Lustre 1.6 Operations Manual

3.3.4 Choosing a Proper Kernel I/O Scheduler
One of the many functions of the Linux kernel (indeed of any OS kernel), is to
provide access to disk storage. The algorithm which decides how the kernel provides
disk access is known as the "I/O Scheduler," or "Elevator." In the 2.6 kernel series,
there are four interchangeable schedulers:
Scheduler
cfq
as
deadline
noop
Description
"Completely Fair Queuing" makes a good default for most workloads on
general-purpose servers. It is not a good choice for Lustre OSS nodes, however,
as it introduces overhead and I/O latency.
"Anticipatory Scheduler" is best for workstations and other systems with slow,
single-spindle storage. It is not at all good for OSS nodes, as it attempts to
aggregate or batch requests in order to improve performance for slow disks.
“Deadline” is a relatively simple scheduler which tries to minimize I/O latency
by re-ordering requests to improve performance. Best for OSS nodes with
"simple" storage, that is software RAID, JBOD, LVM, and so on.
“NOOP” is the most simple scheduler of all, and is really just a single FIFO
queue. It does not attempt to optimize I/O at all, and is best for OSS nodes that
have high-performance storage, that is DDN, Engenio, and so on. This scheduler
may yield the best I/O performance if the storage controller has been carefully
tuned for the I/O patterns of Lustre.
The above observations on the schedulers are just our best advice. We strongly
suggest that you conduct local testing to ensure high performance with Lustre. Also,
note that most distributions ship with either “cfq” or “as” configured as the default
scheduler. Choosing an alternate scheduler is an absolutely necessary step to
optimally configure Lustre for the best performance. The “cfq” and “as” schedulers
should never be used for server platforms.
For more in-depth discussion on choosing an I/O scheduler algorithm for Linux, see:
■ http://www.redhat.com/magazine/008jun05/features/schedulers
■ http://www.novell.com/brainshare/europe/05_presentations/tut303.pdf
■ http://kerneltrap.org/node/3851
Chapter 3 Prerequisites 3-7