Cassandra new-version binaries and previous SSTable format downgrade safety

Restoring a Cassandra node to a previous version after a failed major upgrade is generally unsupported if the new binaries have already written data or modified the system schema. The only fully supported recovery path without data loss is restoring a pre-upgrade snapshot.

The Risks of Direct Downgrades

Cassandra SSTable formats are often incompatible across major versions (e.g., 3.11 to 4.0). If the new binaries started and wrote to disk, they may have:

• Updated SSTable Format: Written data files with a newer version header that the older binary does not recognize.
• Modified Schema: If the upgrade triggered schema changes, the system_schema keyspaces have been updated. Older binaries may fail to start or experience silent corruption when encountering schema definitions they cannot understand.
• Compression/Indexing: Newer versions often introduce default compression algorithms or index structures that the older engine cannot parse.

How to Verify Compatibility

If you must attempt to verify if the data is still readable, you should inspect the SSTable headers and the schema state. Use the sstables-tools utility (included in the Cassandra distribution) to check the file version:

sstables-tools meta /path/to/data/table-name-sdb

Look for the Version field in the output. If this version is higher than the maximum version supported by your target older binary, the node will fail to start.

Recommended Recovery Steps

Since the node already failed mid-upgrade, follow these steps to ensure data integrity:

1. Stop the process: Ensure no Cassandra processes are running to prevent further writes.
2. Clear the data directory: Move the corrupted/upgraded data directory to a backup location.
3. Reinstall Previous Binaries: Reinstall the original stable Cassandra version.
4. Restore from Snapshot: Deploy your pre-upgrade snapshot into the data directory.
5. Repair: Once the node is up, run nodetool repair to ensure consistency across the cluster.

Note: The safety of a downgrade depends entirely on the specific versions. For example, moving back from 4.0 to 3.11 is significantly more dangerous than moving from 3.11 to 3.0.