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> --- 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
