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146 IBM Certification Study Guide AIX HACMP
7.3.1 Tuning the System Using I/O Pacing
Use I/O pacing to tune the system so that system resources are distributed
more equitably during large disk writes. Enabling I/O pacing is required for an
HACMP cluster to behave correctly during large disk writes, and it is strongly
recommended if you anticipate large blocks of disk writes on your HACMP
cluster.
You can enable I/O pacing using the
smit chgsys fastpath to set high- and
low-water marks. These marks are by default set to zero (disabling I/O
pacing) when AIX is installed. While the most efficient high- and low-water
marks vary from system to system, an initial high-water mark of 33 and a
low-water mark of 24 provide a good starting point. These settings only
slightly reduce write times, and consistently generate correct failover
behavior from HACMP for AIX. If a process tries to write to a file at the
high-water mark, it must wait until enough I/O operations have finished to
make the low-water mark. See the
AIX Performance Monitoring & Tuning
Guide
, SC23-2365 for more information on I/O pacing.
7.3.2 Extending the syncd Frequency
Edit the /sbin/rc.boot file to increase the syncd frequency from its default
value of 60 seconds to either 30, 20, or 10 seconds. Increasing the frequency
forces more frequent I/O flushes and reduces the likelihood of triggering the
deadman switch due to heavy I/O traffic.
7.3.3 Increase Amount of Memory for Communications Subsystem
If the output of netstat -m reports that requests for mbufs are being denied,
or if errors indicating
LOW_MBUFS are being logged to the AIX error report,
increase the value associated with “
thewall” network option. The default
value is 25% of the real memory. This can be increased to as much as 50% of
the real memory.
To change this value, add a line similar to the following at the end of the
/etc/rc.net file:
no -o thewall=xxxxx
where
xxxxx
is the value you want to be available for use by the
communications subsystem. For example,
no -o thewall=65536