Tuning I/O resources

Reducing disk I/O by increasing the size of the namei cache

There are two kernel parameters that control the performance of the namei cache:

CACHEENTS: Sets the number of entries in the namei cache. Increasing the size of the namei cache reduces the number of disk accesses required to find inodes associated with pathname components. You can use configure to adjust the value of this parameter as described in ``Using configure to change kernel resources''.
HASHQS: Sets the number of hash queues used to access the namei cache. Having more hash queues speeds finding a pathname component in the cache. You can only adjust the value of this parameter using idtune as described in ``Using idtune to reallocate kernel resources''.

If the hit % value shown by sar -n is consistently low (for example, less than 65%), increase CACHEENTS until hit % increases to a satisfactorily high value (for example, more than 90%). Poor name-lookup performance can result if the value of CACHEENTS is less than twice the grown size of the in-core inode table. As a rough guide, CACHEENTS should be approximately three times the grown size of the in-core inode table reported by the inod-sz field of sar -v.

The value of HASHQS should be at least one third the value of CACHEENTS. For example, if the value of CACHEENTS is 1000, a suitable value for HASHQS would be 334.

Remember that each time you change the value of a kernel parameter, you must relink the kernel and reboot your system for the new value to take effect. See ``Configuring kernel parameters'' for details of how to change the values of kernel parameters.