Trie-based SSTable format replaces BTree SSTables

Cassandra 5.0 introduces trie-based SSTable format (CEP-25), changing on-disk data structure from BTree format. This affects disk space usage, compaction performance, and read access patterns.

Monitor cassandra_sstable_live_disk_space_used for disk space changes. Track cassandra_compaction_size_compacted and cassandra_compaction_tasks_completed to detect compaction performance differences. Plan for SSTable format migration during upgrades. Baseline cassandra_live_ss_table count and cassandra_client_request_read_time_50p.