Quantum 3.1.4.1 manual Latency-testindex-number seconds

StorNext File System Tuning

The Metadata Controller System

•Identify disk performance issues.

If Device throughput is inconsistent or less than expected, it might indicate a slow disk in a stripe group, or that RAID tuning is necessary.

•Identify file fragmentation.

If the extent count “exts” is high, it might indicate a fragmentation problem.This causes the device I/Os to be broken into smaller chunks, which can significantly impact throughput.

•Identify read/modify/write condition.

If buffered VFS writes are causing Device reads, it might be beneficial to match I/O request size to a multiple of the “cachebufsize” (default 64KB; see mount_cvfs man page). Another way to avoid this is by truncating the file before writing.

The cvadmin command includes a latency-testutility for measuring the latency between an FSM and one or more SNFS clients. This utility causes small messages to be exchanged between the FSM and clients as quickly as possible for a brief period of time, and reports the average time it took for each message to receive a response.

The latency-testcommand has the following syntax:

latency-testindex-number [seconds]

latency-test all [seconds]

If an index-numberis specified, the test is run between the currently- selected FSM and the specified client. (Client index numbers are displayed by the cvadmin who command). If all is specified, the test is run against each client in turn.

The test is run for 2 seconds, unless a value for seconds is specified. Here is a sample run:

snadmin (lsi) >	latency-test
Test started on	client	1	(bigsky-node2)... latency 55us
Test started on	client	2	(k4)... latency 163us

There is no rule-of-thumb for “good” or “bad” latency values. Latency can be affected by CPU load or SNFS load on either system, by unrelated Ethernet traffic, or other factors. However, for otherwise idle systems, differences in latency between different systems can indicate differences in hardware performance. (In the example above, the difference is a Gigabit Ethernet and faster CPU versus a 100BaseT Ethernet and a

18 StorNext File System Tuning Guide