Re: Is it possible to transfer a large file between two computers using RDMA UD?

[Terry Toole <toole@photodiagnostic.com>]

On Thu, Jan 2, 2020 at 2:31 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
>
> On Thu, Jan 02, 2020 at 07:25:34PM +0000, Hefty, Sean wrote:
> > > > > Is it possible to transfer a large file, say 25GB, between two computers using
> > > > > RDMA UD, and have an exact copy of the original file on the receiving side? My
> > > > > understanding is that the order of the messages is not guaranteed with UD.
> > > > > But I thought that if I only use one QP I could ensure that the ordering of the
> > > > > data will be predictable.
> > > >
> > > > It is not guaranteed to be in order.
> > > >
> > > > Why would you do this anyhow? The overhead to use RC is pretty small
> > > >
> > > So even if I am using only 1 QP, it is still not guaranteed to be in
> > > order. Ok. Thanks.
> >
> > The spec doesn't guarantee ordering, though I doubt messages would
> > ever be unordered in practice.  You could transfer a sequence number
> > as immediate data to allow the receiver to verify (and correct)
> > packet ordering, and detect lost packets.
>
> They could always become lost though, due to a link BER or
> something. And that is only on IB, not Ethernet.
>
> I don't think you can avoid needing retransmit on the sender...
>

My plan was to try to send large files between two computers.
From many transfers, I was intending to get an estimate of
what types of losses we could reasonably expect. From my reading,
I got the impression that with a point to point connection, the expected
message loss with RDMA UD was quite low.

Terry