The I/O temperature relocation criterion, <IOTEMP>, causes files to be relocated if their I/O temperatures rise above or drop below specified values over a specified period immediately prior to the time at which the fsppadm enforce command was issued. A file's I/O temperature is a measure of the read, write, or total I/O activity against it normalized to the file's size. Higher I/O temperatures indicate higher levels of application activity; lower temperatures indicate lower levels. VxFS computes a file's I/O temperature by dividing the number of bytes transferred to or from it (read, written, or both) during the specified period by its size at the time that the fsppadm enforce command was issued.
As with the other file relocation criteria, <IOTEMP> may be specified with a lower threshold by using the <MIN> element, an upper threshold by using the <MAX> 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 or hours in the past specified in the <PERIOD> element, where a day is a 24 hour period. The default unit of time is days. You can specify hours as the time unit by setting the Units attribute of the <PERIOD> element to hours. Symantec recommends that you specify hours only if you are using solid state disks (SSDs).
See Frequent scans.
For example, if you issued the fsppadm enforce command at 2 PM on Wednesday and you want VxFS to look at file I/O activity for the period between 2 PM on Monday and 2 PM on Wednesday, which is a period of 2 days, you would specify the following <PERIOD> element:
<PERIOD> 2 </PERIOD>
<PERIOD Units="hours"> 3 </PERIOD>
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.