Re: [PATCH v2] Document qemu-img options data_file and data_file_raw

[Max Reitz <mreitz@redhat.com>]

On 04.05.21 01:15, Connor Kuehl wrote:
> On 4/30/21 9:45 AM, Max Reitz wrote:
>>> +  ``data_file_raw``
>>> +    If this option is set to ``on``, QEMU will always keep the external
>>> +    data file consistent as a standalone read-only raw image. It does
>>> +    this by forwarding updates through to the raw image in addition to
>>> +    updating the image metadata. If set to ``off``, QEMU will only
>>> +    update the image metadata without forwarding the changes through
>>> +    to the raw image. The default value is ``off``.
>>
>> Hm, what updates and what changes?  I mean, the first part makes sense (the “It does this by...”), but the second part doesn’t.  qemu will still forward most writes to the data file.  (Not all, but most.)
>>
>> (Also, nit pick: With data_file_raw=off, the data file is not a raw image.  (You still call it that in the penultimate sentence.))
>> When you write data to a qcow2 file with data_file, the data also goes to the data_file, most of the time.  The exception is when it can be handled with a metadata update, i.e. when it's a zero write or discard.
>>
>> In addition, such updates (i.e. zero writes, I presume) not happening to the data file are usually a minor problem.  The real problem is that without data_file_raw, data clusters can be allocated anywhere in the data file, whereas with data_file_raw, they are allocated at their respective guest offset (i.e. the host offset always equals the guest offset).
>>
>> I personally would have been fine with the first sentence, but if we want more of an explanation...  Perhaps:
>>
>> <<EOF
>>
>> If this option is set to ``on``, QEMU will always keep the external data file consistent as a standalone read-only raw image.
>>
>> It does this by effectively forwarding all write accesses that happen to the qcow2 file to the raw data file, including their offsets. Therefore, data that is visible on the qcow2 node (i.e., to the guest) at some offset is visible at the same offset in the raw data file.
>>
>> If this option is ``off``, QEMU will use the data file just to store data in an effectively arbitrary manner.  The file’s content will not make sense without the accompanying qcow2 metadata.  Where data is written will have no relation to its offset as seen by the guest, and some writes (specifically zero writes) may not be forwarded to the data file at all, but will only be handled by modifying qcow2 metadata.
>>
>> In short: With data_file_raw, the data file reads as a valid raw VM image file.  Without it, its content can only be interpreted by reading the accompanying qcow2 metadata.
>>
>> Note that this option only makes the data file valid as a read-only raw image.  You should not write to it, as this may effectively corrupt the qcow2 metadata (for example, dirty bitmaps may become out of sync).
>>
>> EOF
>>
>> This got longer than I wanted it to be.  Hm.  Anyway, what do you think?
> 
> I found it very helpful. I'll incorporate your explanation into the next
> revision.
> 
> I'm wondering what the most appropriate trailer would be for the next
> revision?
> 
> 	Suggested-by: Max [..]
> 	Co-developed-by: Max [..]
> 
> Let me know if you have a strong preference, otherwise I'll go with
> Suggested-by:

I’m fine without any tag (if I merge this patch, it’ll get my S-o-b 
anyway :)), but if any, I’d probably go with a Suggested-by, yes.

Max