HP-UX Memory File System (MemFS) 2.0

Installing, Configuring, Tuning, and Removing HP-UX Memory File System
Tuning Memory File System
Chapter 2 17
Tuning Memory File System
Tuning the MemFS buffer cache size is very important because MemFS stores data and
metadata in the buffer cache. The amount of memory in the buffer cache used for MemFS can
be tuned using the following dynamic tunables on HP-UX 11i v2. These tunables can be
modified without a system reboot.
memfs_bufcache_swappct
memfs_bufcache_swappct is the percentage of buffer cache at which MemFS buffers start
swapping. The default value is 50. The memfs_bufcache_swappct tunable can be as low as 0
and can be as large as 100. If memfs_bufcache_swappct is set to 0, MemFS data buffers
would start swapping immediately to the user process. If memfs_bufcache_swappct is set to
100, MemFS can occupy the entire buffer cache before it starts swapping. The larger
memfs_bufcache_swappct is, the greater the amount of buffer cache that will be reserved for
MemFS pages and the better the performance of MemFS. But this may adversely impact the
performance of other filesystems that use the buffer cache.
memfs_bufcache_metapct
memfs_bufcache_metapct is the maximum percentage of buffer cache which can
accommodate MemFS metadata of all MemFS instances. The default value is 10. The
memfs_bufcache_metapct tunable cannot be less than 10 and cannot be greater than 50.
Tuning memfs_bufcache_metapct to a value as high as 50 may be necessary if the system
primarily uses large MemFS filesystems. The larger memfs_bufcache_metapct is, more of the
buffer cache will be reserved for MemFS metadata pages and hence will allow creation of
more and larger MemFS filesystem instances, but impact the performance of other
filesystems that use the buffer cache. Since MemFS metadata pages can never be swapped out
to the user process, exercise caution when tuning memfs_bufcache_metapct as this may
severely impact normal performance of other filesystems.
WARNING Shrinking the buffer cache (through kctune or sam) when any MemFS
file systems are mounted should NOT be done. Doing so may cause the
system to be unstable. If the buffer cache needs to be down-sized
manually, all MemFS file systems should first be unmounted.
NOTE The memfs_bufcache_swappct and memfs_bufcache_metapct tunables are
dynamic on HP-UX 11i v2 and can be modified without a system reboot.