On 4 Apr 2016, at 15:15, Eric Blake <ebl...@redhat.com> wrote: > qemu already has an existing server implementation option that will > explicitly search the payload of NBD_CMD_WRITE for large blocks of > zeroes, and punch holes in the underlying file. For old clients > that don't know how to use the new NBD_CMD_WRITE_ZEROES, this is a > workaround to keep the server's destination file approximately as > sparse as the client's source. However, for new clients that know > how to explicitly request holes, it is unnecessary overhead; and > can lead to the server punching a hole and risking fragmentation or > future ENOSPC even when the client explicitly wanted to write > zeroes rather than a hole. So it makes sense to let the new > NBD_CMD_FLAG_NO_HOLE work for WRITE as well as WRITE_ZEROES. > > Signed-off-by: Eric Blake <ebl...@redhat.com> Reviewed-by: Alex Bligh <a...@alex.org.uk>
+1 Alex > --- > doc/proto.md | 33 +++++++++++++++++++++++++++------ > 1 file changed, 27 insertions(+), 6 deletions(-) > > diff --git a/doc/proto.md b/doc/proto.md > index 35a3266..fb97217 100644 > --- a/doc/proto.md > +++ b/doc/proto.md > @@ -737,8 +737,14 @@ by a sparse file. With current NBD command set, the > client has to issue > through the wire. The server has to write the data onto disk, effectively > losing the sparseness. > > -To remedy this, a `WRITE_ZEROES` extension is envisioned. This extension adds > -one new command and one new command flag. > +To remedy this, a `WRITE_ZEROES` extension is envisioned. This > +extension adds one new transmission flag, one new command, and one new > +command flag; and refines an existing command. > + > +* `NBD_FLAG_SEND_WRITE_ZEROES` > + > + The server SHOULD set this transmission flag to 1 if the > + `NBD_CMD_WRITE_ZEROES` request is supported. > > * `NBD_CMD_WRITE_ZEROES` > > @@ -772,12 +778,27 @@ The server SHOULD return `ENOSPC` if it receives a > write zeroes request > including one or more sectors beyond the size of the device. It SHOULD > return `EPERM` if it receives a write zeroes request on a read-only export. > > +* `NBD_CMD_WRITE` > + > + By default, the server MAY search for large contiguous blocks of > + all zero content, and use trimming to zero out those portions of > + the write, even if it did not advertise `NBD_FLAG_SEND_TRIM`; but > + it MUST ensure that any trimmed areas of data read back as zero. > + However, the client MAY set the command flag > + `NBD_CMD_FLAG_NO_HOLE` to inform the server that the entire > + written area MUST be fully provisioned, ensuring that future > + writes to the same area will not cause fragmentation or cause > + failure due to insufficient space. Clients SHOULD NOT set this > + flag unless the server advertised `NBD_FLAG_SEND_WRITE_ZEROES` in > + the transmisison flags. > + > The extension adds the following new command flag: > > -- `NBD_CMD_FLAG_NO_HOLE`; valid during `NBD_CMD_WRITE_ZEROES`. > - SHOULD be set to 1 if the client wants to ensure that the server does > - not create a hole. The client MAY send `NBD_CMD_FLAG_NO_HOLE` even > - if `NBD_FLAG_SEND_TRIM` was not set in the transmission flags field. > +- `NBD_CMD_FLAG_NO_HOLE`; valid during `NBD_CMD_WRITE` and > + `NBD_CMD_WRITE_ZEROES`. SHOULD be set to 1 if the client wants to > + ensure that the server does not create a hole. The client MAY send > + `NBD_CMD_FLAG_NO_HOLE` even if `NBD_FLAG_SEND_TRIM` was not set in > + the transmission flags field. > > ### `STRUCTURED_REPLY` extension > > -- > 2.5.5 > -- Alex Bligh
