IBM SG24-5131-00 manual Tuning the System Using I/O Pacing, Extending the syncd Frequency

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 -mreports 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

146 IBM Certification Study Guide AIX HACMP