VxFS 4.1 implements a private buffer cache used exclusively
for metadata. The allocations made for this buffer cache are not static
but grow and shrink during system usage, depending on the load on
the file system. The global (static) tunable vxfs_bc_bufhwm represents the maximum possible size of the VxFS buffer cache.
The maximum size of the metadata buffer cache is set (auto-tuned)
at boot time based on system memory size, provided that the value
of vxfs_bc_bufhwm is set to zero (default).
Like with the tunable vx_ninode, a
large metadata buffer cache can help improve file system performance,
especially during metadata-intensive loads (stat, create, remove,
link, lookup operations).
Systems low on RAM (having typically 1.5 GB/CPU) may not need a large metadata buffer cache if the file system load
is not metadata-intensive or when performance is not critical. In
such circumstances, the value of vxfs_bc_bufhwm can be manually tuned down, subject to a minimum of 6 MB.