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Re: [libvirt] [patch] libvirt documentation - disk cache description



On 05/28/2010 02:15 AM, íÁÒË ëÏÒÅÎÂÅÒÇ wrote:
> The description was got from qemu manual.
> 
> Please review, edit, apply adn update the website :)
> 

Hmm, I don't think we should duplicate the qemu cache value descriptions
in our docs, it might be a better idea to link to qemu docs with a note
to check the hypervisor docs for meaning of the different values.

> diff -u docs/formatdomain.html.in docs.new/formatdomain.html.in
> --- docs/formatdomain.html.in	2010-05-27 11:35:33.995122526 +0600
> +++ docs.new/formatdomain.html.in	2010-05-27 13:36:27.362623268 +0600
> @@ -517,10 +520,30 @@
>        <dt><code>driver</code></dt>
>        <dd>If the hypervisor supports multiple backend drivers, then the optional
>          <code>driver</code> element allows them to be selected. The <code>name</code>
> -        attribute is the primary backend driver name, while the optional <code>type</code>
> -        attribute provides the sub-type. The optional <code>cache</code> attribute
> +        attribute is the primary backend driver name (for example, "qemu"),
> +        while the optional <code>type</code> attribute provides the sub-type.
> +        For qemu, possible values are: "raw", "qcow", "qcow2", "vmdk" and some other.
> +
> +	<p>The optional <code>cache</code> attribute
>          controls the cache mechanism, possible values are "default", "none",
> -        "writethrough" and "writeback". <span class="since">Since 0.1.8</span>
> +        "writethrough" and "writeback".</p>
> +
> +        <p>Writethrough caching means that the host
> +        page cache will be used to read and write data but write notification will
> +        be sent to the guest only when the data	has been reported as written by the
> +        storage subsystem.</p>
> +
> +        <p>Writeback caching will report data writes as completed as soon as
> +        the data is present in the host page cache.  This is safe as long as you trust your host.
> +	If your host crashes or loses power, then the guest may experience data corruption.</p>
> +
> +	<p>The host page cache can be avoided entirely with <code>cache='none'</code>.
> +	This will attempt to do disk IO directly to the guests memory.</p>
> +
> +	<p>As said in qemu-kvm-0.12.3 manual: Some block drivers perform badly with <code>cache='writethrough'</code>, most notably,
> +	qcow2. If performance is more important than correctness, <code>cache='writeback'</code>
> +	should be used with qcow2.</p>
> +        <span class="since">Since 0.1.8</span>

I know this 'since' piece is from the existing docs, but it's not really
correct: it only applies to the <driver> tag in general. We should have
a separate 'since 0.6.0' for the cache attribute.

- Cole


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