Re: [Xen-users] Shared Storage

[John Madden <jmadden@xxxxxxxxxxx>]

> 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.
John





--
John Madden
Sr UNIX Systems Engineer / Office of Technology
Ivy Tech Community College of Indiana
jmadden@xxxxxxxxxxx

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