Re: [RFC PATCH 2/2] qemu-img convert: Fix sparseness detection

[Peter Lieven]

Am 20.04.21 um 18:52 schrieb Vladimir Sementsov-Ogievskiy:
> 20.04.2021 18:04, Kevin Wolf wrote:
>> Am 20.04.2021 um 16:31 hat Vladimir Sementsov-Ogievskiy geschrieben:
>>> 15.04.2021 18:22, Kevin Wolf wrote:
>>>> In order to avoid RMW cycles, is_allocated_sectors() treats zeroed areas
>>>> like non-zero data if the end of the checked area isn't aligned. This
>>>> can improve the efficiency of the conversion and was introduced in
>>>> commit 8dcd3c9b91a.
>>>>
>>>> However, it comes with a correctness problem: qemu-img convert is
>>>> supposed to sparsify areas that contain only zeros, which it doesn't do
>>>> any more. It turns out that this even happens when not only the
>>>> unaligned area is zeroed, but also the blocks before and after it. In
>>>> the bug report, conversion of a fragmented 10G image containing only
>>>> zeros resulted in an image consuming 2.82 GiB even though the expected
>>>> size is only 4 KiB.
>>>>
>>>> As a tradeoff between both, let's ignore zeroed sectors only after
>>>> non-zero data to fix the alignment, but if we're only looking at zeros,
>>>> keep them as such, even if it may mean additional RMW cycles.
>>>>
>>>
>>> Hmm.. If I understand correctly, we are going to do unaligned
>>> write-zero. And that helps.
>>
>> This can happen (mostly raw images on block devices, I think?), but
>> usually it just means skipping the write because we know that the target
>> image is already zeroed.
>>
>> What it does mean is that if the next part is data, we'll have an
>> unaligned data write.
>>
>>> Doesn't that mean that alignment is wrongly detected?
>>
>> The problem is that you can have bdrv_block_status_above() return the
>> same allocation status multiple times in a row, but *pnum can be
>> unaligned for the conversion.
>>
>> We only look at a single range returned by it when detecting the
>> alignment, so it could be that we have zero buffers for both 0-11 and
>> 12-16 and detect two misaligned ranges, when both together are a
>> perfectly aligned zeroed range.
>>
>> In theory we could try to do some lookahead and merge ranges where
>> possible, which should give us the perfect result, but it would make the
>> code considerably more complicated. (Whether we want to merge them
>> doesn't only depend on the block status, but possibly also on the
>> content of a DATA range.)
>>
>> Kevin
>>
>
> Oh, I understand now the problem, thanks for explanation.
>
> Hmm, yes that means, that if the whole buf is zero, is_allocated_sectors must 
> not align it down, to be possibly "merged" with next chunk if it is zero too.
>
> But it's still good to align zeroes down, if data starts somewhere inside the 
> buf, isn't it?
>
> what about something like this:
>
> diff --git a/qemu-img.c b/qemu-img.c
> index babb5573ab..d1704584a0 100644
> --- a/qemu-img.c
> +++ b/qemu-img.c
> @@ -1167,19 +1167,39 @@ static int is_allocated_sectors(const uint8_t *buf, 
> int n, int *pnum,
>          }
>      }
>  
> +    if (i == n) {
> +        /*
> +         * The whole buf is the same.
> +         *
> +         * if it's data, just return it. It's the old behavior.
> +         *
> +         * if it's zero, just return too. It will work good if target is 
> alredy
> +         * zeroed. And if next chunk is zero too we'll have no RMW and no 
> reason
> +         * to write data.
> +         */
> +        *pnum = i;
> +        return !is_zero;
> +    }
> +
>      tail = (sector_num + i) & (alignment - 1);
>      if (tail) {
>          if (is_zero && i <= tail) {
> -            /* treat unallocated areas which only consist
> -             * of a small tail as allocated. */
> +            /*
> +             * For sure next sector after i is data, and it will rewrite this
> +             * tail anyway due to RMW. So, let's just write data now.
> +             */
>              is_zero = false;
>          }
>          if (!is_zero) {
> -            /* align up end offset of allocated areas. */
> +            /* If possible, align up end offset of allocated areas. */
>              i += alignment - tail;
>              i = MIN(i, n);
>          } else {
> -            /* align down end offset of zero areas. */
> +            /*
> +             * For sure next sector after i is data, and it will rewrite this
> +             * tail anyway due to RMW. Better is avoid RMW and write zeroes 
> up
> +             * to aligned bound.
> +             */
>              i -= tail;
>          }
>      }
>
>

I think we forgot to follow up on this. Has anyone tested this suggestion?

Otherwise, I would try to rerun the tests I did with the my old and Kevins 
suggestion.


Peter