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Re: [Qemu-devel] Re: [PATCH] virtio-blk: add SGI_IO passthru support
From: |
Christoph Hellwig |
Subject: |
Re: [Qemu-devel] Re: [PATCH] virtio-blk: add SGI_IO passthru support |
Date: |
Thu, 30 Apr 2009 22:13:50 +0200 |
User-agent: |
Mutt/1.3.28i |
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---
- [Qemu-devel] Re: [PATCH] virtio-blk: add SGI_IO passthru support, (continued)
- [Qemu-devel] Re: [PATCH] virtio-blk: add SGI_IO passthru support, Anthony Liguori, 2009/04/27
- 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 <=
- Re: [Qemu-devel] Re: [PATCH] virtio-blk: add SGI_IO passthru support, Paul Brook, 2009/04/30
- 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