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

[Ronen Hod]

On 06/18/2012 02:14 PM, Dor Laor wrote:
> On 06/18/2012 01:05 PM, Rusty Russell wrote:
> > On Mon, 18 Jun 2012 16:03:23 +0800, Asias He<asias@xxxxxxxxxx>  wrote:
> > > On 06/18/2012 03:46 PM, Rusty Russell wrote:
> > > > On Mon, 18 Jun 2012 14:53:10 +0800, Asias He<asias@xxxxxxxxxx>  wrote:
> > > > > This patch introduces bio-based IO path for virtio-blk.
> > > >
> > > > Why make it optional?
> > >
> > > request-based IO path is useful for users who do not want to bypass the
> > > IO scheduler in guest kernel, e.g. users using spinning disk. For users
> > > using fast disk device, e.g. SSD device, they can use bio-based IO path.
> >
> > Users using a spinning disk still get IO scheduling in the host though.
> > What benefit is there in doing it in the guest as well?
>
> The io scheduler waits for requests to merge and thus batch IOs together. It's not important w.r.t spinning disks since the host can do it but it causes much less vmexits which is the key issue for VMs.

Does it make sense to use the guest's I/O scheduler at all?
- It is not aware of the physical (spinning) disk layout.
- It is not aware of all the host's disk pending requests.
It does have a good side-effect - batching of requests.

Ronen.

> > Cheers,
> > Rusty.
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