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

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Date:	Fri, 27 Jul 2012 10:03:48 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Asias He <asias@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org
Cc:	"Michael S. Tsirkin" <mst@...hat.com>,
	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 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).

I'm not sure what the correct behavior for bio & cacheflush is, if any.

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.

Cheers,
Rusty.
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