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. Now it might be possible to get that latency down to a level where we can ignore it but when doing all this additional work there always will be additional overhead. > > Paul ---end quoted text--- -- 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
