RELOCATE statement

Article:HOWTO59526  |  Created: 2011-10-12  |  Updated: 2011-10-12  |  Article URL http://www.symantec.com/docs/HOWTO59526
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RELOCATE statement

The RELOCATE action statement of file placement policy rules specifies an action that VxFS takes on designated files during periodic scans of the file system, and the circumstances under which the actions should be taken. The fsppadm enforce command is used to scan all or part of a file system for files that should be relocated based on rules in the active placement policy at the time of the scan.

See the fsppadm(1M) manual page.

The fsppadm enforce command scans file systems in path name order. For each file, VxFS identifies the first applicable rule in the active placement policy, as determined by the rules' SELECT statements. If the file resides on a volume specified in the <FROM> clause of one of the rule's RELOCATE statements, and if the file meets the criteria for relocation specified in the statement's <WHEN> clause, the file is scheduled for relocation to a volume in the first placement class listed in the <TO> clause that has space available for the file. The scan that results from issuing the fsppadm enforce command runs to completion before any files are relocated.

The following XML snippet illustrates the general form of the RELOCATE statement:

 <RELOCATE>
   <FROM>
     <SOURCE>
       <CLASS> placement_class_name </CLASS>
     </SOURCE>
     <SOURCE> additional_placement_class_specifications
     </SOURCE>
   </FROM>
   <TO>
     <DESTINATION>
       <CLASS> placement_class_name </CLASS>
       <BALANCE_SIZE Units="units_specifier">
         chunk_size
       </BALANCE_SIZE>
     </DESTINATION>
     <DESTINATION>
       additional_placement_class_specifications
     </DESTINATION>
   </TO>
   <WHEN> relocation_conditions </WHEN>
 </RELOCATE>

A RELOCATE statement contains the following clauses:

<FROM>

An optional clause that contains a list of placement classes from whose volumes designated files should be relocated if the files meet the conditions specified in the <WHEN> clause. No priority is associated with the ordering of placement classes listed in a <FROM> clause. If a file to which the rule applies is located on a volume in any specified placement class, the file is considered for relocation.

If a RELOCATE statement contains a <FROM> clause, VxFS only considers files that reside on volumes in placement classes specified in the clause for relocation. If no <FROM> clause is present, qualifying files are relocated regardless of where the files reside.

<TO>

Indicates the placement classes to which qualifying files should be relocated. Unlike the source placement class list in a FROM clause, placement classes in a <TO> clause are specified in priority order. Files are relocated to volumes in the first specified placement class if possible, to the second if not, and so forth.

The <TO> clause of the RELOCATE statement contains a list of <DESTINATION> XML elements specifying placement classes to whose volumes VxFS relocates qualifying files. Placement classes are specified in priority order. VxFS relocates qualifying files to volumes in the first placement class specified as long as space is available. A <DESTINATION> element may contain an optional <BALANCE_SIZE> modifier sub-element. The <BALANCE_SIZE>modifier indicates that relocated files should be distributed across the volumes of the destination placement class in chunks of the indicated size. For example, if a balance size of one megabyte is specified for a placement class containing three volumes, VxFS relocates the first megabyte the file to the first (lowest indexed) volume in the class, the second megabyte to the second volume, the third megabyte to the third volume, the fourth megabyte to the first volume, and so forth. Using the Units attribute in the <BALANCE_SIZE> XML tag, the chunk value may be specified in the balance size value may be specified in bytes (Units="bytes"), kilobytes (Units="KB"), megabytes (Units="MB"), or gigabytes (Units="GB").

The <BALANCE_SIZE> element distributes the allocation of database files across the volumes in a placement class. In principle, distributing the data in each file across multiple volumes distributes the I/O load across the volumes as well.

<WHEN>

An optional clause that indicates the conditions under which files to which the rule applies should be relocated. Files that have been unaccessed or unmodified for a specified period, reached a certain size, or reached a specific I/O temperature or access temperature level may be relocated. If a RELOCATE statement does not contain a <WHEN> clause, files to which the rule applies are relocated unconditionally.

A <WHEN> clause may be included in a RELOCATE statement to specify that files should be relocated only if any or all of four types of criteria are met. Files can be specified for relocation if they satisfy one or more criteria.

The following are the criteria that can be specified for the <WHEN> clause:

<ACCAGE>

This criterion is met when files are inactive for a designated period or during a designated period relative to the time at which the fsppadm enforce command was issued.

<MODAGE>

This criterion is met when files are unmodified for a designated period or during a designated period relative to the time at which the fsppadm enforce command was issued.

<SIZE>

This criterion is met when files exceed or drop below a designated size or fall within a designated size range.

<IOTEMP>

This criterion is met when files exceed or drop below a designated I/O temperature, or fall within a designated I/O temperature range. A file's I/O temperature is a measure of the I/O activity against it during the period designated by the <PERIOD>element prior to the time at which the fsppadm enforce command was issued.

