Messages in this thread | | | Date | Mon, 30 Jul 2012 08:26:48 +0800 | From | Asias He <> | Subject | Re: [PATCH V3 3/3] virtio-blk: Add bio-based IO path for virtio-blk |
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On 07/29/2012 08:59 PM, Michael S. Tsirkin wrote: > On Sat, Jul 28, 2012 at 10:38:41AM +0800, Asias He wrote: >> On 07/27/2012 08:33 AM, Rusty Russell wrote: >>> On Fri, 13 Jul 2012 16:38:51 +0800, Asias He <asias@redhat.com> wrote: >>>> Add 'virtio_blk.use_bio=1' to kernel cmdline or 'modprobe virtio_blk >>>> use_bio=1' to enable ->make_request_fn() based I/O path. >>> >>> This patch conflicts with Paolo's Bonzini's 'virtio-blk: allow toggling >>> host cache between writeback and writethrough' which is also queued (see >>> linux-next). >> >> Rebased against Paolo's patch in V4. >> >>> I'm not sure what the correct behavior for bio & cacheflush is, if any. >> >> REQ_FLUSH is not supported in the bio path. >> >>> But as to the patch itself: it's a hack. >>> >>> 1) Leaving the guest's admin to turn on the switch is a terrible choice. >>> 2) The block layer should stop merging and sorting when a device is >>> fast, not the driver. >>> 3) I pointed out that slow disks have low IOPS, so why is this >>> conditional? Sure, more guest exits, but it's still a small number >>> for a slow device. >>> 4) The only case where we want merging is on a slow device when the host >>> isn't doing it. >>> >>> Now, despite this, I'm prepared to commit it. But in my mind it's a >>> hack: we should aim for use_bio to be based on a feature bit fed from >>> the host, and use the module parameter only if we want to override it. >> >> OK. A feature bit from host sound like a choice but a switch is also >> needed on host side. > > qemu automatically gives you the ability to control > any feature bit.
Automatically?
>> And for other OS, e.g. Windows, the bio thing >> does not apply at all. > > Let's try to define when it's a good idea. Is it a hint to guest that > backend handles small accesses efficiently so ok to disable batching?
Yes. It's also a hint for latency reduction.
>> Anyway, I have to admit that adding a module parameter here is not >> the best choice. Let's think more. >> >> -- >> Asias
-- Asias
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