Re: [Xen-devel] Question about using zero-copy for disk I/O in Xen.

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> I have an impression the zero-copy technology is frequently tied up with
> network I/O. Recently I found people in Xen community start talking
> zero-copy for disk I/O as well (e.g. scsi front-end/back-end driver and
> blktap in Xen). So is there any difference for zero-copy between disk I/O
> and network I/O? O_DIRECT can be used to eliminate the kernel buffer copy,
> isn't it counted as part of zero-copy for disk I/O?


I don't have the context for the discussions you saw (were they on the mailing 
list?  I may have missed them...) but I suspect they were referring to the 
fact that there is no copy in the disk -> dom0 -> guest path.  This is 
because dom0 does disk DMA direct to domU pages.  The guest may still perform 
a copy in order to move data between its buffer cache and userspace.

When people talk about network IO being zero copy in a more general case (i.e. 
not just Xen-specific) it tends to refer to the need to copy data on its way 
to the userspace consumers.

Cheers,
Mark

-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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