Re: NBD prefetch read
On Tue, Mar 20, 2018 at 08:22:31PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> 20.03.2018 19:58, Wouter Verhelst wrote:
> > On Tue, Mar 20, 2018 at 11:57:46AM +0300, Vladimir Sementsov-Ogievskiy wrote:
> > > 19.03.2018 17:39, Eric Blake wrote:
> > > > Can you demonstrate an actual sequence of commands sent over the wire,
> > > > for how it would be useful?
> > > - we initialize two drives A and B in qemu and setup copy-on-read for them.
> > > - client send a sequence of READAHEAD commands, and data is copied from A to
> > > B on read from B, in corresponding sequence.
> > >
> > > So, here B is a cache in terms of the PRE-FETCH command.
> > This sounds very similar to what xNBD does
> > (https://bitbucket.org/hirofuchi/xnbd/wiki/Home). Can you confirm?
> >
> > If so, I suppose it makes sense to add the current behaviour of xNBD to
> > the spec, rather than inventing our own thing.
> >
>
> Hm, what do you mean on this page? "Scenario 2 (Simple proxy server,
> distributed Copy-on-Write)" ?
Well, I don't necessarily mean the implementation details, so much as
the general concept :-)
You are talking about live migration of storage; that is what xNBD
implements and has in production. Does it not make sense to at least
look at what they're doing, so that we can possibly implement something
compatible?
> It looks similar, but there is no control channel with READAHEAD. The idea
> is no data is send through the control channel.
Eh, NBD has no control channel? I'm not sure what you're talking about
here.
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