Re: [Qemu-devel] [Nbd] [PATCH 1/2] NBD proto: add WRITE_ZEROES extension

[Wouter Verhelst]

Hi,

On Wed, Mar 23, 2016 at 05:16:01PM +0300, Denis V. Lunev wrote:
> From: Pavel Borzenkov <address@hidden>
> 
> There exist some cases when a client knows that the data it is going to
> write is all zeroes. Such cases include mirroring or backing up a device
> implemented by a sparse file.
> 
> With current NBD command set, the client has to issue NBD_CMD_WRITE
> command with zeroed payload and transfer these zero bytes through the
> wire. The server has to write the data onto disk, effectively denying
> the sparseness.
> 
> To remedy this, the patch adds WRITE_ZEROES extension with one new
> NBD_CMD_WRITE_ZEROES command.
> 
> Signed-off-by: Pavel Borzenkov <address@hidden>
> Reviewed-by: Roman Kagan <address@hidden>
> Signed-off-by: Denis V. Lunev <address@hidden>
> CC: Wouter Verhelst <address@hidden>
> CC: Paolo Bonzini <address@hidden>
> CC: Kevin Wolf <address@hidden>
> CC: Stefan Hajnoczi <address@hidden>
> ---
>  doc/proto.md | 44 ++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 44 insertions(+)
> 
> diff --git a/doc/proto.md b/doc/proto.md
> index 463ef8a..cda213c 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
>  
> @@ -471,6 +473,10 @@ 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)
> +
> +    Defined by the experimental `WRITE_ZEROES` extension; see below.
> +
>  * Other requests
>  
>      Some third-party implementations may require additional protocol
> @@ -594,6 +600,44 @@ option reply type.
>        message if they do not also send it as a reply to the
>        `NBD_OPT_SELECT` message.
>  
> +### `WRITE_ZEROES` extension
> +
> +There exist some cases when a client knows that the data it is going to write
> +is all zeroes. Such cases include mirroring or backing up a device 
> implemented
> +by a sparse file. With current NBD command set, the client has to issue
> +`NBD_CMD_WRITE` command with zeroed payload and transfer these zero bytes
> +through the wire. The server has to write the data onto disk, effectively
> +denying the sparseness.
> +
> +To remedy this, a `WRITE_ZEROES` extension is envisioned. This extension adds
> +one new command with two command flags.
> +
> +* `NBD_CMD_WRITE_ZEROES` (6)
> +
> +    A write request with no payload. Length and offset define the location
> +    and amount of data to be zeroed.
> +
> +    The server MUST zero out the data on disk, and then send the reply
> +    message. The server MAY send the reply message before the data has
> +    reached permanent storage.
> +
> +    If the `NBD_FLAG_SEND_FUA` flag ("Force Unit Access") was set in the
> +    export flags field, the client MAY set the flag `NBD_CMD_FLAG_FUA` (bit 
> 0)
> +    in the command flags field. If this flag was set, the server MUST NOT 
> send
> +    the reply until it has ensured that the newly-zeroed data has reached
> +    permanent storage.
> +
> +    If the flag `NBD_CMD_FLAG_MAY_TRIM` (bit 1) was set by the client in the
> +    command flags field, the server MAY use trimming to zero out the area,
> +    but it MUST ensure that the data reads back as zero.
> +
> +    If an error occurs, the server SHOULD set the appropriate error code
> +    in the error field. The server MAY then close the connection.
> +
> +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.
> +

So, the semantics of your proposed WRITE_ZEROES are exactly the same as
the WRITE command, except that no payload is sent?

In that case, I think it's slightly more sensible if we don't add a new
command, but instead just have an NBD_CMD_FLAG_ZEROES added to the WRITE
command instead. After all, they're going to be (mostly) the same
anyway.

Did you propose a separate command for a specific reason that I'm
missing (or forgetting), or is that just an oversight?

-- 
< ron> I mean, the main *practical* problem with C++, is there's like a dozen
       people in the world who think they really understand all of its rules,
       and pretty much all of them are just lying to themselves too.
 -- #debian-devel, OFTC, 2016-02-12