[Qemu-devel] Re: [PATCH 0/3] add TRIM/UNMAP support, v4

[Christoph Hellwig <hch@lst.de>]

This patchset adds support for the SCSI WRITE SAME with unmap command,
which allows reclaiming free space from a backing image.

The user facing implementation is pretty complete, but not really
efficient because the underlying bdrv_discard implementation doesn't
use the aio implementation yet.  The reason for that is that the SCSI
layer doesn't really allow any asynchronous commands except for
READ/WRITE by design.  The only support backend so far is the XFS hole
punching ioctl, but others can be added easily when they become
available.  A virtio implementation for a discard command would also
be pretty easy, but until we actually support a better backend then
a plain sparse file it's not worth using for production enviroments
anyway, but more for playing with the thin provisioning infrastructure,
or observing guest behaviour when TRIM / unmap is supported.

If the support is enabled and the backend doesn't support hole punching
the WRITE SAME command becomes a no-op so that migration from
hosts supporting or not supporting it works.

Version 4:
	- incorporate Kevin's review comments for the scsi patch
	- update to the current block queue
	- drop ide TRIM support, as it doesn't easily port to the
	  refactoring of the IDE code

Version 3:
 	- refactor IDE dma support code
 	- proper brace obsfucation
 	- fix compile without xfs headers
 	- use bool instead of int for a one-byte flag
 
Version 2:
 	- replace tabs with spaces
 	- return -ENOMEDIUM from bdrv_discard if there's no driver
 	  assigned
 	- actually list the TP EVPD page as supported when querying
 	  for supported EVPD pages