[libvirt] [PATCH v4 1/2] conf, docs: Add qcow2 cache configuration support

[Peter Krempa]

On Thu, Sep 14, 2017 at 13:02:46 +0800, Liu Qing wrote:
> On Wed, Sep 13, 2017 at 01:20:03PM +0200, Peter Krempa wrote:
> > On Wed, Sep 13, 2017 at 17:21:23 +0800, Liu Qing wrote:

[...]

> > [1] There's discussion I can link to for other tuning parameters. The
> > gist is that allowing users to set something withoug giving them
> > guidance is pointless since they might not use it. Also if the guidance
> > is strict (e.g. a formula, libvirt or qemu should set the defaults
> > properly and not force users to do the calculation)
> The guidance could be found in doc/qcow2-cache.txt in qemu source code.
> As John Ferlan suggested I have added the file locaton in formatdomain.html.in. 

Well, if the guidance is absolute (use this cache size and it's okay)
then we should implement it automatically (don't allow users
to set it.)

I'm asking whether there are some catches why not to do it automatically.
E.g whether there's an possibility that users would do something *else*
as described by the document and what would be the reasons for it.

One of the reasons might be memory consumption of the cache as it
looks like you need 1 MiB of memory to fully cover a 8GiB image.

Also the problem is that those parameters are qemu-isms so we should be
aware when modelling them in the XML since they can change and may not
map well to any other technology.

Also how does the cache impact read-performance from the backing layers?
We might need to set the cache for the backing layers as well.
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