Veritas File System v5.0.1 Administrator's Guide for HP-UX 11i v3 (5900-0082, June 2010)

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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