Adding non-TTL rows into a table that is mostly TTL-ed does harm the compaction process and causes disk size to grow.

Here is the summarized reason:

• Tombstones Aren't Cleared: When TTL data expires, it becomes a "tombstone" (a deletion marker). For these tombstones to be permanently deleted during compaction, the entire data partition they belong to must be eligible for cleanup.

• Non-TTL Rows Keep Partitions Alive: The non-TTL rows you inserted will never expire. They act as permanent anchors within their data partitions, preventing the database from ever considering the partition fully reclaimable.

• The Result: The compaction process is forced to keep the tombstones for all the expired rows within those mixed partitions, leading directly to:

  • Increased Disk Usage: From undeleted tombstones.

  • Slower Read Performance: Queries have to scan over a growing number of tombstones.