Re: [Qemu-devel] Re: [PATCH] virtio-blk: add SGI_IO passthru support

[Paul Brook <paul@codesourcery.com>]

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