Re: [Qemu-devel] [PATCH v2 5/7] QAPI: add command for live block commit, 'block-commit'

[Eric Blake]

On 09/26/2012 08:25 AM, Jeff Cody wrote:

>>> +# @top:    #optional The file name of the backing image within the image 
>>> chain,
>>> +#                    which contains the topmost data to be committed down.
>>> +#                    If not specified, this is one layer below the active
>>> +#                    layer (i.e. active->backing_hd).
>>
>> Why isn't active the default any more? I know, we don't support it yet,
>> but long term this is what makes most sense as a default.
>>
> 
> Eric had a similar question, and asked if anyone had any preference -
> this was my response:
> 
> ---
> 
> I guess I don't have a strong preference either - I originally had it
> the other way, but then that meant the default in the current
> implementation was actually an error.
> 
> Also, I assumed (danger!) that the most common use of commit would be a
> snapshot, followed by a commit of active->backing_hd. With that
> assumption, it seemed like a sane default.

Actually, I envision my personal use case to be:

Take a snapshot, do an experiment in the guest (such as install a
questionable package), and then either roll back to the snapshot
(experiment failed) or commit the active (experiment worked, no need to
have a snapshot any more); either way, taking the snapshot created a new
temporary file name, and after I make my decision on whether to commit
or discard the snapshot, I want to get back to the original file name.
Since the snapshot effectively created a temporary file name, I'd rather
not have to know the name of that temporary file just to pass it to an
explicit 'top' argument when committing the active layer.

Having to specify 'top' to avoid an error when not committing the active
layer is not as bad as getting the defaults wrong for when we do add
support for committing the active layer.

> ---
> 
> I can certainly revert back to having the active layer be the top, if
> that is the preference.

Given that it has been asked again, I'd say yes, go ahead and revert to
the behavior of defaulting 'top' to the active layer.

-- 
Eric Blake   address@hidden    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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