Veritas Storage Foundation™ for Oracle 5.0.1 Administrator's Guide

Warning: The write_throttle parameter is useful in
special situations where a computer system has a
combination of a lot of memory and slow storage devices.
In this configuration, sync operations (such as fsync())
may take so long to complete that the system appears to
hang. This behavior occurs because the file system is
creating dirty pages (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 pages per file that a file system will generate before
flushing the pages to disk. After the number of dirty pages
for a file reaches the write_throttle threshold, the file
system starts flushing pages to disk even if free memory is
still available. The default value of write_throttle
typically generates a lot of dirty pages, 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 pages is limited, so sync
operations will complete much faster.
Because lowering write_throttle can 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
recommended that you avoid changing the value of
write_throttle unless your system has a a large amount
of physical memory and slow storage devices.
write_throttle
If the file system is being used with VxVM, it is recommended that you set the
VxFS I/O parameters to default values based on the volume geometry.
If the file system is being used with a hardware disk array or volume manager
other than VxVM, align the parameters to match the geometry of the logical disk.
With striping or RAID-5, it is common to set read_pref_io to the stripe unit size
and read_nstream to the number of columns in the stripe. For striping arrays,
use the same values for write_pref_io and write_nstream, but for RAID-5 arrays,
set write_pref_io to the full stripe size and write_nstream to 1.
For an application to do efficient disk I/O, it should issue read requests that are
equal to the product of read_nstream multiplied by read_pref_io. Generally, any
multiple or factor of read_nstream multiplied by read_pref_io should be a good
size for performance. For writing, the same rule of thumb applies to the
write_pref_io and write_nstream parameters. When tuning a file system, the
best thing to do is try out the tuning parameters under a real-life workload.
307Tuning for performance
About tuning VxFS