Database / ScyllaDB Interview questions
How do you troubleshoot high read latency in ScyllaDB?
- Check for wide partitions - a large partition takes longer to scan and can dominate latency for that key.
- Check tombstone counts in query tracing - excessive tombstones force reads to skip past dead data.
- Review the consistency level - QUORUM or ALL reads pay more coordination latency than ONE or LOCAL_QUORUM.
- Look at SSTable count per read - a high count (common with STCS under heavy churn) suggests switching to LCS or tuning compaction.
- Check cache hit rates - frequent misses on the row/key cache push more reads to disk.
- Confirm the driver is shard-aware - a non-shard-aware driver adds an internal hop on every request.
ScyllaDB's built-in tracing (TRACING ON in cqlsh, or the Monitoring Stack's per-query breakdown) is usually the fastest way to see exactly where time is going for a specific slow query, rather than guessing from cluster-wide metrics alone.
What does excessive tombstone-skipping during a read usually indicate?
What tool shows exactly where time is spent for one specific slow query?
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