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

[Pavel Borzenkov]

On Wed, Mar 23, 2016 at 06:21:16PM +0100, Wouter Verhelst wrote:
> 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?

No, there is no specific reason. Looks like NBD_CMD_FLAG_ZEROES fits the
spec and implementations nicely. So I'll rewrite the extension and add
the flag instead of the whole command.

> 
> -- 
> < 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
> 
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