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

From: Wouter Verhelst <w@uter.be>
To: Pavel Borzenkov <pborzenkov@virtuozzo.com>
Cc: Kevin Wolf <kwolf@redhat.com>,
	nbd-general@lists.sourceforge.net, qemu-devel@nongnu.org,
	Stefan Hajnoczi <stefanha@redhat.com>,
	"Denis V. Lunev" <den@openvz.org>,
	Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [Qemu-devel] [Nbd] [PATCH 2/2] NBD proto: add GET_LBA_STATUS extension
Date: Thu, 24 Mar 2016 09:41:29 +0100	[thread overview]
Message-ID: <20160324084129.GG1590@grep.be> (raw)
In-Reply-To: <20160324082552.GB24831@phobos.sw.ru>

On Thu, Mar 24, 2016 at 11:25:52AM +0300, Pavel Borzenkov wrote:
> On Wed, Mar 23, 2016 at 07:14:54PM +0100, Kevin Wolf wrote:
> > Am 23.03.2016 um 18:58 hat Wouter Verhelst geschrieben:
> > > On Wed, Mar 23, 2016 at 05:16:02PM +0300, Denis V. Lunev wrote:
> > > > +    the provisioning state of the device. The following provisionnig states
> > > > +    are defined for the command:
> > > > +
> > > > +      - `NBD_STATE_ALLOCATED` (0x0), LBA extent is present on the block device;
> > > > +      - `NBD_STATE_ZEROED` (0x1), LBA extent is present on the block device
> > > > +        and contains zeroes;
> > > 
> > > Presumably this should be "contains only zeroes"?
> > > 
> > > Also, this may end up being a fairly expensive call for the server to
> > > process. Is it really useful?
> > 
> > I think we need to make clear that this is meant as an optimisation and
> > it's always a valid option for a server to return NBD_STATE_ALLOCATED
> > even if the contents is zeroed.
> > 
> > It is definitely useful if the server has a means to efficiently find
> > out the allocation status (e.g. SEEK_HOLE). In that case the client may
> > be able to avoid reading the block and sending it over the network, or
> > when making a copy, it could use it to keep the target file sparse. If
> > the client can't take advantage, we didn't have much overhead, so it's
> > fine.
> 
> Yes, that was the idea. I'll add a note that the server may return
> NBD_STATE_ALLOCATED instead of NBD_STATE_ZEROED if it has not means to
> efficiently differentiate allocated blocks with zeroes from allocated
> blocks with non-zeroed content.

Okay, that alleviates my concerns.

In that case it might be useful if the server could say something along
the lines of "I know it's allocated, but I didn't check whether there's
anything non-zero in there"? The client can then decide to do nothing
with that information; but the more useful information is sent along,
the better...

-- 
< 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