Re: [PATCH 1/2] qemu: introduce locking option for disk source of qemu

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On Sat, Jan 23, 2021 at 11:12:15AM +0100, Peter Krempa wrote:
> On Fri, Jan 22, 2021 at 21:32:05 -0500, Masayoshi Mizuma wrote:
> > From: Masayoshi Mizuma <m.mizuma@xxxxxxxxxxxxxx>
> > 
> > Introduce locking option for disk source of qemu.
> > It may be useful to avoid file lock issues.
> 
> Lock issues usually mean that you are doing something wrong.
> 
> > locking option supports three switches; 'auto', 'on' and 'off'.
> > The default behaivor will work if locking option isn't set.
> > 
> > Example of the usage:
> > 
> >   <disk type='file' device='disk'>
> >     <driver name='qemu' type='qcow2' cache='none'/>
> >     <source file='/tmp/QEMUGuest1.img' locking='off'/>
> >     <target dev='hda' bus='ide'/>
> >     <address type='drive' controller='0' bus='0' target='0' unit='0'/>
> >   </disk>
> > 
> > Signed-off-by: Masayoshi Mizuma <m.mizuma@xxxxxxxxxxxxxx>
> > ---
> >  docs/schemas/domaincommon.rng  |  9 +++++++++
> >  src/conf/domain_conf.c         |  8 ++++++++
> >  src/conf/storage_source_conf.c |  9 +++++++++
> >  src/conf/storage_source_conf.h | 14 ++++++++++++++
> >  src/libvirt_private.syms       |  1 +
> >  src/qemu/qemu_block.c          |  5 +++++
> >  6 files changed, 46 insertions(+)
> 
> Few quick notes:
> 
> - I'm not entirely persuaded that we should at this at all
>     - qemu locking works to prevent user mistakes
>     - you really need to know what to expect if you use images which are
>       used by multiple VMs
> 
>     - you really need to add some form of justification, the current one
>       definitely is insufficient and thus I'm inclined to NACKing the
>       concept of enabling setting of the locks completely

I agree - we should be providing higher level conceptual logic that
indirectly maps into the different locking states.

For example we have <readonly/> and <shareable/> elements that
influence the type of locks held.  A flag to unconditionally
disable locking entirely feels like it is just hacking around
some undefined problem that should be fixed in a better way.


Regards,
Daniel
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