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Re: [Qemu-devel] [RFC 5/7] qcow2: implement lazy refcounts

From: Kevin Wolf
Subject: Re: [Qemu-devel] [RFC 5/7] qcow2: implement lazy refcounts
Date: Mon, 25 Jun 2012 17:01:09 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:12.0) Gecko/20120430 Thunderbird/12.0.1

Am 22.06.2012 17:08, schrieb Stefan Hajnoczi:
> Lazy refcounts is a performance optimization for qcow2 that postpones
> refcount metadata updates and instead marks the image dirty.  In the
> case of crash or power failure the image will be left in a dirty state
> and repaired next time it is opened.
> Reducing metadata I/O is important for cache=writethrough and
> cache=directsync because these modes guarantee that data is on disk
> after each write (hence we cannot take advantage of caching updates in
> RAM).  Refcount metadata is not needed for guest->file block address
> translation and therefore does not need to be on-disk at the time of
> write completion - this is the motivation behind the lazy refcount
> optimization.
> The lazy refcount optimization must be enabled at image creation time:
>   qemu-img create -f qcow2 -o compat=1.1,lazy_refcounts=on a.qcow2 10G
>   qemu-system-x86_64 -drive if=virtio,file=a.qcow2,cache=writethrough
> Signed-off-by: Stefan Hajnoczi <address@hidden>

This one requires a feature table entry as well.


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