rcache & wcache by sar

[Lynn Hsu_2]

Hello,
Can someone comment on what this SAR output
mean on rcache and wcache? I have a plan to increase buffer cache. Currently it is at 144MB. I'd like to get input from the group.

Thanks in advance,

Lynn

[RAC_1]

In sar -b output, you need to look at %read and %write values. I would expect to see more 80% write hits. This would mean effective use of assigned buffer cache.

Before you increase it, you would want to look at this and then increase. 144MB looks on smaller side, but also depends what apps/program you run on your system.

[A. Clay Stephenson]

The % cache hits rates are really in the "it depends" catagory and your statement of increasing from 144MiB is meaningless without context. How much physical memory is in your box? Are you using lots of raw i/o? If so, your cache can be on the small side and your current cache size may be entirely reasonable. It also matters what version of HP-UX you are running. In any event, the general approach is not to look at the absolute values of the cache hit rates but rather how much gain do you get for an increase in the size of the cache. Initially, small increases yield large improvements but eventually the improvements become very small (what economists refer to as diminshed marginal utility) (on older versions of HP-UX less actually was more) and you have then gone beyond the "sweet spot". In any event, you never want to increase buffer cache or shared memory to the point where excessive page outs are triggered because the performance impact from the swapping will be at least 100x worse than any gains you get from cache hit improvement.