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

[Asias He <asias@redhat.com>]

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. And for other OS, e.g. Windows, the bio thing does 
not apply at all.

Anyway, I have to admit that adding a module parameter here is not the 
best choice. Let's think more.

-- 
Asias