When appending new data to a file whose last block is a partial block, the diskspace quota usage is not correctly update. For example, suppose the block size is 1024 bytes, and a file has size 1536 bytes (1.5 blocks). If we then append another 1024 bytes to the file, the diskspace usage for this file will not be updated to (2560 * replication) as expected, but (2048 * replication).
The cause of the issue is that in FSNamesystem#commitOrCompleteLastBlock, we have
This code assumes that the last block of the file has never been completed before, thus is always counted with the preferred block size in quota computation.