Veritas File System 5.0 AdministratorÆs Guide, HP-UX 11i v3, First Edition, May 2008

element, or as a range by using both. However, I/O temperature is dimensionless
and therefore has no specification for units.
VxFS computes files' I/O temperatures over the period between the time when
the fsppadm enforce command was issued and the number of days in the past
specified in the <PERIOD> element, where a day is a 24 hour period. For example,
if the fsppadm enforce command was issued at 2 PM on Wednesday, and a
<PERIOD> value of 2 was specified, VxFS looks at file I/O activity for the period
between 2 PM on Monday and 2 PM on Wednesday. The number of days specified
in the <PERIOD> element should not exceed one or two weeks due to the disk space
used by the File Change Log (FCL) file.
I/O temperature is a softer measure of I/O activity than access age. With access
age, a single access to a file resets the file's atime to the current time. In contrast,
a file's I/O temperature decreases gradually as time passes without the file being
accessed, and increases gradually as the file is accessed periodically. For example,
if a new 10 megabyte file is read completely five times on Monday and fsppadm
enforce runs at midnight, the file's two-day I/O temperature will be five and its
access age in days will be zero. If the file is read once on Tuesday, the file's access
age in days at midnight will be zero, and its two-day I/O temperature will have
dropped to three. If the file is read once on Wednesday, the file's access age at
midnight will still be zero, but its two-day I/O temperature will have dropped to
one, as the influence of Monday's I/O will have disappeared.
If the intention of a file placement policy is to keep files in place, such as on top-tier
storage devices, as long as the files are being accessed at all, then access age is
the more appropriate relocation criterion. However, if the intention is to relocate
files as the I/O load on them decreases, then I/O temperature is more appropriate.
The case for upward relocation is similar. If files that have been relocated to
lower-tier storage devices due to infrequent access experience renewed application
activity, then it may be appropriate to relocate those files to top-tier devices. A
policy rule that uses access age with a low <MAX> value, that is, the interval between
fsppadm enforce runs, as a relocation criterion will cause files to be relocated
that have been accessed even once during the interval. Conversely, a policy that
uses I/O temperature with a <MIN> value will only relocate files that have
experienced a sustained level of activity over the period of interest.
RELOCATE statement examples
The following example illustrates an unconditional relocation statement, which
is the simplest form of the RELOCATE policy rule statement:
<RELOCATE>
<FROM>
165Dynamic Storage Tiering
File placement policy rules