Lustre 1.6 Operations Manual

10.2 Insights into Disk Performance
Measurement
Several tips and insights for disk performance measurement are provided below.
Some of this information is specific to RAID arrays and/or the Linux RAID
implementation.
■ Performance is limited by the slowest disk.
Benchmark all disks individually. We have frequently encountered situations
where drive performance was not consistent for all devices in the array.
Note – On a Thumper system with the out-of-tree Marvell driver, disk ordering is
not preserved between boots (but controller ordering is). Perform a sgp_dd survey
and create arrays without rebooting in-between.
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Disks and arrays are very sensitive to request size.
To identify the optimal request size for a given disk, benchmark the disk with
different record sizes ranging from 4 KB to 1 to 2 MB.
By default, the maximum size of a request is quite small.
To properly handle I/O request sizes greater than 256 KB, the current Linux
kernel needs either a driver patch or several changes in the block layer defaults,
namely MAX_SECTORS, MAX_PHYS_SEGMENTS and MAX_HW_SEGMENTS.
Select the best I/O scheduler for your setup.
Try different I/O schedulers (kernel parameter elevator= on old kernels or echo
>/sys/block//queue/scheduler), because they behave
differently depending on storage and load. Benchmark all I/O schedulers and
select the best one for your setup. For further information on I/O schedulers,
refer to:
http://www.linuxjournal.com/article/6931
http://www.redhat.com/magazine/008jun05/features/schedulers/
Chapter 10 RAID 10-5