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

From: Max Reitz
Subject: Re: [PATCH 09/17] block: Refactor bdrv_has_zero_init{,_truncate}
Date: Wed, 5 Feb 2020 18:55:48 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1

On 05.02.20 15:07, Eric Blake wrote:
> On 2/5/20 1:51 AM, Vladimir Sementsov-Ogievskiy wrote:
>>>>> +typedef enum {
>>>>> +    /*
>>>>> +     * bdrv_known_zeroes() should include this bit if the contents of
>>>>> +     * a freshly-created image with no backing file reads as all
>>>>> +     * zeroes without any additional effort.  If .bdrv_co_truncate is
>>>>> +     * set, then this must be clear if BDRV_ZERO_TRUNCATE is clear.
>>>> 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.
>> This means, that if truncation bit is set, than create bit should be
>> set.. But
>> here we say that if truncation is clear, than create bit must be clear.
> Max, did we get the logic backwards?

Or maybe my explanation was just wrong.

Because nobody actually forces a driver to use truncate to ensure that
an newly created file will be 0.  Hm.  And more importantly, you can’t
use truncate with PREALLOC_MODE_OFF when you want to create an image
with preallocation.

Let’s see.  The offending commit message says:

> No .bdrv_has_zero_init() implementation returns 1 if growing the file
> would add non-zero areas (at least with PREALLOC_MODE_OFF), so using it
> in lieu of this new function was always safe.
> But on the other hand, it is possible that growing an image that is not
> zero-initialized would still add a zero-initialized area, like when
> using nonpreallocating truncation on a preallocated image.  For callers
> that care only about truncation, not about creation with potential
> preallocation, this new function is useful.

So I suppose the explanation is just the preallocation mode alone;
has_zero_init() is for the image’s actual preallocation mode, whereas
has_zero_init_truncate() is forced to PREALLOC_MODE_OFF.  As such, the
latter is less strict than the former.  So the former cannot be true
when the latter is false.

>>>> 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.
>> ah yes, due to qcow2 zero clusters.
> Hmm. Do we actually set the zero flag on unallocated clusters when
> resizing a qcow2 image?

No.  They are just unallocated, i.e. zero.  (Nodes with backing files
never return true for bdrv_has_zero_init_truncate anyway).

> That would be an O(n) operation (we have to
> visit the L2 entry for each added cluster, even if only to set the zero
> cluster bit).  Or do we instead just rely on the fact that qcow2 is
> inherently sparse, and that when you resize the guest-visible size
> without writing any new clusters, then it is only subsequent guest
> access to those addresses that finally allocate clusters, making resize
> O(1) (update the qcow2 metadata cluster, but not any L2 tables) while
> still reading 0 from the new data.  To some extent, that's what the
> allocation mode is supposed to control.
> What about with external data images, where a resize in guest-visible
> length requires a resize of the underlying data image?  There, we DO
> have to worry about whether the data image resizes with zeroes (as in
> the filesystem) or with random data (as in a block device).

Well, partially: Namely, only with data_file_raw.  Because otherwise the
clusters are still unallocated and thus read as zero.  So yes, then we
do have to worry about that.

With data_file_raw, we have an obligation to make the data file return
the same data as the qcow2 file, so, um.  I wonder whether we actually
take any care of this yet.  If you have some external data file without
zero_init(_truncate), do get zeroes when reading from the qcow2 node,
but non-zeroes when reading from the raw data file?  That would be OK
without data_file_raw, but not with it.  I suppose I’ll have to test it.


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