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

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

> Il 26/01/2011 22:24, James Harper ha scritto:
> >>
> >> 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.
> 
> the overhead is 10% on a gigabit link and when you speak about
resources
> overhead you have mention also the CPU overhead on the storage side.

I don't know the exact size of the iSCSI header, but to be 10% of a
gigabit link it would have to be 900 bytes, and I'm pretty sure it's
much less. If you weren't using jumbo frames then maybe 10% might be
realistic, but that's hardly an enterprise scenario.

> If you check the datasheets of brands like emc you can see that the
same
> storage platform is sold in iSCSI and FC version ...on the first one
you
> can use less than half the servers you can use with the last one.
> 
> Every new entry level storage is based on std hardware without any hw
> acceleration ...for example EMC AX storages are simply xeon servers.
> 

Well if EMC are selling workstation grade cards with no TCP offload at
all then I'm not surprised that the performance is so poor.

James


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