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Re: Allowing > 32 bit lengths for NBD_CMD_TRIM, NBD_CMD_WRITE_ZEROES

On Thu, Sep 06, 2018 at 12:55:49PM +0200, Wouter Verhelst wrote:
> > (4) Allow a different request message format, differentiated by using
> > a different magic (! NBD_REQUEST_MAGIC).  We could use the opportunity
> > to widen a few fields and reserve space for future expansion.
> > Similar to (1).
> I was originally opposed to this, but have since come to realize that
> that (being opposed) may have been a mistake. We probably should have
> used the opportunity to change the request format when we changed the
> response format, too.
> It's not too late to introduce a request format change, though. In that
> case, I would suggest:
> - magic (32 bit value): different magic number, indeed.
> - request length (32 bit value): length of header and data (I don't
>   think it makes sense to have requests that are >4GiB, but could be
>   persuaded otherwise).

I think the way to think about this is in terms of time rather than
request size.  How long does it take to transmit a 4GB or larger
request, given current & expected future improvements in network

I would say if it would always be longer than [number picked at
random] 20 ms then limiting it to 4GB is fine, because otherwise the
TCP connection is blocked for too long while one request is being
transmitted causing interactivity problems.

This actually ties in nicely with another conversation I was having
recently.  I was promoting NBD to an iSCSI user and they said
something to the effect of “but but but with iSER [iSCSI Extensions
for RDMA] we get awesome performance and NBD can't match that”.  Which
is quite likely to be true.  It would be nice to provide a
standardized RDMA transport for NBD.

If we assume that NBD over RDMA was a thing, and you have the latest
and most expensive 12x XDR Infiniband hardware, and we were able to
saturate the link, then 4 GB could be transmitted in 13 ms!  That's
less than my arbitrarily picked limit above :-)

(Agree with the rest of your email and the limits proposed.)


Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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