The work in SERVER-80124 led to this specific scenario:

• Compaction is executed on a table where blocks from the end of the file can be re-written to the start of the file.
• In parallel and, before the blocks selected by compaction are re-written, data is inserted into those blocks at the start of the file by another writer.
  • This can happen as the first-fit strategy is set on the file as soon as compaction starts.
• Compaction does little work and keeps trying which impacts performance.

The test should implement the scenario described above and understand why compaction continues and where are the blocks re-written.

In theory, if the previously freed blocks are no longer available, it is possible that compaction may extend the file to rewrite the selected blocks bu t this needs to be proven.