Support discarding of unused sectors

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-levelling 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

To support SSDs and other devices that benefit from knowing unused sectors, the blkdev_issue_discard() function has recently been integrated into the Linux kernel (2.6.30rcX).

The filesystem needs to call that function when it knows sectors become unused.

Linux filesystems known to already support blkdev_issue_discard() (when available) are: ext4, GFS2, swap.

Related Standards

Data Set Management Commands Proposal for ATA8-ACS2

Deterministic TRIM Proposal for ATA8-ACS2

Other Documents

• An interesting Article at Anandtech that (while being a little verbose and Windows-oriented) explains many aspects of SSD performance, including the prospective benefits of the ATA TRIM command (at page 10).

• Patch adding the blkdev_issue_discard() feature to GFS2

• Presentation on a comparison between different filesystems performance on SSDs (including XFS)