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

From: Badari Pulavarty <pbadari@gmail.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: linux-kernel@vger.kernel.org, rusty@rustcorp.com.au, kvm@vger.kernel.org
Subject: Re: [PATCH] virtio-blk: allow toggling host cache between writeback and writethrough
Date: Thu, 05 Jul 2012 11:39:10 -0700	[thread overview]
Message-ID: <1341513550.5508.7.camel@badari-desktop> (raw)
In-Reply-To: <1341321577-24435-1-git-send-email-pbonzini@redhat.com>

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