mirror: allow customizing the granularity

The desired granularity may be very different depending on the kind of
operation (e.g. continuous replication vs. collapse-to-raw) and whether
the VM is expected to perform lots of I/O while mirroring is in progress.

Allow the user to customize it, while providing a sane default so that
in general there will be no extra allocated space in the target compared
to the source.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

qmp-commands.hx

@@ -938,7 +938,8 @@ EQMP
     {
         .name       = "drive-mirror",
         .args_type  = "sync:s,device:B,target:s,speed:i?,mode:s?,format:s?,"
-                      "on-source-error:s?,on-target-error:s?",
+                      "on-source-error:s?,on-target-error:s?,"
+                      "granularity:i?",
         .mhandler.cmd_new = qmp_marshal_input_drive_mirror,
     },
 
@@ -962,6 +963,7 @@ Arguments:
   file/device (NewImageMode, optional, default 'absolute-paths')
 - "speed": maximum speed of the streaming job, in bytes per second
   (json-int)
+- "granularity": granularity of the dirty bitmap, in bytes (json-int, optional)
 - "sync": what parts of the disk image should be copied to the destination;
   possibilities include "full" for all the disk, "top" for only the sectors
   allocated in the topmost image, or "none" to only replicate new I/O
@@ -971,6 +973,10 @@ Arguments:
 - "on-target-error": the action to take on an error on the target
   (BlockdevOnError, default 'report')
 
+The default value of the granularity is the image cluster size clamped
+between 4096 and 65536, if the image format defines one.  If the format
+does not define a cluster size, the default value of the granularity
+is 65536.
 
 
 Example: