Manage your compactions before they manage you! This document discusses compaction in Cassandra databases. It covers why compaction is needed, different compaction strategies like size-tiered, levelled, and time-series, how to tune strategies, monitoring compactions, and disk tuning tips. The key messages are to select the right strategy for your usage, monitor compactions closely, and tune disk configuration, as compactions involve heavy I/O that must be well managed.

1. Manage your compactions before they manage you!

2. About Pythian
18 Years of Data
infrastructure
•management consulting
•200+ Top brands
•6000+ databases under
•management
•Over 300 DBA’s, in 29
countries
•Top 5% of DBA work force, 9
•Oracle ACE’s, 2 Microsoft
MVP’ 2© 2015. All Rights Reserved.

3. About Me
•Cassandra Consultant
–First contact was 0.8
•Cassandra MVP & Datastax
Certified Architect
•Lisbon Cassandra Meetup
•Passion for distributed
systems
•Loves a good challenge
•Waterpolo is my sport
•@cjrolo
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4. 1 Why Compact
2 Compaction strategies and tuning
3 General tuning
4 Take aways, compacted
5 Q&A
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5. Compaction Impact
•High I/O usage
•Temporary increase on disk space
•High CPU usage
•Increase latency on operations!
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6. Commonly heard
•"My system is compacting 100% of the time"
•"All disk I/O is used by compaction"
•"Compaction is far behind"
•"Hundreds or thousands of SSTables!"
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7. •Quick recap on the write path
–Commitlog -> Memtable -> SSTable
–SSTables are immutable
Why do we compact?
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8. •Tombstones
•Row duplicates
•Rows spread across multiple SSTables
•Consolidation of data is imperative
Why do we compact? (2)
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9. Size-tiered compaction
•The original compaction type!
•"This strategy triggers a minor compaction when there are a
number of similar sized SSTables on disk as configured by the
table subproperty, min_threshold. A minor compaction does not
involve all the tables in a keyspace"
•When to use: write once data, write heavy scenarios, limited I/O
available
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10. Tuning size-tiered
•CQL Properties:
–bucket_high
–bucket_low
–cold_reads_to_omit
–max_threshold
–min_threshold
–min_sstable_size
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11. Levelled compaction
•Strategy appeared in Cassandra 1.0.
•"The leveled compaction strategy creates SSTables of a fixed,
relatively small size (160 MB by default) that are grouped into
levels. Within each level, SSTables are guaranteed to be non-
overlapping. Each level (L0, L1, L2 and so on) is 10 times as large
as the previous."
•When to use: Random reads, Reads that are latency sensitive,
highly updated rows
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12. Tuning levelled compaction
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•CQL Properties:
–sstable_size_in_mb

13. Time-series compaction
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•Available since Cassandra 2.0.11, 2.1.1
•"DateTieredCompactionStrategy stores data written within a
certain period of time in the same SSTable"
•When to use? Time series data!

14. •Time-Series Tuning
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•CQL Properties:
–base_time_seconds
–max_sstable_age_days
–max_threshold
–min_threshold
–timestamp_resolution

15. DTCS Quirks...
•Out-of-order data
–Hints
–Clients not in sync
–Repairs
–Someone inserted out-of-order data...
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16. Monitoring Compaction
•CFStats
•Nodetool Compactionstats
•CompactionManagerMBean:
–CompletedTasks: Number of completed compactions since the last start of this Cassandra
instance
–PendingTasks: Number of estimated tasks remaining to perform
–ColumnFamilyInProgress: The table currently being compacted.
–BytesTotalInProgress: Total number of data bytes (index and filter are not included) being
compacted.
–BytesCompacted: The progress of the current compaction.
•Strace
•iotop
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17. Disk Tuning
•Compaction means large I/O
•Big RAID stripes
•SSDs!!
•Dedicated non-striped disks
•No SAN/NAS
•I/O scheduler can have some impact
•Some linux settings can be used for emergencies.
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18. Take Aways, compacted
•What did we learn here...
–Selecting the proper compaction strategy can improve your cluster
performance
•Doing the opposite can create serious issues...
–Monitor your compactions!
–You can try compactions strategies out without changing your tables!
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19. Q&A
•Thanks for listening!
•Questions?
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20. Thank you