Compaction works on the last 10%/20% of the file if it fits in the first 90%/80% of the file. However, while this condition might not be satisfied, we could still try to move blocks to make potentially bigger holes.

If we manage to move blocks towards the start of the file, even though we may not be able to truncate the file, this means free space for incoming data (in this case, the file size does not increase), or it can lead to bigger holes and the data compaction could not move at first could be moved now.