Adjusting the Buffer Cache (bufhwm)

The bufhwm variable, set in the /etc/system file, controls the maximum amount of memory allocated to the buffer cache and is specified in Kbytes. The default value of bufhwm is 0, which allows up to 2 percent of system memory to be used. This can be increased up to 20 percent and may need to be increased to 10 percent for a dedicated NFS file server with a relatively small memory system. On a larger system, the bufhwm variable may need to be limited to prevent the system from running out of the operating system kernel virtual address space.

The buffer cache is used to cache inode, indirect block, and cylinder group related disk I/O only. The following is an example of a buffer cache ( bufhwm) setting in the /etc/system file that can handle up to 10 Mbytes of cache. This is the highest value to which you should set bufhwm.

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set bufhwm=10240

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You can monitor the buffer cache using sar -b (see the following code example), which reports a read (%rcache) and a write hit rate (%wcache) for the buffer cache.

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# sar -b 5 10
SunOS hostname 5.2 Generic sun4c    08/06/93
23:43:39 bread/s lread/s %rcache bwrit/s lwrit/s %wcache pread/s pwrit/s
Average        0      25     100       3      22      88       0       0

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If a significant number of reads and writes per second occur (greater than 50) and if the read hit rate (%rcache) falls below 90 percent, or if the write hit rate (%wcache) falls below 65 percent, increase the buffer cache size, bufhwm.

In the previous sar -b 5 10 command output, the read hit rate (%rcache) and the write hit rate (%wcache) did not fall below 90 percent or 65 percent respectively.

Following are descriptions of the arguments to the sar command.

Table 4-6 Descriptions of the Arguments to the sar Command

b	Checks buffer activity
5	Time, every 5 seconds (must be at least 5 seconds)
10	Number of times the command gathers statistics

Your system will prevent you from increasing the buffer cache to an unacceptably high level. Signs of increasing buffer cache size include:

• Hung server

• Device drivers that suffer from a shortage of operating system kernel virtual memory