Veritas™ File System 5.0.1 Administrator's Guide

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:

<FROM>

</SOURCE>

</FROM>

<TO>

</DESTINATION>

</TO>

</RELOCATE>

163Dynamic Storage Tiering

File placement policy rules