[Sheepdog] Reclaim free space using ATA discard

[Wido den Hollander]

Hi,

Since kernel version 2.6.33 Linux supports the "discard" commando,
mainly used by SSD's for their wear-leveling and performance.

It's also called TRIM: http://en.wikipedia.org/wiki/TRIM

At the moment only Btrfs and EXT4 support trim/discard, but this could
be a nice feature for sheepdog.

If sheepdog reads the "discard" messages from the kernel in the VM you
could figure out which blocks are free again and shrink the used space
automatically.

Some more information about TRIM/discard:
* http://lwn.net/Articles/293658/
* http://en.wikipedia.org/wiki/Btrfs#Features
* http://www.archivum.info/qemu-devel@nongnu.org/2009-03/01144/%
28Qemu-devel%29-Re:-%28patch-2-7%29-qemu:-separate-thread-for-io.html


Might be a suggestion for Sheepdog?

-- 
Met vriendelijke groet,

Wido den Hollander
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