Veritas™ File System 5.0.1 Administrator's Guide

files that have experienced I/O activity in the recent past to be relocated to
higher performing, perhaps more failure tolerant storage, ACCAGE is too coarse
a filter. For example, in a rule specifying that files on tier2 volumes that have
been accessed within the last three days should be relocated to tier1 volumes,
no distinction is made between a file that was browsed by a single user and a
file that actually was used intensively by applications.
DST implements the concept of I/O temperature and access temperature to
overcome these deficiencies. A file's I/O temperature is equal to the number of
bytes transferred to or from it over a specified period of time divided by the size
of the file. For example, if a file occupies one megabyte of storage at the time of
an fsppadm enforce operation and the data in the file has been completely read
or written 15 times within the last three days, VxFS calculates its 3-day average
I/O temperature to be 5 (15 MB of I/O ÷ 1 MB file size ÷ 3 days).
Similarly, a file's average access temperature is the number of read or write
requests made to it over a specified number of 24-hour periods divided by the
number of periods. Unlike I/O temperature, access temperature is unrelated to
file size. A large file to which 20 I/O requests are made over a 2-day period has
the same average access temperature as a small file accessed 20 times over a 2-day
period.
If a file system's active placement policy includes any <IOTEMP> or <ACCESSTEMP>
clauses, VxFS begins policy enforcement by using information in the file system's
FCL file to calculate average I/O activity against all files in the file system during
the longest <PERIOD> specified in the policy. Shorter specified periods are ignored.
VxFS uses these calculations to qualify files for I/O temperature-based relocation
and deletion.
See “About the File Change Log file” on page 114.
Note: If FCL is turned off, I/O temperature-based relocation will not be accurate.
When you invoke the fsppadm enforce command, the command displays a
warning if the FCL is turned off.
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
Dynamic Storage Tiering
Calculating I/O temperature and access temperature
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