Re: [Qemu-devel] [Nbd] [RFC 1/1] nbd (specification): add NBD_CMD_WRITE_ZEROES command

From: Wouter Verhelst <w@uter.be>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>,
	"nbd-general@lists.sourceforge.net"
	<nbd-general@lists.sourceforge.net>,
	"qemu-devel@nongnu.org" <qemu-devel@nongnu.org>,
	"Denis V. Lunev" <den@openvz.org>
Subject: Re: [Qemu-devel] [Nbd] [RFC 1/1] nbd (specification): add NBD_CMD_WRITE_ZEROES command
Date: Sun, 6 Mar 2016 11:28:30 +0100	[thread overview]
Message-ID: <20160306102830.GB13207@grep.be> (raw)
In-Reply-To: <56D995AE.1080805@redhat.com>

Hi all,

On Fri, Mar 04, 2016 at 03:03:26PM +0100, Paolo Bonzini wrote:
> NBD-wise, I think the TRIM command is good as it is, and
> NBD_CMD_WRITE_ZEROES should be added like Den is doing.
> 
> It also makes sense to use trimming to implement NBD_CMD_WRITE_ZEROES,
> but it should be explicitly requested by the user.  For this, my
> suggestion is that NBD_CMD_WRITE_ZEROES should have an
> NBD_FLAG_TRY_TRIM flag in bit 16.  If specified, the backend can use a
> zero-writing mechanism that trims, _but_ it must ensure that the bytes
> read as zero.  If it cannot ensure that, it must not trim and it
> should instead do a full write.  This is similar to the SCSI command
> WRITE SAME (when the command payload is all zeroes).  Like Kevin said,
> it also happens to map nicely to the QEMU block device layer.

That seems like a sensible approach, yes.

Would one of you who feels strongly about this be willing to write up a
proposed spec for that? I could do it, but since I'm not 100% sure I
understand all the specific requirements, I'm uncomfortable doing so.

If that doesn't raise any obvious issues that I'm aware of, I'd be happy
to add it to the "experimental" section of the proto.md file.

(speaking of which, I notice that the STARTTLS patches got merged into
qemu, so I've moved the description of that to the main body and out of
the "experimental" part of that document)

-- 
< 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