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Re: [Qemu-devel] [PATCH] qcow2: make cache=unsafe usable

From: Alexander Graf
Subject: Re: [Qemu-devel] [PATCH] qcow2: make cache=unsafe usable
Date: Wed, 7 Sep 2011 11:47:22 +0200

On 07.09.2011, at 11:24, Avi Kivity wrote:

> Currently cache=unsafe is unsafe to the point of unusability - the
> caches are never written to disk except on exit so anything except
> an orderly exit -- including live migration -- leaves the disk image
> corrupted.
> Fix by interpreting flush requests and doing everything except flushing
> the underlying file.  The contents of the metadata cache are transferred
> to the host pagecache, so that qemu aborts keep the disk in a consistent
> state, and live migration (on the same host, or if using a coherent
> filesystem) works.

Yes, I've seen breakage with cache=unsafe and qcow2 myself. Thus semantically, 
the patch seems very reasonable to me. However, I'll leave it to Kevin to 
decide if it's a good idea to just unset random flags in open() or if we want 
to have something more expressive there :)


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