[Qemu-devel] Re: Notes on block I/O data integrity

[Rusty Russell <rusty@rustcorp.com.au>]

On Wed, 26 Aug 2009 03:41:20 am Christoph Hellwig wrote:
> As various people wanted to know how the various data integrity patches
> I've send out recently play together here's a small writeup on what
> issues we have in QEMU and how to fix it:

Classic mail.  Thanks for the massive and coherent clue injection!

> Action plan for the guest drivers:
> 
>  - virtio-blk needs to advertise ordered queue by default.
>    This makes cache=writethrough safe on virtio.

From a guest POV, that's "we don't know, let's say we're ordered because that
may make us safer".  Of course, it may not help: how much does it cost to
drain the queue?

The bug, IMHO is that we *should* know.  And in future I'd like to fix that,
either by adding an VIRTIO_BLK_F_ORDERED feature, or a VIRTIO_BLK_F_UNORDERED
feature.

> Action plan for QEMU:
> 
>  - IDE needs to set the write cache enabled bit
>  - virtio needs to implement a cache flush command and advertise it
>    (also needs a small change to the host driver)

So, virtio-blk needs to be enhanced for this as well.

Thanks!
Rusty.