io-scheduler tuning for better read/write ratio

From: Ralf Gross
Date: Tue Jun 16 2009 - 12:08:30 EST


I'm trying to tune the kernel/io-scheduler for better read/write
ratio on a Areca RAID0 device (4 disks, kernel 2.6.26, xfs fs).

I can get 200 MB/s seq. writes and about the same for seq. reads. My
problem is that if there are reads _and_ writes on this device, the
write throughput is much higher than the read throughput (40 MB/s
read, 90 MB/s write).

The deadline scheduler sounded like the way to go for getting better
read results, but reagardless which parameter I change, the ratio keeps
the same.

cfq, noop.. different paramter settings, but alway the same result.

short: is there a way to tune the kernel/scheduler settings in a way
to get a higher read throughput? Writes are not that important,
basicially there are only two 30 GB files on the device/filesystem
that are used to spool data for two LTO-4 tape drives. So I need a
certain read speed to keep both drives streaming. While the data gets
written to one file, I need at least 50 MB/s for reading from the
other file and sent to the tape drive.

Thanks, Ralf
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