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

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Date:	Sun, 29 Jul 2012 15:59:48 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Asias He <asias@...hat.com>
Cc:	Rusty Russell <rusty@...tcorp.com.au>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	Christoph Hellwig <hch@....de>,
	Minchan Kim <minchan.kim@...il.com>,
	Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH V3 3/3] virtio-blk: Add bio-based IO path for virtio-blk

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@...hat.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.

> 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?

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