StorNext File System Tuning
The Metadata Controller System
18 StorNext File System Tuning Guide
• 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-test utility 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-test command has the following syntax:
latency-test index-number [seconds]
latency-test all [seconds]
If an
index-number is 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 af fected by CPU load or SNFS l oad on ei ther sys tem, by u nrelate d
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