Re: qcow2: Zero-initialization of external data files

[Eric Blake <eblake@redhat.com>]

On 4/9/20 8:42 AM, Eric Blake wrote:

>>> I'd argue that requiring the user to pre-zero the raw data file is
>>> undesirable; and that we should instead fix our code to not report the
>>> image as reading all zeroes when creating with data_file_raw=on.
>>
>> OK.  I think that could be achieved by just enforcing @preallocation to
>> be at least “metadata” whenever @data-file-raw is set.  Would that make
>> sense?
> 
> Is a preallocation of metadata sufficient to report things correctly? If 
> so, it seems like a reasonable compromise to me.  I was more envisioning 
> a fix elsewhere: if we are reporting block status of what looks like an 
> unallocated cluster, but data-file-raw is set, we change our answer to 
> instead report it as allocated with unknown contents.  But with 
> preallocation, you either force the qcow2 file to list no cluster as 
> unallocated (which matches the fact that the raw image really is fully 
> allocated) while not touching the raw image, or you can go one step 
> further and request full preallocation to wipe the raw image to 0 in the 
> process.

What happens when an operation attempts to unmap things?  Do we reject 
all unmap operations when data-file-raw is set (thus leaving a cluster 
marked as allocated at all times, if we can first guarantee that 
preallocation set things up that way)?

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