Re: [Xen-users] iscsi vs nfs for xen VMs

[Pasi Kärkkäinen <pasik@xxxxxx>]

On Thu, Jan 27, 2011 at 09:35:38AM +0100, Adi Kriegisch wrote:
> Hi!
> 
> > > iSCSI tipically has a quite big overhead due to the protocol, FC, SAS,
> > > native infiniband, AoE have very low overhead.
> > > 
> > 
> > For iSCSI vs AoE, that isn't as true as you might think. TCP offload can
> > take care of a lot of the overhead. Any server class network adapter
> > these days should allow you to send 60kb packets to the network adapter
> > and it will take care of the segmentation, while AoE would be limited to
> > MTU sized packets. With AoE you need to checksum every packet yourself
> > while with iSCSI it is taken care of by the network adapter.
> What AoE actually does is sending a frame per block. Block size is 4K -- so
> no need for fragmentation. The overhead is pretty low, because we're
> talking about Ethernet frames.
> Most iSCSI issues I have seen are with reordering of packages due to
> transmission across several interfaces. So what most people recommend is to
> keep the number of interfaces to two. To keep performance up this means you
> have to use 10G, FC or similar which is quite expensive -- especially if
> you'd like to have a HA SAN network (HSRP and stuff like that is required).
> 
> AoE does not suffer from those issues: Using 6 GBit interfaces is no
> problem at all, load balancing will happen automatically, as the load is
> distributed equally across all available interfaces. HA is very simple:
> just use two switches and connect one half of the interfaces to one switch
> and the other half to the other switch. (It is recommended to use switches
> that can do jumbo frames and flow control)
> IMHO most of the current recommendations and practises surrounding iSCSI
> are there to overcome the shortcomings of the protocol. AoE is way more
> robust and easier to handle.
> 

iSCSI does not have problems using multiple gige interfaces.
Just setup multipathing properly.

-- Pasi


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