Diagnosing Problems in Production - Cassandra
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1) The document discusses various tools for diagnosing problems in Cassandra production environments, including OpsCenter for monitoring, application metrics collection with Statsd/Graphite, and log aggregation with Splunk or Logstash. 2) Some common issues covered are incorrect server times causing data inconsistencies, tombstone overhead slowing queries, not using the proper snitch, and disk space not being reclaimed on new nodes. 3) Diagnostic tools described are htop, iostat, vmstat, dstat, strace, tcpdump, and nodetool for investigating process activity, disk usage, memory, networking, and Cassandra-specific statistics. GC profiling and query tracing are also recommended.
Diagnosing Problems in Production - Cassandra
- 1. ©2013 DataStax Confidential. Do not distribute without consent. Jon Haddad, Technical Evangelist @rustyrazorblade Diagnosing Problems in Production 1
- 3. DataStax OpsCenter • Will help with 90% of problems you encounter • Should be first place you look when there's an issue • Community version is free • Enterprise version has additional features
- 4. Server Monitoring & Alerts • Monit • monitor processes • monitor disk usage • send alerts • Munin / collectd • system perf statistics • Nagios / Icinga • Various 3rd party services • Use whatever works for you
- 5. Application Metrics • Statsd / Graphite • Grafana • Gather constant metrics from your application • Measure anything & everything • Microtimers, counters • Graph events • user signup • error rates • Cassandra Metrics Integration • jmxtrans
- 6. Log Aggregation • Hosted - Splunk, Loggly • OSS - Logstash + Kibana, Greylog • Many more… • For best results all logs should be aggregated here • Oh yeah, and log your errors.
- 7. Gotchas
- 8. Incorrect Server Times • Everything is written with a timestamp • Last write wins • Usually supplied by coordinator • Can also be supplied by client • What if your timestamps are wrong because your clocks are off? • Always install ntpd! server time: 10 server time: 20 INSERT real time: 12 DELETE real time: 15 insert:20 delete:10
- 9. Tombstones • Tombstones are a marker that data no longer exists • Tombstones have a timestamp just like normal data • They say "at time X, this no longer exists"
- 10. Tombstone Hell • Queries on partitions with a lot of tombstones require a lot of filtering • This can be reaaaaaaally slow • Consider: • 100,000 rows in a partition • 99,999 are tombstones • How long to get a single row? • Cassandra is not a queue! read 99,999 tombstones finally get the right data
- 11. Not using a Snitch • Snitch lets us distribute data in a fault tolerant way • Changing this with a large cluster is time consuming • Dynamic Snitching • use the fastest replica for reads • RackInferring (uses IP to pick replicas) • DC aware • PropertyFileSnitch (cassandra-topology.properties) • EC2Snitch & EC2MultiRegion • GoogleCloudSnitch • GossipingPropertyFileSnitch (recommended)
- 12. Version Mismatch • SSTable format changed between versions, making streaming incompatible • Version mismatch can break bootstrap, repair, and decommission • Introducing new nodes? Stick w/ the same version • Upgrade nodes in place • One at a time • One rack / AZ at a time (requires proper snitch)
- 13. Disk Space not Reclaimed • When you add new nodes, data is streamed from existing nodes • … but it's not deleted from them after • You need to run a nodetool cleanup • Otherwise you'll run out of space just by adding nodes
- 14. Using Shared Storage • Single point of failure • High latency • Expensive • Performance is about latency • Can increase throughput with more disks • Avoid EBS, SAN, NAS
- 15. Compaction • Compaction merges SSTables • Too much compaction? • Opscenter provides insight into compaction cluster wide • nodetool • compactionhistory • getcompactionthroughput • Leveled vs Size Tiered • Leveled on SSD + Read Heavy • Size tiered on Spinning rust • Size tiered is great for write heavy time series workloads
- 16. Diagnostic Tools
- 17. htop • Process overview - nicer than top
- 18. iostat • Disk stats • Queue size, wait times • Ignore %util
- 19. vmstat • virtual memory statistics • Am I swapping? • Reports at an interval, to an optional count
- 20. dstat • Flexible look at network, CPU, memory, disk
- 21. strace • What is my process doing? • See all system calls • Filterable with -e • Can attach to running processes
- 22. tcpdump • Watch network traffic
- 23. nodetool tpstats • What's blocked? • MemtableFlushWriter? - Slow disks! • also leads to GC issues • Dropped mutations? • need repair!
- 24. Histograms • proxyhistograms • High level read and write times • Includes network latency • cfhistograms <keyspace> <table> • reports stats for single table on a single node • Used to identify tables with performance problems
- 25. Query Tracing
- 27. JVM GC Overview • What is garbage collection? • Manual vs automatic memory management • Generational garbage collection (ParNew & CMS) • New Generation • Old Generation
- 28. New Generation • New objects are created in the new gen (eden) • Comprised of Eden & 2 survivor spaces (SurvivorRatio) • Space identified by HEAP_NEWSIZE in cassandra-env.sh • Historically limited to 800MB
- 29. Minor GC • Occurs when Eden fills up • Stop the world • Dead objects are removed • Copy current survivor to empty survivor • Live objects are promoted into survivor (S0 & S1) then old gen • Survivor objects promoted to old gen (MaxTenuringThreshold) • Spillover promoted to old gen • Removing objects is fast, promoting objects is slow
- 30. Old Generation • Objects are promoted to new gen from old gen • Major GC • Mostly concurrent • 2 short stop the world pauses
- 31. Full GC • Occurs when old gen fills up or objects can’t be promoted • Stop the world • Collects all generations • Defragments old gen • These are bad! • Massive pauses
- 32. Workload 1: Write Heavy • Objects promoted: Memtables • New gen too big • Remember: promoting objects is slow! • Huge new gen = potentially a lot of promotion new gen old gen too much promotion
- 33. Workload 2: Read Heavy • Short lived objects being promoted into old gen • Lots of minor GCs • Read heavy workloads on SSD • Results in frequent full GC new gen old gen (full of short lived objects) early promotion fills up quickly
- 34. GC Profiling • Opscenter gc stats • Look for correlations between gc spikes and read/write latency • Cassandra GC Logging • Can be activated in cassandra-env.sh • jstat • prints gc activity
- 35. GC Profiling • What to look out for: • Long, multi-second pauses • Caused by Full GCs. Old gen is filling up faster than the concurrent GC can keep up with it. Typically means garbage is being promoted out of the new gen too soon • Long minor GC • Many of the objects in the new gen are being promoted to the old gen. • Most commonly caused by new gen being too big • Sometimes caused by objects being promoted prematurely
- 36. How much does it matter?
- 37. Stuff is broken, fix it!
- 38. Narrow Down the Problem • Is it even Cassandra? Check your metrics! • Nodes flapping / failing • Check ops center • Dig into system metrics • Slow queries • Find your bottleneck • Check system stats • JVM GC • Compaction • Histograms • Tracing
- 39. ©2013 DataStax Confidential. Do not distribute without consent. 39