Re: [Nbd] [PATCH 2/2] NBD proto: add GET_LBA_STATUS extension
- To: Kevin Wolf <kwolf@...696...>
- Cc: nbd-general@lists.sourceforge.net, "Denis V. Lunev" <den@...2317...>, qemu-devel@...530..., Stefan Hajnoczi <stefanha@...696...>, Paolo Bonzini <pbonzini@...696...>, Wouter Verhelst <w@...112...>
- Subject: Re: [Nbd] [PATCH 2/2] NBD proto: add GET_LBA_STATUS extension
- From: Pavel Borzenkov <pborzenkov@...2319...>
- Date: Thu, 24 Mar 2016 11:25:52 +0300
- Message-id: <20160324082552.GB24831@...2346...>
- In-reply-to: <20160323181454.GI4126@...2331...>
- References: <1458742562-30624-1-git-send-email-den@...2317...> <1458742562-30624-3-git-send-email-den@...2317...> <20160323175834.GC2467@...3...> <20160323181454.GI4126@...2331...>
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 type of information required by the client is passed to server in the
> > > + command flags field. If the server does not implement requested type or
> > > + have no means to express it, it MUST NOT return an error, but instead MUST
> > > + return a single LBA status descriptor with *offset* and *length* equal to
> > > + the *offset* and *length* from request, and *status* set to `0`.
> > > +
> > > + The following request types are currently defined for the command:
> > > +
> > > + 1. Block provisioning state
> > > +
> > > + Upon receiving an `NBD_CMD_GET_LBA_STATUS` command with command flags
> > > + field set to `NBD_FLAG_GET_ALLOCATED` (0x0), the server MUST return
> >
> > I prefer to have a non-zero flag value.
> >
> > > + 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.
>
> It's less clear in a case where the server needs to read in the block
> and scan its contents. It could still be a net win if the next thing the
> client does is retrieving the block: The server would still have the
> cost of reading the block, but it wouldn't be transferred. But when the
> client doesn't follow up with a read, it's quite a bit of overhead that
> we had for no benefit. Returning NBD_STATE_ALLOCATED might be more
> appropriate in this case than scanning for zeros.
>
> Kevin
> ------------------------------------------------------------------------------
> Transform Data into Opportunity.
> Accelerate data analysis in your applications with
> Intel Data Analytics Acceleration Library.
> Click to learn more.
> http://pubads.g.doubleclick.net/gampad/clk?id=278785351&iu=/4140
> _______________________________________________
> Nbd-general mailing list
> Nbd-general@lists.sourceforge.net
> https://lists.sourceforge.net/lists/listinfo/nbd-general
Reply to: