咨询Cassandra中sstablemetadata输出的Dropped timestamps含义

Let's break this down clearly:

The Dropped timestamps field (displayed as epoch timestamps) in the sstablemetadata tool's output refers to the set of timestamp thresholds for deleted data (tombstones) that Cassandra has confirmed are safe to permanently remove from the target SSTable.

Here's a deeper dive into what this means in practice:

• When you delete data in Cassandra, it doesn't get erased immediately. Instead, Cassandra writes a tombstone to mark the data as deleted. Tombstones stick around for the duration of your cluster's gc_grace_seconds setting to ensure all nodes in the cluster have synced the deletion.
• Once a tombstone exceeds the gc_grace_seconds window and the SSTable goes through a compaction cycle, Cassandra will clean up those expired tombstones. The Dropped timestamps entry logs the timestamps associated with these cleaned-up tombstones. Put simply: any deletion operations with timestamps older than these values have been fully removed from the SSTable and won't interfere with future queries or compactions.
• This metric is useful for ops teams to verify that tombstone cleanup is working as expected, check if gc_grace_seconds is configured appropriately, and get visibility into the SSTable's cleanup history.

For example, if you see Dropped timestamps: [1620000000, 1630000000] in the output, that means all tombstones with timestamps before those epoch values have been successfully purged from the SSTable.

内容的提问来源于stack exchange，提问作者Payal