Implement support for flush, fua and rotational.

[nbd.git] / doc / proto.txt

diff --git a/doc/proto.txt b/doc/proto.txt
index fe5e819..d173365 100644 (file)
--- a/doc/proto.txt
+++ b/doc/proto.txt
@@ -26,8 +26,8 @@ server during the handshake.
 There are two message types in the data pushing phase: the request, and
 the response.
 
 There are two message types in the data pushing phase: the request, and
 the response.
 

-There are three request types in the data pushing phase: NBD_CMD_READ,
-NBD_CMD_WRITE, and NBD_CMD_DISC (disconnect).
+There are four request types in the data pushing phase: NBD_CMD_READ,
+NBD_CMD_WRITE, NBD_CMD_DISC (disconnect), and NBD_CMD_FLUSH.

 
 The request is sent by the client; the response by the server. A request
 header consists a 32 bit magic number (magic), a 32 bit field denoting
 
 The request is sent by the client; the response by the server. A request
 header consists a 32 bit magic number (magic), a 32 bit field denoting

@@ -50,6 +50,16 @@ we change that to asynchronous handling, handling the disconnect request
 will probably be postponed until there are no other outstanding
 requests.
 
 will probably be postponed until there are no other outstanding
 requests.
 

+A flush request will not be sent unless NBD_FLAG_SEND_FLUSH is set,
+and indicates the backing file should be fdatasync()'d to disk.
+
+The top 16 bits of the request are flags. NBD_CMD_FLAG_FUA implies
+a force unit access, and can currently only be usefully combined
+with NBD_CMD_WRITE. This is implementing using sync_file_range
+if present, else by fdatasync() of that file (note not all files
+in a multifile environment). NBD_CMD_FLAG_FUA will not be set
+unless NBD_FLAG_SEND_FUA is set.
+

 There are two versions of the negotiation: the 'old' style (nbd <=
 2.9.16) and the 'new' style (nbd >= 2.9.17, though due to a bug it does
 not work with anything below 2.9.18). What follows is a description of
 There are two versions of the negotiation: the 'old' style (nbd <=
 2.9.16) and the 'new' style (nbd >= 2.9.17, though due to a bug it does
 not work with anything below 2.9.18). What follows is a description of