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

From: Wouter Verhelst
Subject: Re: [Qemu-devel] [Nbd] [PATCH 1/2] NBD proto: add WRITE_ZEROES extension
Date: Wed, 23 Mar 2016 18:40:15 +0100
User-agent: Mutt/1.5.24 (2015-08-30)

On Wed, Mar 23, 2016 at 09:14:10AM -0600, Eric Blake wrote:
> On 03/23/2016 08:16 AM, Denis V. Lunev wrote:
> [1] Oh, you ARE adding this to the "Experimental extensions" section of
> the document, so your wording IS correct.  I guess the idea is that we
> write up the documentation in the experimental section, tweak qemu to
> implement it both as NBD client and as NBD server (since qemu has code
> that can serve in both positions), see how well it worked, and THEN do a
> followup patch to proto.md to move the text into the non-experimental
> section, along with any tweaks learned during the qemu patches.

That's the way I accept specification changes now, yes: define an
experimental spec first, wait for a first implementation (whether that's
qemu or the reference implementation is not as relevant), and only then
move it to the normative part of the spec.

> > +one new command with two command flags.
> > +
> > +
> > +    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.
> Do we want to add:
> The server SHOULD return EINVAL if the client set NBD_CMD_FLAG_FUA but
> the export flags did not include NBD_FLAG_SEND_FUA.

Well, if the export flags didn't include NBD_FLAG_SEND_FUA, that means
the server may not implement that flag, and hence the server may not
know what to do with it.

It's probably a good idea to send a particular error (EINVAL sounds
good, and is what the current reference nbd-server implementation
sends[1]) for commands or flags the server implementation doesn't know
about, but the spec doesn't currently say that explicitly.

[1] actually, the current server sends EINVAL for commands the server
doesn't know about, but silently ignores flags the server doesn't know

> > +    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.
> Bug in the existing spec: The constant NBD_CMD_FLAG_FUA is mentioned,
> but never defined with a fixed value.  Your text above defines it to
> 'bit 0' in the command flags field - is that correct?

(checks code) Yes, it is.

> If so, should we add a section to the document that lists the bit
> values of all supported command flags?

Currently that's only the FUA flag, but yes, I'll do so.

> Meanwhile, your proposed text hardcodes NBD_CMD_FLAG_MAY_TRIM to 'bit
> 1'; that might also be worth adding into the same new section of the
> document documenting all supported command flags.
> Do we want to require that the server has negotiated the
> NBD_FLAG_SEND_TRIM export flag prior to allowing the
> NBD_CMD_FLAG_MAY_TRIM flag in this command?
> Possibly-related bug in the existing spec: Should the text of the
> NBD_CMD_TRIM (4) command mention the desired server behavior if the
> client sends NBD_CMD_TRIM even though NBD_FLAG_SEND_TRIM was not
> negotiated in the export flags?

Not sure that's a good idea. The export flag is there to say "I support
it". If the server doesn't support a feature, the client shouldn't use
that feature. If the client *does* use it, it ends up in undefined
behaviour-land (because the server may be from whatever source).

> Similarly, should the text of the NBD_CMD_FLUSH () command mention the
> desired server behavior if the client sends NBD_CMD_FLUSH even though
> NBD_FLAG_SEND_FLUSH was not negotiated in the export flags?  At least
> NBD_CMD_FLUSH recommended that the client must not send the command if
> the feature was not negotiated.

I suppose it would make sense to make the section on NBD_CMD_TRIM have a
similar notice, yes.

> > +    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.
> Should we add a paragraph stating that the client MUST NOT send
> the export options?  Similarly, should we suggest that the server reply
> with EINVAL if it knows about the command, but the client issues the
> command in spite of not negotiating it?  Should we enhance the
> documentation in the "Error values" heading to mention that EINVAL
> should be used in general for any client command not expected by the server?

Yes, probably. As above, the current nbd-server implementation does so,
and I'll make that explicit.

< 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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