Veritas File System 5.0 Administrator's Guide (September 2006)
As its name implies, the File Change Log records information about changes made
to files in a VxFS file system. In addition to recording creations, deletions,
extensions, the FCL periodically captures the cumulative amount of I/O activity
(number of bytes read and written) on a file-by-file basis. File I/O activity is
recorded in the FCL each time a file is opened or closed, as well as at timed intervals
to capture information about files that remain open for long periods.
If a file system's active file placement policy contains <IOTEMP> clauses, execution
of the fsppadm enforce command begins with a scan of the FCL to extract I/O
activity information over the period of interest for the policy. The period of interest
is the interval between the time at which the fsppadm enforce command was
issued and that time minus the largest interval value specified in any <PERIOD>
element in the active policy.
For files with I/O activity during the largest interval, VxFS computes an
approximation of the amount of read, write, and total data transfer (the sum of
the two) activity by subtracting the I/O levels in the oldest FCL record that pertains
to the file from those in the newest. It then computes each file's I/O temperature
by dividing its I/O activity by its size at Tscan. Dividing by file size is an implicit
acknowledgement that relocating larger files consumes more I/O resources than
relocating smaller ones. Using this algorithm requires that larger files must have
more activity against them in order to reach a given I/O temperature, and thereby
justify the resource cost of relocation.
While this computation is an approximation in several ways, it represents an easy
to compute, and more importantly, unbiased estimate of relative recent I/O activity
upon which reasonable relocation decisions can be based.
File relocation and deletion decisions can be based on read, write, or total I/O
activity.
The following XML snippet illustrates the use of IOTEMP in a policy rule to specify
relocation of low activity files from tier1 volumes to tier2 volumes:
<RELOCATE>
<FROM>
<SOURCE>
<CLASS>tier1</CLASS>
</SOURCE>
</FROM>
<TO>
<DESTINATION>
<CLASS>tier2</CLASS>
</DESTINATION>
</TO>
<WHEN>
Dynamic Storage Tiering
Calculating I/O temperature and access temperature
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