See Calculating I/O temperature and access temperature.

<ACCESSTEMP>

This criterion is met when files exceed or drop below a specified average access temperature, or fall within a specified access temperature range. A file's access temperature is similar to its I/O temperature, except that access temperature is computed using the number of I/O requests to the file, rather than the number of bytes transferred.

Note:

The use of <IOTEMP> and <ACCESSTEMP> for data placement on VxFS servers that are used as NFS servers may not be very effective due to NFS caching. NFS client side caching and the way that NFS works can result in I/O initiated from an NFS client not producing NFS server side I/O. As such, any temperature measurements in place on the server side will not correctly reflect the I/O behavior that is specified by the placement policy.

If the server is solely used as an NFS server, this problem can potentially be mitigated by suitably adjusting or lowering the temperature thresholds. However, adjusting the thresholds may not always create the desired effect. In addition, if the same mount point is used both as an NFS export as well as a local mount, the temperature-based placement decisions will not be very effective due to the NFS cache skew.

The following XML snippet illustrates the general form of the <WHEN> clause in a RELOCATE statement:

 <WHEN>
   <ACCAGE Units="units_value">
     <MIN Flags="comparison_operator">
       min_access_age</MIN>
     <MAX Flags="comparison_operator">
       max_access_age</MAX>
   </ACCAGE>
   <MODAGE Units="units_value">
     <MIN Flags="comparison_operator">
       min_modification_age</MIN>
     <MAX Flags="comparison_operator">
       max_modification_age</MAX>
   </MODAGE>
   <SIZE " Units="units_value">
     <MIN Flags="comparison_operator">
       min_size</MIN>
     <MAX Flags="comparison_operator">
       max_size</MAX>
   </SIZE>
   <IOTEMP Type="read_write_preference" Prefer="temperature_preference">
     <MIN Flags="comparison_operator">
       min_I/O_temperature</MIN>
     <MAX Flags="comparison_operator">
       max_I/O_temperature</MAX>
     <PERIOD Units="days_or_hours"> days_or_hours_of_interest </PERIOD>
   </IOTEMP>
   <ACCESSTEMP Type="read_write_preference"
    Prefer="temperature_preference">
     <MIN Flags="comparison_operator">
       min_access_temperature</MIN>
     <MAX Flags="comparison_operator">
       max_access_temperature</MAX>
     <PERIOD Units="days_or_hours"> days_or_hours_of_interest </PERIOD>
   </ACCESSTEMP>
 </WHEN>

The access age (<ACCAGE>) element refers to the amount of time since a file was last accessed. VxFS computes access age by subtracting a file's time of last access, atime, from the time when the fsppadm enforce command was issued. The <MIN> and <MAX> XML elements in an <ACCAGE> clause, denote the minimum and maximum access age thresholds for relocation, respectively. These elements are optional, but at least one must be included. Using the Units XML attribute, the <MIN> and <MAX> elements may be specified in the following units:

hours

Hours

days

Days. A day is considered to be 24 hours prior to the time that the fsppadm enforce command was issued.

Both the <MIN> and <MAX> elements require Flags attributes to direct their operation.

For <MIN>, the following Flags attributes values may be specified:

gt

The time of last access must be greater than the specified interval.

eq

The time of last access must be equal to the specified interval.

gteq

The time of last access must be greater than or equal to the specified interval.

For <MAX>, the following Flags attributes values may be specified.

lt

The time of last access must be less than the specified interval.

lteq

The time of last access must be less than or equal to the specified interval.

Including a <MIN> element in a <WHEN> clause causes VxFS to relocate files to which the rule applies that have been inactive for longer than the specified interval. Such a rule would typically be used to relocate inactive files to less expensive storage tiers. Conversely, including <MAX> causes files accessed within the specified interval to be relocated. It would typically be used to move inactive files against which activity had recommenced to higher performance or more reliable storage. Including both <MIN> and <MAX> causes VxFS to relocate files whose access age lies between the two.

The modification age relocation criterion, <MODAGE>, is similar to access age, except that files' POSIX mtime values are used in computations. You would typically specify the <MODAGE> criterion to cause relocation of recently modified files to higher performance or more reliable storage tiers in anticipation that the files would be accessed recurrently in the near future.

The file size relocation criterion, <SIZE>, causes files to be relocated if the files are larger or smaller than the values specified in the <MIN> and <MAX> relocation criteria, respectively, at the time that the fsppadm enforce command was issued. Specifying both criteria causes VxFS to schedule relocation for files whose sizes lie between the two. Using the Units attribute, threshold file sizes may be specified in the following units:

bytes

Bytes

KB

Kilobytes

MB

Megabytes

GB

Gigabytes

See Specifying the I/O temperature relocation criterion

See Prefer attribute

See Average I/O activity

See RELOCATE statement examples

See File placement policy rules


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Article URL http://www.symantec.com/docs/HOWTO59526


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