RE: [Xen-users] Shared Storage

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

> 
> > I would honestly prefer to manage hundred of LVs instead of hundreds
of
> > LUNs. I'm just concerned about the iSCSI bottleneck (if any) if I
were
> > to create an LVM VG using a single iSCSI LUN for about 50 - 100 LVs.
Any
> > advice is appreciated.
> 
> I'd be more concerned over iSCSI itself being able to scale based on
> your workload, especially if you're doing it over GbE.  Even 50 VMs
> doing relatively little though concurrently could cause problems given
> the nature of iSCSI (TCP overhead, latency of ethernet, etc).  My feel
> for this is that the fewer-LUN-more-LV route would be more efficient
> because you'll have fewer block device queues and multipath call-outs
> and such but that's just a guess on my part.  Test it out, see which
one
> is better.  If there's  no difference, go with the one that's easier
to
> manage.
> 

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.

James


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