[Qemu-block] [PATCH] doc: Propose NBD_FLAG_INIT_ZEROES extension

[Eric Blake]

While not directly related to NBD_CMD_WRITE_ZEROES, the qemu
team discovered that it is useful if a server can advertise
whether an export is in a known-all-zeroes state at the time
the client connects.

Signed-off-by: Eric Blake <address@hidden>
---
 doc/proto.md | 5 +++++
 1 file changed, 5 insertions(+)

This replaces the following qemu patch attempt:
https://lists.gnu.org/archive/html/qemu-devel/2016-12/msg00357.html
which tried to add NBD_CMD_HAS_ZERO_INIT with poor semantics. The
semantics in this proposal should be much better.

Patch is to the merge of the master branch and the
extension-write-zeroes branch.  By the way, qemu 2.8 is due
to be released "real soon now", and implements NBD_CMD_WRITE_ZEROES,
so maybe it is time to consider promoting the extension-write-zeroes
branch into master.

diff --git a/doc/proto.md b/doc/proto.md
index afe71fc..7e4ec7f 100644
--- a/doc/proto.md
+++ b/doc/proto.md
@@ -697,6 +697,11 @@ The field has the following format:
   the export.
 - bit 9, `NBD_FLAG_SEND_BLOCK_STATUS`: defined by the experimental
   `BLOCK_STATUS` 
[extension](https://github.com/NetworkBlockDevice/nbd/blob/extension-blockstatus/doc/proto.md).
+- bit 10, `NBD_FLAG_INIT_ZEROES`: Indicates that the server guarantees
+  that at the time transmission phase begins, all offsets within the
+  export read as zero bytes.  Clients MAY use this information to
+  avoid writing to sections of the export that should still read as
+  zero after the client is done writing.

 Clients SHOULD ignore unknown flags.

-- 
2.9.3