Support discarding of unused sectors

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Rationale

For magnetic disks, DRAM and many other physical media used as block devices, it does not matter for the device to know which sectors are actually unused by the filesystem and therefore contain irrelevant content.

This is quite different for Solid State Disks (SSDs) built from Flash Memory: To avoid performance degradation from the dynamic wear-leveling algorithms, there has to be a clever management of "unused" blocks in the SSD controller.

To let the SSD controller know which sectors contain irrelevant data, the ATA TRIM command has been standardized, and now the first SSDs are available that implement that command (e.g. the SSDs using the Indilinx Barefoot controller, from firmware revision 1370 onwards).

What needs to be done

This work has been implemented in Linux since v2.6.38, Jan 18 2011.

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