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

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On 07/27/2012 08:33 AM, Rusty Russell wrote:
On Fri, 13 Jul 2012 16:38:51 +0800, Asias He <asias@xxxxxxxxxx> 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
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