Re: [Nbd] [PATCH v3] doc: Add NBD_CMD_BLOCK_STATUS extension
On Tue, Nov 29, 2016 at 06:07:56PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> 29.11.2016 17:52, Alex Bligh wrote:
> > Vladimir,
> >> if the bitmap is 010101010101 we will have too many descriptors.
> >> For example, 16tb disk, 64k granularity -> 2G of descriptors
> >> payload.
> > Yep. And the cost of consuming and retrying is quite high. One
> > option would be for the client to realise this is a possibility, and
> > not request the entire extent map for a 16TB disk, as it might be
> > very large! Even if the client worked at e.g. a 64MB level (where
> > they'd get a maximum of 1024 extents per reply), this isn't going to
> > noticeably increase the round trip timing. One issue here is that to
> > determine a 'reasonable' size, the client needs to know the minimum
> > length of any extent.
>
> and with this approach we will in turn have overhead of too many
> requests for 00000000 or 11111111 bitmaps.
This is why my proposal suggests the server may abort the sent extents
after X extents (sixteen, but that number can certainly change) have
been sent. After all, the server will have a better view on what's going
to be costly in terms of "number of extents".
> > I think the answer is probably a 'maximum number of extents' in the request packet.
> >
> > Of course with statuses in extent, the final extent could be
> > represented as 'I don't know, break this bit into a separate
> > request' status.
> >
>
> With such predefined status, we can postpone creating extended requests,
> have number of extents restricted by server and have sum of extents
> lengths be equal to request length.
--
< 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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