Veritas™ File System 5.0.1 Administrator's Guide

Table 2-1
Tunable VxFS I/O parameters (continued)
DescriptionParameter
The write_throttle parameter is useful in special
situations where a computer system has a combination
of a large amount of memory and slow storage devices.
In this configuration, sync operations, such as fsync(),
may take long enough to complete that a system appears
to hang. This behavior occurs because the file system is
creating dirty buffers (in-memory updates) faster than
they can be asynchronously flushed to disk without
slowing system performance.
Lowering the value of write_throttle limits the
number of dirty buffers per file that a file system
generates before flushing the buffers to disk. After the
number of dirty buffers for a file reaches the
write_throttle threshold, the file system starts
flushing buffers to disk even if free memory is still
available.
The default value of write_throttle is zero, which puts
no limit on the number of dirty buffers per file. If
non-zero, VxFS limits the number of dirty buffers per file
to write_throttle buffers.
The default value typically generates a large number of
dirty buffers, but maintains fast user writes. Depending
on the speed of the storage device, if you lower
write_throttle, user write performance may suffer,
but the number of dirty buffers is limited, so sync
operations complete much faster.
Because lowering write_throttle may in some cases
delay write requests (for example, lowering
write_throttle may increase the file disk queue to the
max_diskq value, delaying user writes until the disk
queue decreases), it is advisable not to change the value
of write_throttle unless your system has a
combination of large physical memory and slow storage
devices.
write_throttle
File system tuning guidelines
If the file system is being used with VxVM, it is advisable to let the VxFS I/O
parameters be set to default values based on the volume geometry.
VxFS performance: creating, mounting, and tuning file systems
Tuning I/O
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