[libvirt] [PATCH 1/2] conf: add disk cache tuning parameters after qemu

[Nikolay Shirokovskiy]

On 26.09.2016 12:01, Peter Krempa wrote:
> On Mon, Sep 26, 2016 at 11:43:58 +0300, Nikolay Shirokovskiy wrote:
>> xml example:
>>
>> <disk ...>
>>   <cachetune writeback='on' direct='off' no_flush='on'/>
> 
> Aren't writeback and direct mutually exclusive?
> 
>> </disk>
> 
> What's wrong with 
> 
>  <disk ...>
>    <driver ... cache=''>?
> 
> 
> See https://libvirt.org/formatdomain.html#elementsDisks
> 
> 
> This commit message really does not describe your motivation for the
> addition.

Sorry, I forget to write down the motivation.

Internally qemu uses 3 orthogonal flags to configure disk cache. They
are exposed in interface too quite long time ago (29c4e2b). All-in-one
cache parameter that libvirt uses now do not provide all possible
values (5 vs 8). For example absent combination of direct + no flush (no sync)
is useful to speed up tests.

So I decided to provide new possibilities just as qemu does - keep
old parameter and use new ones to tune over.