IBM SG24-5131-00 사용자 설명서

IBM Certification Study Guide  AIX HACMP

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.

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.

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