HP-UX 11i v3 Installation and Update Guide, September 2008 (Update 3 Release)

B Controlling Memory Utilization of VxFS 4.1
This appendix discusses the effect of two VxFS tunables, vx_ninode and
vxfs_bc_bufhwm, on system memory consumption and provides guidelines on setting
them for machines with relatively low RAM.
What You Will Find in This Appendix
• “Introduction” (page 125)
• “Controlling the inode Cache” (page 125)
• “Controlling the Buffer Cache” (page 126)
• “Conclusion” (page 126)
Introduction
VxFS 4.1 caches objects in memory to improve performance. Most of the memory
consumed by VxFS is used to cache inodes (in the inode cache) and metadata (in the
buffer cache). The sizes of these caches and the behavior of VxFS are controlled by a
set of tunables. You can tailor the performance of VxFS to meet a variety of usage
scenarios while taking into account variations in machine configurations via the use of
these tunables.
The default settings of these tunables are meant to provide good performance for typical
deployment configurations. However, these default values can result in the VxFS driver
consuming more memory, especially when the file systems are under heavy file system
load. For machines low on RAM, these tunables may need to be manually turned down
depending on the expected use of the machine and the performance required of the
file system.
VxFS 4.1 exposes two global tunables, vx_ninode and vxfs_bc_bufhwm, that control
the size of the inode cache and buffer cache, respectively, and thereby affect system
memory consumption by the file system driver.
This appendix discusses when and why the sizes of the inode and buffer caches need
to be tuned down from their default values in certain configurations. The following
sections describe these tunables in detail as well as the effects of changing their default
values.
Controlling the inode Cache
As a matter of course, VxFS file systems allocate and free up inodes as required by the
load on the file system. VxFS caches these inodes for better performance (faster lookups).
In general, larger inode caches help file systems perform better for file server and web
server loads. The global (static) tunable vx_ninode represents the maximum possible
size of the VxFS inode cache.
Normally, the size of the inode cache is decided (auto-tuned) at boot time by VxFS
depending on the amount of physical memory in the machine, provided that the value
of vx_ninode is set to zero (default).
Introduction 125