Re: [PATCH 1/3] qcow2: Require that the virtual size is a multiple of the sector size

[Alberto Garcia]

On Wed 08 Jan 2020 08:46:11 PM CET, Nir Soffer wrote:
>> However when an image is opened the virtual size is rounded down,
>> which means that trying to access the last few advertised bytes will
>> result in an error. As seen above QEMU cannot create such images and
>> there's no good use case that would require us to try to handle them
>> so let's just treat them as unsupported.
>
> Making error impossible is best.
>
> Can we require multiple of 4k to avoid unaligned read/write at the end
> of an image aligned to 512 bytes on storage with 4k sector size?

I wouldn't force that on the user. The only reason why I'm not allowing
the non-sector-aligned case is because it's currently broken, but I
wouldn't have a problem with it if it was working fine.

>> +echo
>> +echo "== Image size not a multiple of the sector size =="
>> +_make_test_img 64k
>
> Logging the change here would make the test and the output more clear:
>
>     echo "modifying virtual size to 65535"

Ok

>> +== Image size not a multiple of the sector size ==
>> +Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=65536
>> +qemu-io: can't open device TEST_DIR/t.qcow2: Virtual size is not a multiple 
>> of 512
>
> The output is confusing, looks like we created image with aligned
> size, and on the next line it complains about the size.

Ok, I can also replace the "qemu-io write" with a "qemu-img info" or
something like that to make it a bit less confusing (i.e. the error
happens already when you open the image, not when you write to it).

Berto