Report from: https://asktug.com/t/topic/65715/12

If users only set the TiFlash replica number of a table to zero but not drop that table. The data in TiFlash will only mark deleted by deleteRange.
Usually, the data out of dated will be compacted by MergeDelta or other compaction tasks. But all those tasks are triggered by the coprocessor read request or data written from Raft. If users set the TiFlash replica number to zero, then the outdated data won't be compacted and the disk space is wasted until users drop that table/partition in TiDB.