Re: [PATCH v4 07/30] qcow2: Document the Extended L2 Entries feature

[Eric Blake]

On 4/15/20 2:11 PM, Alberto Garcia wrote:
> On Fri 10 Apr 2020 11:29:59 AM CEST, Vladimir Sementsov-Ogievskiy wrote:
> > > Should we also document that extended L2 entries are incompatible
> > > with raw external files? [...] After all, when raw external file is
> > > enabled, the entire image is allocated, at which point subclusters
> > > don't make much sense.
> >
> > It still may cache information about zeroed subclusters: gives more
> > detailed block-status.

That's a good point about one reason why it might be useful.

> So shall I forbid extended_l2 + data_file_raw then?
>
> I wonder, if the only problem is that it's just not very useful, does it
> make sense to add additional complexity and restrictions to the code
> simply to prevent the user from making a sub-optimal choice?

At this point, I'm not seeing a technical reason why we have to forbid 
subclusters with data-file-raw.  Mixing may be inefficient compared to 
using raw-data-file without subclusters, but inefficiencies are not 
worth the code bloat to forbid the combination.  If we come up with a 
scenario where the mix would cause data corruption, that's a different 
story, but I'm not seeing such a reason at the moment.

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
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