On Wed, 08 Dec 2004 17:41:45 -0500, Guy wrote:
> I also tried changing /proc/sys/vm/max-readahead.
> I tried the default of 31, 0 and 127. All gave me about the same
> performance.
>
> I started testing the speed with the dd command below. It complete in about
> 12.9 seconds. None of the read ahead changes seem to affect my speed.
> Everything is now set to 0, still 12.9 seconds.
> 12.9 seconds = about 79.38 MB/sec.
>
> time dd if=/dev/md2 of=/dev/null bs=1024k count=1024
I'm running kernel 2.6.8; I found the readahead setting had a pretty
dramatic effect. I set readahead for all the drives and their
partitions to zero:
blockdev --setra 0 /dev/{hdc,hdg,sda,hdc5,hdg5,sda5}
I tested various readahead values for the array device by reading 1GB
of data from the device using this procedure:
blockdev --flushbufs /dev/md1
blockdev --setra $readahead /dev/md1
dd if=/dev/md1 of=/dev/null bs=1024k count=1024
These are the results:
RA transfer rate (B/s)
---------------
0: 15768513
128: 33680867
256: 42982770
512: 59223248
1024: 78590551
2048: 81918844
4096: 82386839
We seem to reach the point of diminishing returns at 1024 readahead,
~80MB/sec throughput. To recap, this is with three Seagate Barracuda
drives, two of which are 80GB PATA, the other a 120GB SATA, in a RAID5
configuration. 256 was the default readahead value. The chunk size
on my array is 32k. I don't know if that has an effect or not.
-Steve
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