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