Managing system performance

Adjusting filesystem parameters

The filesystem parameter NAUTOUP specifies the time, in seconds, for automatic filesystem updates (for both the buffer cache and filesystem pages). A system buffer is written to the hard disk when it has been resident in memory for the interval specified by the NAUTOUP parameter. Specifying a smaller limit increases system reliability (by writing the buffer caches and filesystem pages to disk more frequently) and decreases system performance. Specifying a larger limit increases system performance at the expense of reliability.

The parameter BUFHWM specifies the amount of memory that can be used by the transfer of filesystem structure data such as inodes, indirect blocks, and cylinder groups. Setting BUFHWM too low causes excess filesystem activity to flush the buffer before it can be re-used. Setting BUFHWM too high reduces the page pool size and can increase paging. In general terms, BUFHWM should grow proportionately with memory size. The value of BUFHWM is autotuned and it should not have to be retuned. However, you might want to retune BUFHWM for a specific condition, for example, to reduce the value of BUFHWM so there is more memory for use by X-windows. Check sar -b before and after the change to ensure that the %rcache and %wcache have not changed dramatically.

The parameter NHBUF specifies the size of the hash table used to locate a buffer, given a device number and a block number. The value must be a power of ``2'' from ``32'' to ``1024'', inclusive. This value should be about one quarter of the total buffers available. Typically, a value between 1/8 and 1/4 of BUFHWM is sufficient.

The parameter NPBUF specifies the number of physical I/O buffers to allocate; one is needed for each active physical read or write. There is no rule for adjusting NPBUF. However, if you expect a lot of I/O and filesystem activity, improvement can be gained by raising NPBUF.

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SCO OpenServer Release 6.0.0 -- 05 June 2007