Re: [PATCH 09/17] block: Refactor bdrv_has_zero_init{,_truncate}

[Max Reitz <mreitz@redhat.com>]

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On 04.02.20 18:51, Eric Blake wrote:
> On 2/4/20 11:42 AM, Max Reitz wrote:
> 
>>>
>>> I understand that this is preexisting logic, but could I ask: why?
>>> What's wrong
>>> if driver can guarantee that created file is all-zero, but is not sure
>>> about
>>> file resizing? I agree that it's normal for these flags to have the same
>>> value,
>>> but what is the reason for this restriction?..
>>
>> If areas added by truncation (or growth, rather) are always zero, then
>> the file can always be created with size 0 and grown from there.  Thus,
>> images where truncation adds zeroed areas will generally always be zero
>> after creation.
>>
>>> So, the only possible combination of flags, when they differs, is
>>> create=0 and
>>> truncate=1.. How is it possible?
>>
>> For preallocated qcow2 images, it depends on the storage whether they
>> are actually 0 after creation.  Hence qcow2_has_zero_init() then defers
>> to bdrv_has_zero_init() of s->data_file->bs.
>>
>> But when you truncate them (with PREALLOC_MODE_OFF, as
>> BlockDriver.bdrv_has_zero_init_truncate()’s comment explains), the new
>> area is always going to be 0, regardless of initial preallocation.
>>
>>
>> I just noticed a bug there, though: Encrypted qcow2 images will not see
>> areas added through growth as 0.  Hence, qcow2’s
>> bdrv_has_zero_init_truncate() implementation should not return true
>> unconditionally, but only for unencrypted images.
> 
> Hence patch 5 earlier in the series :)

Ah, good. :-)

Max


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