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RE: [Xen-users] disk backend performance

Hi James,

Am Samstag, den 29.11.2008, 14:18 +1100 schrieb James Harper: 
> > 
> > So: over a dedicated cable with jumbo frames it is actually smarter to
> use
> > ISCSI than AOE? Is that your conclusion?
> My conclusion was more along the lines of "don't assume that just
> because iSCSI uses TCP/IP that it is inferior performance-wise to AoE".
> I don't believe that there currently exist any hardware implemented AoE
> adapters, but there are hardware iSCSI adapters (HBA's).

Yeah, but only Gbit. Intel has actually hid away from TCP offload
engines because of the tremendous costs (and resultant pricing) that
goes along with it. 
So 10GBit-Adapters have no TCOE atm.

> With a software implementation, Ethernet headers have to be constructed
> per packet (+IP+TCP for iSCSI), error detection has to be implemented
> for AoE, and packetization has to be done in software (less so for iSCSI
> as most hardware has TCP Large Send Offload which from a software point
> of view allows sending of 64k TCP packets). Any network adapter that
> you'd use in a server these days has TX and RX checksum offloading
> available, so you get checksumming of your TCP packets for free when
> using software iSCSI, while for AoE you need to calculate checksums
> manually in software.
> With a hardware iSCSI HBA, the O/S just has to say to the card 'read x
> sectors starting at y and put them in memory here'. The card does the
> rest. Of course, to do this you have to buy an iSCSI HBA, which
> increases the cost of the solution somewhat.
> If you want to use non-disk devices then iSCSI is probably a better
> choice - a robotic tape library is more likely to work over iSCSI
> without problems than it will over AoE, although my information on that
> sort of thing may be out of date...
> A protocol called HyperSCSI exists, which is basically SCSI over
> Ethernet, but I don't know how available it is.

HyperSCSI died somewhere 2002. It seemed to have a good chance to become
the optimal tradeoff for simple SANs. May be someone decided to
commercialize it deeply ;)
But there are Still Ethernet-Storage-Techniques like AoE,
SCSIoverEthernet (XoE) or the coming FCoE-Standard.

http://www.open-fcoe.org/ is interesting, too. FCoE seems to be intended
as the FC camp's response to iSCSI. Besides running over raw ethernet
versus TCP/IP, I'm guessing the main difference would be in how they
integrate with the rest of a data center...

> James


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