Red Hat Knowledgebase

Releases Found:

Red Hat Enterprise Linux 4, Red Hat Enterprise Linux 5

Resolution:

When using Red Hat Enterprise Linux 4 or Red Hat Enterprise Linux 5, the default scheduler cfq is ideal in most cases because it performs well on nearly all workloads.

However, if a VOD server runs with the cfq scheduler, its average I/O latency will increase when the amount of connections on the server increases. If I/O latency is too high the clients' video streams will not play smoothly.

In the case above, The VOD servers should use the "deadline" I/O scheduler to allow servers to optimize I/O requests. For a VOD server, minimizing I/O latency is more important than maximizing I/O throughput.

The I/O scheduler can be selected at boot time using the "elevator" kernel parameter. In the following example, the system has been configured to use the deadline scheduler in the grub.conf file.

title Red Hat Enterprise Linux Server (2.6.18-8.el5)
root (hd0,0)
kernel /vmlinuz-2.6.18-8.el5 ro root=/dev/vg0/lv0 elevator=deadline
initrd /initrd-2.6.18-8.el5.img

In Red Hat Enterprise Linux 5, it is also possible to change the I/O scheduler for a particular disk on the fly.

# cat /sys/block/sdb/queue/scheduler
noop anticipatory deadline [cfq]
# echo 'deadline' > /sys/block/sdb/queue/scheduler
# cat /sys/block/sdb/queue/scheduler
noop anticipatory [deadline] cfq

The following are the tunable files for the deadline scheduler. They can be tuned to any suitable value according to hardware performance and software requirement.

/sys/block/DEVNAME/queue/iosched/read_expire
/sys/block/DEVNAME/queue/iosched/write_expire
/sys/block/DEVNAME/queue/iosched/fifo_batch
/sys/block/DEVNAME/queue/iosched/write_starved
/sys/block/DEVNAME/queue/iosched/front_merges

DEVNAME is the name of block device (such as sda, sdb, hda, etc)

A detailed description of the deadline I/O scheduler can be found at: /usr/share/doc/kernel-[version]/Documentation/block/deadline-iosched.txt.