Re: [PATCH] virtio-blk: allow toggling host cache between writeback and writethrough

Badari Pulavarty
2012-07-05T18:36:26Z

On Tue, 2012-07-03 at 15:19 +0200, Paolo Bonzini wrote:
> This patch adds support for the new VIRTIO_BLK_F_CONFIG_WCE feature,
> which exposes the cache mode in the configuration space and lets the
> driver modify it. The cache mode is exposed via sysfs.
>
> Even if the host does not support the new feature, the cache mode is
> visible (thanks to the existing VIRTIO_BLK_F_WCE), but not modifiable.
>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---

Hi Paolo,

Curious - What is the host side change to support this ? QEMU would
close and re-open the device/file with the corresponding flags
(O_SYNC) ?

And also, is there a way to expose cache=none (O_DIRECT) to the guest ?
Our cluster filesystem folks need a way to verify/guarantee that
virtio-blk device has cache=none selected at host. Otherwise, they
can not support a cluster filesystem running inside a VM (on
virtio-blk).

Thoughts ?

Thanks,
Badari