Phillip Susi wrote:
Did you direct the program to use O_DIRECT?
I'm just using the s/w (iorate/bonnie++) with default options - I'm no
expert. I could try though.
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 left the stripe size at the default, which, I believe, is 64K bytes;
same as your fakeraid below.
I did play with 'blockdev --setra' too.
I noticed it was 256 with a single disk, and, with s/w raid, it
increased by 256 for each extra disk in the array. IE for the raid 0
array with 4 drives, it was 1024.
With h/w raid, however, it did not increase when I added disks. Should I
use 'blockdev --setra 320' (ie 64 x 5 = 320, since we're now running
RAID5 on 5 drives)?
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.
I think that makes sense (which is a change in this RAID performance
business :( ).
Thanks.
Max.
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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