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[RFC] Implement discard for AIO on certain file systems #167

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[RFC] Implement discard for AIO on certain file systems #167

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Commits on Oct 7, 2015

  1. Add support for discard to tapdisk 

    With this commit, tapdisk is able to understand and pass-through discard
request to tapdisk drivers which support it.

Each discard messages on the xen blkif is handled as follows:
1. xenio_blkif_get_request() gets discard requests from the ring. It decodes
   the request depending on the blkif protocol type and converts
   them into generic blkif_request_discard_t using blkif_get_req_discard.
2. tapdisk_xenblkif_make_vbd_request() iterates the message counter
   blkback_stats.st_ds_req for discards.
3. tapdisk_xenblkif_parse_request_discard() converts the discard request into
   a td_vbd_request with a start sector (sec) and a length
   (discard_nr_sectors).
3. The td_vbd_request is encapsulated into a td_request_t and is sanity
   checked in tapdisk_image_check_td_request, tapdisk_image_check_request and
   the new td_queue_discard method.
4. Ultimately the request is handled in td_queue_discard. If the tapdisk
   driver implements td_queue_discard, the request is passed through to that.
   If not, the request is failed with -EOPNOTSUPP.

This commit has been dev-tested using:
* v8 Windows PV drivers that include XenDisk and thereby implement discard
* Linux xen-blkfront that implements discard

Signed-off-by: Robert Breker <[email protected]>
    Robert Breker committed Oct 7, 2015
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  2. Add discard support to the block_aio driver 

    This commit enables discard for images accessed using block_aio:
- For discard support, the image must be stored on a filesystem that supports
  hole punching. To determine this, there is a whitelist in
  util.c:is_hole_punching_supported_for_fd(), that filters based on the
  kernel version and filesystem.
- Discard request are handled as synchronous hole punches into the image.

This commit has been dev-tested using:
* v8 Windows PV drivers that include XenDisk and thereby implement discard
* Linux xen-blkfront that implements discard

Signed-off-by: Robert Breker <[email protected]>
    Robert Breker committed Oct 7, 2015
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  3. Have tapback announce the discard capability for vbds 

    The discard capability is announced for vbds if the following two conditions
apply:
* The backing tapdisk indicates discard support for the image that is
  attached to the vbd (i.e. block-aio is used in combination with a capable
  filesystem)
* tapback is not run with the --nodiscard overwrite

If discard is enabled, the discard granularity (discard_granularity) is
statically announced as the sector size of the vbd. The alignment
(discard_alignment) is statically announced as 0.
Secure discard (discard_secure) is never guaranteed.

This commit has been dev-tested using:
* v8 Windows PV drivers that include XenDisk and thereby implement discard
* Linux xen-blkfront that implements discard

Signed-off-by: Robert Breker <[email protected]>
    Robert Breker committed Oct 7, 2015
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