Question about using zero-copy for disk I/O in Xen.

Question about using zero-copy for disk I/O in Xen.

[Liang Yang]

Hello,

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?

Thanks,

Liang

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

[Mark Williamson]

> 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

-- 
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