VERITAS File System 4.1 Administrator's Guide

VxFS Performance: Creating, Mounting, and Tuning File Systems
Kernel Tunables
Chapter 2 53
The initial value of vx_bc_bufhwm is zero. When the operating system reboots, VxFS sets the
value of vx_bc_bufhwm based on the amount of system memory. You can explicitly reset the
value of vx_bc_bufhwm by changing the value of vxfs_bc_bufhwm using the sam or kctune
commands (see the sam(1M) and kctune(1M) manual pages). You can also set the value by
adding an entry to the system configuration file. For example, the following entry:
vxfs_bc_bufhwm vx_bc_bufhwm 300000
sets the high water mark to 300 megabytes. The change takes effect after you rebuild the
HP-UX kernel using the mk_kernel command. You specify the vx_bc_bufhwm tunable in units
of kilobytes. The minimum value is 6144.
Increasing the value of vx_bc_bufhwm increases the VxFS buffer cache immediately, allowing
a greater amount of memory to be used to cache VxFS metadata. Decreasing the value of
vx_bc_bufhwm decreases the VxFS buffer cache to the specified value. This frees memory
such that the amount of memory used for buffer cache is lower than the specified value of
vx_bc_bufhwm.
Typically, the default value computed by VxFS based on the amount of system memory
ensures good system performance across a wide range of applications. For application loads
that cause frequent file system metadata changes on the system (for example, a high rate of
file creation or deletion, or accessing large directories), changing the value of vx_bc_bufhwm
may improve performance.
You can use the vxfsstat command to monitor buffer cache statistics and inode cache usage
(see the vxfsstat(1M) manual page).
Number of Links to a File
In VxFS, the number of possible links to a file is determined by the vx_maxlink global
tunable. The default value of vx_maxlink is 32767, the maximum value is 65535. This is a
static tunable.
You can set the value of vx_maxlink using the sam or kctune commands (see the sam(1M)
and kctune(1M) manual pages), or by adding an entry to the system configuration file as
shown in the following example:
vxfs_maxlink vx_maxlink 40000
This sets the value of vx_maxlink to 40,000 links.
VxFS Inode Free Time Lag
In VxFS, an inode is put on a freelist if it is not being used. The memory space for this unused
inode can be freed it it stays on the freelist for a specified amount of time.