COMPRESS statement

The COMPRESS statement in a file placement policy rule specifies in-place file compression on multi-volume or single-volume file systems. The placement policy becomes assigned to the selected file, and allocation for the compressed extents is done from the same tier specified in the <SOURCE> element of the <FROM> clause. SmartTier performs in-place compression of the entire file, even if the file spans across multiple tiers.

Note:

SmartTier does not schedule compression activity. If you did not integrate your Veritas Storage Foundation product with the Veritas Operations Manager (VOM), then you must automate compression activity by using techniques such as scheduling through cron jobs.

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

 <COMPRESS>
   <FROM>
     <SOURCE>
       <CLASS> placement_class_name </CLASS>
     </SOURCE>
     <SOURCE> additional_placement_class_specifications
     </SOURCE>
   </FROM>
   <WHEN> compression_conditions </WHEN>
 </COMPRESS>

A COMPRESS statement contains the following clauses:

<FROM>

An optional clause that contains a list of placement classes from whose volumes designated files should be compressed 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 compression.

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

<WHEN>

An optional clause that indicates the conditions under which files to which the rule applies should be compressed. 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 compressed. If a COMPRESS statement does not contain a <WHEN> clause, files to which the rule applies are compressed unconditionally.

A <WHEN> clause may be included in a COMPRESS statement to specify that files should be compressed only if any or all of four types of criteria are met. Files can be specified for compression 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 COMPRESS 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 compression, 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 compress files to which the rule applies that have been inactive for longer than the specified interval. Such a rule would typically be used to compress inactive files to less expensive storage tiers. Conversely, including <MAX> causes files accessed within the specified interval to be compressed. 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 compress files whose access age lies between the two.

The modification age compression 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 compression 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 compression criterion, <SIZE>, causes files to be compressed if the files are larger or smaller than the values specified in the <MIN> and <MAX> compression criteria, respectively, at the time that the fsppadm enforce command was issued. Specifying both criteria causes VxFS to schedule compression 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