On Thursday 30 April 2009, Christoph Hellwig wrote: > On Wed, Apr 29, 2009 at 12:37:20PM +0100, Paul Brook wrote: > > How exactly does it introduce additional latency? A scsi command block is > > hardly large or complicated. Are you suggesting that a 16/32byte scsi > > command takes significantly longer to process than a 16byte virtio > > command descriptor? I'd expect any extra processing to be a small > > fraction of the host syscall latency, let alone the latency of the > > physical host adapter. It probably even fits on the same CPU cache line. > > Encoding the scsi CDB is additional work but I would be surprised it it > is mesurable. Just using scsi cdbs would be simple enough, the bigger > issue is emulating a full blown scsi bus because then you need to do all > kinds queueing decisions at target levels etc and drag in a complicated > scsi stack and not just a simple block driver in the guest. And at > least on current linux kernels that does introduce mesurable latency. Only if you emulate a crufty old parallel scsi bus, and that's just silly. One of the nice things about scsi is it separates the command set from the transport layer. cf. USB mass-storage, SAS, SBP2(firewire), and probably several others I've forgotten. Paul -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
