Details:
- Write size should be exactly csum_block_size (4k by default)
- Write should be made into a new object (unallocated space)
- In this case, the checksum of the block was calculated as if the block was
padded with extra (2^32 - size) zero bytes due to a simple integer overflow
- As a cherry on the cake, such calculation was 'slightly' slow because it
was processing almost 4 GB of zeroes for a small write
Vitaliy Filippov