Veritas Storage Foundation 5.1 SP1 Advanced Features Administrator"s Guide (5900-1503, April 2011)

Calculating I/O temperature and access temperature
An important application of VxFS SmartTier is automating the relocation of
inactive files to lower cost storage. If a file has not been accessed for the period
of time specified in the <ACCAGE> element, a scan of the file system should schedule
the file for relocation to a lower tier of storage. But, time since last access is
inadequate as the only criterion for activity-based relocation.
Why time since last access is inadequate as the only criterion for activity-based
relocation:
■ Access age is a binary measure. The time since last access of a file is computed
by subtracting the time at which the fsppadm enforce command is issued
from the POSIX atime in the file's metadata. If a file is opened the day before
the fsppadm enforce command, its time since last access is one day, even
though it may have been inactive for the month preceding. If the intent of a
policy rule is to relocate inactive files to lower tier volumes, it will perform
badly against files that happen to be accessed, however casually, within the
interval defined by the value of the <ACCAGE> pa-rameter.
■ Access age is a poor indicator of resumption of significant activity. Using
ACCAGE, the time since last access, as a criterion for relocating inactive files to
lower tier volumes may fail to schedule some relocations that should be
performed, but at least this method results in less relocation activity than
necessary. Using ACCAGE as a criterion for relocating previously inactive files
that have become active is worse, because this method is likely to schedule
relocation activity that is not warranted. If a policy rule's intent is to cause
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.
SmartTier 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
369Administering SmartTier
Calculating I/O temperature and access temperature