Performing a measure-only reallocation scan of a LUN or volume

A measure-only reallocation scan is similar to a normal reallocation scan except that only the check phase is performed. It allows the optimization of the LUN, file, or volume to be tracked over time or measured ad-hoc.

Considerations

A measure-only reallocation scan checks the layout of a LUN, file, or volume. If the layout measurement becomes less optimal than the threshold (specified by the -t threshold option), or if a portion of the data is very poorly optimized, the log message advises you to consider performing a LUN, file, or volume reallocation (using the reallocate start command) to optimize the layout.

For scheduled measure-only reallocation scans, the optimization of the last completed check is saved and may be viewed at any time by using reallocate status.

Additional detailed information about the layout of the LUN, file, or volume is logged if you use the -l logfile option.

Step

Enter the following command: reallocate measure [-l logfile] [-t threshold] [-i interval] [-o] pathname | /vol/volname
- -l logfile is the file where information about the layout is recorded. If logfile is specified, information about the layout is recorded in the file.
- -t threshold is a number between 3 (layout is moderately optimal) and 10 (layout is not optimal). The default is 4. When the layout becomes less optimal than the threshold level, the layout of the LUN, file, or volume is considered unoptimized, and the log message advises you to consider performing a LUN, file, or volume reallocation.
  Note: Because Data ONTAP also includes “hot spots” in its calculation of whether to start a reallocation, the log message might advise you to consider performing a reallocation when the average optimization is better than the threshold but a small percentage of the total data is very poorly optimized.
- -i interval is the interval, in minutes, hours, or days, at which the scan is performed. A measure-only reallocation scan runs periodically at a system-defined interval, but depending on the system configuration and write/read workload, you can change the job interval with the -i option. You specify the interval as follows: [m | h | d]
  For example, 30m is a 30-minute interval.
  
  The countdown to the next scan begins only after the first scan is complete. For example, if the interval is 24 hours and a scan starts at midnight and lasts for an hour, the next scan begins at 1:00 a.m. the next day—24 hours after the first scan is completed.
- -o performs the scan only once, after which the scan is automatically removed from the system.
Example
The following example measures the optimization of the dblun LUN once and records detailed information about the measurement in the measure_log_dblun log. reallocate measure -o -l /vol/logs/measure_log_dblun/vol/dbvol/dblun

After a measure-only reallocation scan, the optimization information is logged via EMS in the system log files.

Parent topic: How you manage reallocation scans