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

[Christoph Hellwig <hch@lst.de>]

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
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