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

[Paul Brook]

On Wednesday 29 April 2009, Christoph Hellwig wrote:
> On Wed, Apr 29, 2009 at 12:11:19PM +0100, Paul Brook wrote:
> > Is this actually measurably faster, or just infinitesimally faster in
> > theory?
>
> On normal disks it's rather theoretical.  On high-end SSDs and arrays the
> impact is noticeable, mostly due to the additional latency.

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.

Paul