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Right I/O scheduler for hardware RAID
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Joined: 10 Sep 2003
Posts: 16

PostPosted: Sun May 14, 2006 3:19 pm    Post subject: Right I/O scheduler for hardware RAID Reply with quote

Linux 2.6 - I/O schedulers
I was having some serious I/O slow downs with 2.6.11 and my 3ware raid card. One of the problems was the I/O scheduler. The kernel builds with the "anticipatory" I/O scheduler as the default. However, if you read the documentation in the kernel on this scheduler in "Documentation/block/as-iosched.txt" you find the following:

"Also, users with hardware RAID controllers, doing striping, may find
highly variable performance results with using the as-iosched. The
as-iosched anticipatory implementation is based on the notion that a disk
device has only one physical seeking head. A striped RAID controller
actually has a head for each physical device in the logical RAID device."

Choosing an I/O Scheduler for Red Hat® Enterprise Linux® 4 and the 2.6 Kernel
The Completely Fair Queuing (CFQ) scheduler is the default algorthim in Red Hat Enterprise Linux 4. As the name implies, CFQ maintains a scalable per-process I/O queue and attempts to distribute the available I/O bandwidth equally among all I/O requests. CFQ is well suited for mid-to-large multi-processor systems and for systems which require balanced I/O performance over multiple LUNs and I/O controllers.

I/O Benchmarking Guide

3Ware specific topic
Performance/Optimization: How can 3ware read performance be increased in Linux

Selecting Scheduler
For example, if your raid device is "/dev/sda", you can see the current scheduler setting by typing:
cat /sys/block/sda/queue/scheduler

You can change it to "cfq" type typing:
echo "cfq" > /sys/block/sda/queue/scheduler



Pinpoint the append line!

This is a little bit outdated but it's still interesting.
Performance Tips for Kernel 2.6

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Joined: 27 Feb 2004
Posts: 1186

PostPosted: Sun Jun 11, 2006 11:46 pm    Post subject: Reply with quote


I've found this:
ANTICIPATORY (AS): This is a decent scheduler, it works well with all file systems, Only use it on older drives, particulary drives that spin at 5400rpm or less (many laptop hard drives)
DEADLINE: This is a surprising scheduler, it is light and awesome. It performs the best on most file systems, including reiser4. Only use it on drives that spin faster than 5400rpm
CFQ: Only use it on ext3, this scheduler doesn't work as good as AS or Deadline on anything else.

Than again, I guess this is desktop system related...
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