ufs_LW and ufs_HW

Description

ufs_HW specifies the number of bytes outstanding on a single file barrier value. If the number of bytes outstanding is greater than this value and ufs_WRITES is set, then the write is deferred. The write is deferred by putting the thread issuing the write to sleep on a condition variable.

ufs_LW is the barrier for the number of bytes outstanding on a single file below which the condition variable on which other sleeping processes are toggled. When a write completes and the number of bytes is less than ufs_LW, then the condition variable is toggled, which causes all threads waiting on the variable to awaken and try to issue their writes.

Data Type

Signed integer

Default

8 x 1024 x 1024 for ufs_LW and 16 x 1024 x 1024 for ufs_HW

Range

0 to MAXINT

Units

Bytes

Dynamic?

Yes

Validation

None

Implicit

ufs_LW and ufs_HW have meaning only if ufs_WRITES is not equal to zero. ufs_HW and ufs_LW should be changed together to avoid needless churning when processes awaken and find that either they cannot issue a write (when ufs_LW and ufs_HW are too close) or they might have waited longer than necessary (when ufs_LW and ufs_HW are too far apart).

When to Change

Consider changing these values when file systems consist of striped volumes. The aggregate bandwidth available can easily exceed the current value of ufs_HW. Unfortunately, this parameter is not a per-file system setting.

You might also consider changing this parameter when ufs_throttles is a non-trivial number. Currently, ufs_throttles can only be accessed with a kernel debugger.

Commitment Level

Unstable