Storage Foundation 7.4.1 Administrator's Guide - Linux

Average I/O activity criteria

The Average criteria allows you to specify the value of the I/O temperature as a ratio of per-file activity that occurs over the time specified by the <PERIOD> element compared to the overall file system activity that occurs over a longer period of time. The <PERIOD> element in the RELOCATE criteria specifies the a number of hours or days immediately before the time of the scan. During that time, the I/O statistics that are collected are used to process the files that are being scanned. Since I/O activity can change over time, collect the average I/O activity over a longer duration than the <PERIOD> value itself, which is by default 24 hours. Doing so lets you compute an average temperature of the whole file system. Veritas recommends that you specify an Average attribute value only if you are using solid state disks (SSDs).

See Average I/O activity with solid state disks.

The following placement policy snippet gives an example of the Average criteria:

   <RELOCATE>
      ...
      <WHEN>
         <IOTEMP Type="nrbytes" Prefer="high" Average="*">
            <MIN Flags="gteq"> 1.5 </MIN>
            <PERIOD Units="hours"> 6 </PERIOD>
         </IOTEMP>
      </WHEN>
   </RELOCATE>

In the snippet, VxFS relocates any file whose read IOTEMP over the last 6 hours is 1.5 times that of all the active files in the whole file system over the last 24 hours. This Average criteria is more intuitive and easier to specify than the absolute values.

The following formula computes the read IOTEMP of a given file:

IOTEMP = (bytes of the file that are read in the PERIOD) /
         (PERIOD in hours * size of the file in bytes)

The write and read/write IOTEMP are also computed accordingly.

The following formula computes the average read IOTEMP:

Average IOTEMP = (bytes read of all active files in the last h hours) /
                 (h * size of all the active files in bytes)

h is 24 hours by default. The average write and read/write IOTEMP are also computed accordingly.

In the example snippet, the value 1.5 is the multiple of average read IOTEMP over the last 24 hours across the whole file system, or rather across all of the active inodes whose activity is still available in the File Change Log (FCL) file at the time of the scan. Thus, the files' read IOTEMP activity over the last 6 hours is compared against 1.5 times that of the last 24 hours average activity to make the relocation decision. Using this method eliminates the need to give a specific number for the <IOTEMP> or <ACCESSTEMP> criteria, and instead lets you specify a multiple of the Average temperature. Keeping this averaging period longer than the specified <PERIOD> value normalizes the effects of any spikes and lulls in the file activity.

You can also use the Average criteria with the <ACCESSTEMP> criteria. The purpose and usage are the same.

You determine the type of the average by whether you specify the Average criteria with the <IOTEMP> or with the <ACCESSTEMP> criteria. The Average criteria can be any of the following types, depending on the criteria used:

The default Average is a 24 hour average temperature, which is the total of all of the temperatures available up to the last 24 hours in the FCL file, divided by the number of files for which such I/O statistics still exist in the FCL file. You can override the number of hours by specifying the AveragePeriod attribute in the <PLACEMENT_POLICY> element. Veritas recommends that you specify an AveragePeriod attribute value only if you are using solid state disks (SSDs).

The following example statement causes the average file system activity be collected and computed over a period of 30 hours instead of the default 24 hours:

<PLACEMENT_POLICY Name="Policy1" Version="5.1" AveragePeriod="30">