Re: [PATCH 1/2] qcow2: Force preallocation with data-file-raw

[Max Reitz]

On 19.06.20 18:47, Alberto Garcia wrote:
> On Fri 19 Jun 2020 12:40:11 PM CEST, Max Reitz wrote:
>> +    if (qcow2_opts->data_file_raw &&
>> +        qcow2_opts->preallocation == PREALLOC_MODE_OFF)
>> +    {
>> +        /*
>> +         * data-file-raw means that "the external data file can be
>> +         * read as a consistent standalone raw image without looking
>> +         * at the qcow2 metadata."  It does not say that the metadata
>> +         * must be ignored, though (and the qcow2 driver in fact does
>> +         * not ignore it), so the L1/L2 tables must be present and
>> +         * give a 1:1 mapping, so you get the same result regardless
>> +         * of whether you look at the metadata or whether you ignore
>> +         * it.
>> +         */
>> +        qcow2_opts->preallocation = PREALLOC_MODE_METADATA;
> 
> I'm not convinced by this,

Why not?

This is how I read the spec.  Furthermore, I see two problems that we
have right now that are fixed by this patch (namely (1) using a device
file as the external data file, which may have non-zero data at
creation; and (2) assigning a backing file at runtime must not show the
data).

> but your comment made me think of another
> possible alternative: in qcow2_get_cluster_offset(), if the cluster is
> unallocated and we are using a raw data file then we return _ZERO_PLAIN:
> 
> --- a/block/qcow2-cluster.c
> +++ b/block/qcow2-cluster.c
> @@ -654,6 +654,10 @@ out:
>      assert(bytes_available - offset_in_cluster <= UINT_MAX);
>      *bytes = bytes_available - offset_in_cluster;
>  
> +    if (type == QCOW2_CLUSTER_UNALLOCATED && data_file_is_raw(bs)) {
> +        type = QCOW2_CLUSTER_ZERO_PLAIN;
> +    }
> +
>      return type;
> 
> You could even add a '&& bs->backing' to the condition and emit a
> warning to make it more explicit.

No, this is wrong.  This still wouldn’t fix the problem of having a
device file as the external data file, when it already has non-zero data
during creation.  (Reading the qcow2 file would return zeroes, but
reading the device would not.)

So it would need to be QCOW2_CLUSTER_NORMAL.  Which is kind of the
point, when you think about it – with data-file-raw, all clusters must
always effectively be QCOW2_CLUSTER_NORMAL and be mapped 1:1.

Well, and that’s in turn the point of this patch.

I interpret the spec in that the metadata can be ignored, but it does
not need to be ignored.  So the L1/L2 tables must be 1:1 mapping of
QCOW2_CLUSTER_NORMAL entries.

We could also choose to interpret it as “With data-file-raw, the L1/L2
tables must be ignored”.  In that case, our qcow2 driver would need to
be modified to indeed fully ignore the L1/L2 tables with data-file-raw.
 (I certainly don’t interpret the spec this way, but I suppose we could
call it a bug fix and amend it.)

Max

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