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Re: [Qemu-devel] Re: [PATCH] virtio-blk: add SGI_IO passthru support
From: |
Paul Brook |
Subject: |
Re: [Qemu-devel] Re: [PATCH] virtio-blk: add SGI_IO passthru support |
Date: |
Thu, 30 Apr 2009 22:49:19 +0100 |
User-agent: |
KMail/1.9.9 |
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
- Re: [Qemu-devel] Re: [PATCH] virtio-blk: add SGI_IO passthru support, (continued)
- Re: [Qemu-devel] Re: [PATCH] virtio-blk: add SGI_IO passthru support, Christoph Hellwig, 2009/04/28
- Re: [Qemu-devel] Re: [PATCH] virtio-blk: add SGI_IO passthru support, Anthony Liguori, 2009/04/28
- Re: [Qemu-devel] Re: [PATCH] virtio-blk: add SGI_IO passthru support, Christian Borntraeger, 2009/04/28
- Re: [Qemu-devel] Re: [PATCH] virtio-blk: add SGI_IO passthru support, Christoph Hellwig, 2009/04/29
- Re: [Qemu-devel] Re: [PATCH] virtio-blk: add SGI_IO passthru support, Paul Brook, 2009/04/29
- Re: [Qemu-devel] Re: [PATCH] virtio-blk: add SGI_IO passthru support, Christian Borntraeger, 2009/04/29
- Re: [Qemu-devel] Re: [PATCH] virtio-blk: add SGI_IO passthru support, Paul Brook, 2009/04/29
- Re: [Qemu-devel] Re: [PATCH] virtio-blk: add SGI_IO passthru support, Christoph Hellwig, 2009/04/29
- Re: [Qemu-devel] Re: [PATCH] virtio-blk: add SGI_IO passthru support, Paul Brook, 2009/04/29
- Re: [Qemu-devel] Re: [PATCH] virtio-blk: add SGI_IO passthru support, Christoph Hellwig, 2009/04/30
- Re: [Qemu-devel] Re: [PATCH] virtio-blk: add SGI_IO passthru support,
Paul Brook <=
- Re: [Qemu-devel] Re: [PATCH] virtio-blk: add SGI_IO passthru support, Javier Guerra, 2009/04/30
- Re: [Qemu-devel] Re: [PATCH] virtio-blk: add SGI_IO passthru support, Christian Borntraeger, 2009/04/28
- Re: [Qemu-devel] Re: [PATCH] virtio-blk: add SGI_IO passthru support, Christoph Hellwig, 2009/04/29
- Re: [Qemu-devel] Re: [PATCH] virtio-blk: add SGI_IO passthru support, Christian Borntraeger, 2009/04/29
[Qemu-devel] [PATCH 2/2] virtio-blk: add SG_IO passthru support, Christoph Hellwig, 2009/04/28