storage: Add TRIM algorithm to storage volume API

This new algorithm adds support for wiping volumes using TRIM.

It does not overwrite all the data in a volume, but it tells the
backing storage pool/driver that all bytes in a volume can be
discarded.

It depends on the backing storage pool how this is handled.

A SCSI backend might send UNMAP commands to remove all data present
on a LUN.

A Ceph backend might use rbd_discard() to instruct the Ceph cluster
that all data on that RBD volume can be discarded.

Signed-off-by: Wido den Hollander <wido@widodh.nl>

tools/virsh-volume.c

@@ -906,7 +906,7 @@ static const vshCmdOptDef opts_vol_wipe[] = {
 VIR_ENUM_DECL(virStorageVolWipeAlgorithm)
 VIR_ENUM_IMPL(virStorageVolWipeAlgorithm, VIR_STORAGE_VOL_WIPE_ALG_LAST,
               "zero", "nnsa", "dod", "bsi", "gutmann", "schneier",
-              "pfitzner7", "pfitzner33", "random");
+              "pfitzner7", "pfitzner33", "random", "trim");
 
 static bool
 cmdVolWipe(vshControl *ctl, const vshCmd *cmd)

tools/virsh.pod

@@ -3546,9 +3546,19 @@ B<Supported algorithms>
   pfitzner7  - Roy Pfitzner's 7-random-pass method: random x7.
   pfitzner33 - Roy Pfitzner's 33-random-pass method: random x33.
   random     - 1-pass pattern: random.
+  trim       - 1-pass trimming the volume using TRIM or DISCARD
 
-B<Note>: The availability of algorithms may be limited by the version
-of the C<scrub> binary installed on the host.
+B<Note>: The C<scrub> binary will be used to handle the 'nnsa', 'dod',
+'bsi', 'gutmann', 'schneier', 'pfitzner7' and 'pfitzner33' algorithms.
+The availability of the algorithms may be limited by the version of
+the C<scrub> binary installed on the host. The 'zero' algorithm will
+write zeroes to the entire volume. For some volumes, such as sparse
+or rbd volumes, this may result in completely filling the volume with
+zeroes making it appear to be completely full. As an alternative, the
+'trim' algorithm does not overwrite all the data in a volume, rather
+it expects the storage driver to be able to discard all bytes in a
+volume. It is up to the storage driver to handle how the discarding
+occurs. Not all storage drivers or volume types can support 'trim'.
 
 =item B<vol-dumpxml> [I<--pool> I<pool-or-uuid>] I<vol-name-or-key-or-path>