Prefer mechanism

Article:HOWTO59543  |  Created: 2011-10-12  |  Updated: 2011-10-12  |  Article URL
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Prefer mechanism

You can now specify a value for the Prefer attribute for the <IOTEMP> and <ACCESSTEMP> criteria, which gives preference to relocating files.

See Prefer attribute.

In case of a solid state disk (SSD)-based tier, you might want to relocate a file to an SSD as soon as there is a marked increase in the I/O activity. However, once Veritas File System (VxFS) has relocated the file to an SSD, it may be beneficial to keep the file on the SSD as long as the activity remains high to avoid frequent thrashing. You want to watch the activity for some time longer than the time that you watched the activity when you relocated the file to the SSD before you decide to move the file off of the SSD.

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

         <IOTEMP Type="nrbytes" Prefer="high">
            <MIN Flags="gteq"> 3.4 </MIN>
            <PERIOD Units="hours"> 6 </PERIOD>

If there are a number of files whose I/O temperature is greater than the given minimum value, the files with the higher temperature are first subject to the RELOCATE operation before the files with the lower temperature. This is particularly useful in case of SSDs, which are limited in size and are expensive. As such, you generally want to use SSDs for the most active files.

See Using SmartTier with solid state disks

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