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

[Liu Qing]

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:
> > Random write IOPS will drop dramatically if qcow2 l2 cache could not
> > cover the whole disk. This patch gives libvirt user a chance to adjust
> > the qcow2 cache configuration.
> > 
> > Three new qcow2 driver parameters are added. They are l2-cache-size,
> > refcount-cache-size and cache-clean-interval.
> > 
> > The following are from qcow2-cache.txt.
> > The amount of virtual disk that can be mapped by the L2 and refcount
> > caches (in bytes) is:
> > disk_size = l2_cache_size * cluster_size / 8
> > disk_size = refcount_cache_size * cluster_size * 8 / refcount_bits
> > 
> > The parameter "cache-clean-interval" defines an interval (in seconds).
> > All cache entries that haven't been accessed during that interval are
> > removed from memory.
> > 
> > Signed-off-by: Liu Qing <liuqing at huayun.com>
> > ---
> > Change since v3: a) copy qcow2 cache configurion from source to backing$
> >                     to backing file source.$
> 
> This looks like one of the tuning parameters which really is hard for
> users to set and thus it should be justified properly if we need it. [1]
> 
> From the commit message above it looks like that there are guidelines
> how to set them. Can't we just make them implicit and not expose
> anything to tune the settings? Does it make sense to do so? Are there
> any drawbacks?
> 
> If any of them need to be configured by the user, please describe that
> in detail why it's necessary.
> 
> Peter
> 
> [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.