Liang Yang
2007-Feb-26 23:17 UTC
[Xen-devel] Question about using zero-copy for disk I/O in Xen.
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 _______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel
Mark Williamson
2007-Feb-27 12:02 UTC
[Xen-users] Re: [Xen-devel] Question about using zero-copy for disk I/O in Xen.
Hi, To avoid cross posting (which some people don''t like) I''m replying to this mail on the xen-devel list as it''s a technical question. Cheers, Mark On Monday 26 February 2007 23:17, Liang Yang wrote:> 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 > > > _______________________________________________ > Xen-devel mailing list > Xen-devel@lists.xensource.com > http://lists.xensource.com/xen-devel-- 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! _______________________________________________ Xen-users mailing list Xen-users@lists.xensource.com http://lists.xensource.com/xen-users
Mark Williamson
2007-Feb-27 12:05 UTC
Re: [Xen-devel] Question about using zero-copy for disk I/O in Xen.
> 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! _______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel