernels.

For example, I get 135MB/s on 2.6.8, but I typically get ~90MB/s on later
kernels.

I'm using this :

<http://www.sharcnet.ca/~hahn/iorate.c>

to measure the iorate. I'm using the debian distribution. The h/w is a MegaRAID
320-2. The array I'm measuring is a RAID0 of 4 Fujitsu Max3073NC 15Krpm drives.

The later kernels I've been using are :

2.6.12-1-686-smp
2.6.14-2-686-smp
2.6.15-1-686-smp

The kernel which gives us the best results is :

2.6.8-2-386

(note that it's not an smp kernel)

I'm testing on an otherwise idle system.

Any ideas to why this might be? Any other advice/help?

Thanks!

Max.
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