[Nbd] [RFC 1/1] nbd (specification): add NBD_CMD_WRITE_ZEROES command

["Denis V. Lunev" <den@...2317...>]

This patch proposes a new command to reduce the amount of data passed
through the wire when it is known that the data is all zeroes. This
functionality is generally useful for mirroring or backup operations.

Currently available NBD_CMD_TRIM command can not be used as the
specification explicitely says that "a client MUST NOT make any
assumptions about the contents of the export affected by this
[NBD_CMD_TRIM] command, until overwriting it again with `NBD_CMD_WRITE`"

Particular use case could be the following:

QEMU project uses own implementation of NBD server to transfer data
in between different instances of QEMU. Typically we tranfer VM virtual
disks over this channel. VM virtual disks are sparse and thus the
efficiency of backup and mirroring operations could be improved a lot.

Signed-off-by: Denis V. Lunev <den@...2317...>
---
 doc/proto.md | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/doc/proto.md b/doc/proto.md
index 43065b7..c94751a 100644
--- a/doc/proto.md
+++ b/doc/proto.md
@@ -241,6 +241,8 @@ immediately after the global flags field in oldstyle negotiation:
   schedule I/O accesses as for a rotational medium
 - bit 5, `NBD_FLAG_SEND_TRIM`; should be set to 1 if the server supports
   `NBD_CMD_TRIM` commands
+- bit 6, `NBD_FLAG_SEND_WRITE_ZEROES`; should be set to 1 if the server
+  supports `NBD_CMD_WRITE_ZEROES` commands
 
 ##### Client flags
 
@@ -446,6 +448,11 @@ The following request types exist:
     about the contents of the export affected by this command, until
     overwriting it again with `NBD_CMD_WRITE`.
 
+* `NBD_CMD_WRITE_ZEROES` (6)
+
+    A request to write zeroes. The command is functional equivalent of
+    the NBD_WRITE_COMMAND but without payload sent through the channel.
+
 * Other requests
 
     Some third-party implementations may require additional protocol
-- 
2.1.4