Re: [PATCH V3 3/3] virtio-blk: Add bio-based IO path for virtio-blk

[Asias He <asias@redhat.com>]

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