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

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Message-Id: <1341513550.5508.7.camel@badari-desktop>
Date:	Thu, 05 Jul 2012 11:39:10 -0700
From:	Badari Pulavarty <pbadari@...il.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	linux-kernel@...r.kernel.org, rusty@...tcorp.com.au,
	kvm@...r.kernel.org
Subject: Re: [PATCH] virtio-blk: allow toggling host cache between
	writeback and writethrough

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@...hat.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

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