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@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. 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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
