RE: [Xen-users] Shared Storage

["James Harper" <james.harper@xxxxxxxxxxxxxxxx>]

> 
> > TCP offload helps a lot in the case of iSCSI. You get the ability to
> > send ~60KB of data at once, and your packets are checksummed for
free. I
> > don't actually know what Linux support for RSS is like but if it is
any
> > good you also get automatic distribution of rx workload across
multiple
> > CPU's too, but that only works if you have multiple TCP connections
in
> > flight at once (eg the multiple LUN scenario).
> >
> > I don't know what iSCSI offload involves... maybe that can help
further.
> 
> Offload is great, but it's still TCP.  The protocol itself is heavy on
> overhead so your 1gbit pipe ends up being, you know, .8gbit storage
and
> .2gbit protocol (I don't know what the exact numbers are).  Offload
> keeps this away from your CPUs but doesn't do anything on the pipe
> itself.  iSCSI's great when you can't afford FC or FCoE (which still
> isn't as good as FC) or when you need to do things over a WAN, but it
> has its cost.
> 

For your numbers to make any sense at all, that would be 1800 bytes of
header for a 9000 byte packet. That doesn't seem even remotely right.
Even 180 bytes of header (0.02) seems a bit excessive in terms of raw
numbers, while still being pretty efficient in terms of ratio (0.98)

James


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