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

[Peter Lieven]

Am 19.05.21 um 18:48 schrieb Kevin Wolf:
> Am 19.05.2021 um 15:24 hat Peter Lieven geschrieben:
>> 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.
> I noticed earlier this week that these patches are still in my
> development branch, but didn't actually pick it up again yet. So feel
> free to try it out.


It seems this time I forgot to follow up. Is this topic still open?


Best,

Peter