An important application of VxFS Dynamic Storage Tiering 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 relocat--ion 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.
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.
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 is the interval between the time at which the fsppadm enforce command was issued and that time minus the largest interval value specified in any <PERIOD> element in the active policy.
For files with I/O activity during the largest interval, VxFS computes an approximation of the amount of read, write, and total data transfer (the sum of the two) activity by subtracting the I/O levels in the oldest FCL record that pertains to the file from those in the newest. It then computes each file's I/O temperature by dividing its I/O activity by its size at Tscan. Dividing by file size is an implicit acknowledgement that relocating larger files consumes more I/O resources than relocating smaller ones. Using this algorithm requires that larger files must have more activity against them in order to reach a given I/O temperature, and thereby justify the resource cost of relocation.
While this computation is an approximation in several ways, it represents an easy to compute, and more importantly, unbiased estimate of relative recent I/O activity upon which reasonable relocation decisions can be based.
<RELOCATE> <FROM> <SOURCE> <CLASS>tier1</CLASS> </SOURCE> </FROM> <TO> <DESTINATION> <CLASS>tier2</CLASS> </DESTINATION> </TO> <WHEN> <IOTEMP Type="nrbytes"> <MAX Flags="lt">3</MAX> <PERIOD Units="days">4</PERIOD> </IOTEMP> </WHEN> </RELOCATE>
This snippet specifies that files to which the rule applies should be relocated from
tier1 volumes to
tier2 volumes if their I/O temperatures fall below 3 over a period of 4 days. The Type="nrbytes" XML attribute specifies that total data transfer activity, which is the the sum of bytes read and bytes written, should be used in the computation. For example, a 50 megabyte file that experienced less than 150 megabytes of data transfer over the 4-day period immediately preceding the fsppadm enforce scan would be a candidate for relocation. VxFS considers files that experience no activity over the period of interest to have an I/O temperature of zero. VxFS relocates qualifying files in the order in which it encounters the files in its scan of the file system directory tree.
Using I/O temperature or access temperature rather than a binary indication of activity, such as the POSIX atime or mtime, minimizes the chance of not relocating files that were only accessed occasionally during the period of interest. A large file that has had only a few bytes transferred to or from it would have a low I/O temperature, and would therefore be a candidate for relocation to
tier2 volumes, even if the activity was very recent.
But, the greater value of I/O temperature or access temperature as a file relocation criterion lies in upward relocation: detecting increasing levels of I/O activity against files that had previously been relocated to lower tiers in a storage hierarchy due to inactivity or low temperatures, and relocating them to higher tiers in the storage hierarchy.
<RELOCATE> <FROM> <SOURCE> <CLASS>tier2</CLASS> </SOURCE> </FROM> <TO> <DESTINATION> <CLASS>tier1</CLASS> </DESTINATION> </TO> <WHEN> <IOTEMP Type="nrbytes"> <MAX Flags="gt">5</MAX> <PERIOD Units="days">2</PERIOD> </IOTEMP> </WHEN> </RELOCATE>
The <RELOCATE> statement specifies that files on
tier2 volumes whose I/O temperature as calculated using the number of bytes read is above 5 over a 2-day period are to be relocated to
tier1 volumes. Bytes written to the file during the period of interest are not part of this calculation.
Using I/O temperature rather than a binary indicator of activity as a criterion for file relocation gives administrators a granular level of control over automated file relocation that can be used to attune policies to application requirements. For example, specifying a large value in the <PERIOD> element of an upward relocation statement prevents files from being relocated unless I/O activity against them is sustained. Alternatively, specifying a high temperature and a short period tends to relocate files based on short-term intensity of I/O activity against them.
I/O temperature and access temperature utilize the
sqlite3 database for building a temporary table indexed on an inode. This temporary table is used to filter files based on I/O temperature and access temperature. The temporary table is stored in the database file
.__fsppadm_fcliotemp.db, which resides in the
lost+found directory of the mount point.