Compaction is the consequence of the Log-Structured Merge-Tree engine used by Cassandra. Starting with the SizeTieredCompactionStrategy, we added the LeveledCompactionStrategy and recently the DateTieredCompactionStrategy it has always required some care and feeding. In this talk Aaron Morton, Co-Founder and Principal Consultant at The Last Pickle, will discuss the different strategies, their options, and when to use them.

1. SOUTH BAY CASSANDRA USERS NOVEMBER 2014
COMPACTION, COMPACTION,
EVERYWHERE.
Aaron Morton
@aaronmorton
Co-Founder & Principal Consultant
Licensed under a Creative Commons Attribution-NonCommercial 3.0 New Zealand License

2. About The Last Pickle.
Work with clients to deliver and improve Apache Cassandra
based solutions.
Apache Cassandra Committer, DataStax MVP, Apache
Usergrid Committer.
Based in New Zealand & USA.

3. Compaction?
STCS
LCS
DTCS

4. Compaction?
Because reasons.

5. No compaction?
Row fragmentation would
result in dramatically increased
read latency.

6. No compaction?
Increased file count would
increase memory usage.

7. No compaction?
Overwrites and deletions
would result in wasted disk
space.

8. Compaction?
Yes.

9. Compaction?
Log Structured Merge Tree.

10. Compaction?
Creating new files when
flushing to disk improves
performance and reduces
complexity.

11. Compaction?
SSTable 1
foo:
dishwasher (ts 10):
tomato
purple (ts 10):
cromulent
SSTable 2
foo:
frink (ts 20):
flayven
monkey (ts 10):
embiggins
SSTable 3 SSTable 4
foo:
dishwasher (ts 15):
tomacco
SSTable 5

12. Demo.

13. nodetool cfhistograms foo bar
SSTables per Read
1 sstables: 149
2 sstables: 62
3 sstables: 65
4 sstables: 50
5 sstables: 45
6 sstables: 44
7 sstables: 76
8 sstables: 72
10 sstables: 305
12 sstables: 390

14. Compaction?
STCS
LCS
DTCS

15. SizeTieredCompactionStrategy
The first compaction strategy.
Group files of a similar size for
compaction.

16. SizeTieredCompactionStrategy
Works well when data is
written to initially and then
only read from.

17. STCS - After flush.
Tier 0 ( < 50 MB) Tier 1 ~125MB

18. STCS - Compaction Starts
Tier 0 ( < 50 MB) Tier 1 ~125MB

19. STCS - New SSTable
Tier 0 ( < 50 MB) Tier 1 ~125MB

20. STCS - Purge old SSTables
Tier 0 ( < 50 MB) Tier 1 ~125MB

21. STCS - Compaction Starts (again)
Tier 0 ( < 50 MB) Tier 1 ~125MB

22. STCS - Final State
Tier 0 ( < 50 MB) Tier 1 ~200MB Tier 2 ~800MB

23. STCS - min_sstable_size
Maximum size of SSTables in
the “small” bucket.
Default 50

24. STCS - bucket_low
Lower bound of the bucket
size compared to the average
size in the bucket.
Default 0.5

25. STCS - bucket_high
Upper bound of the bucket
size compared to the average
size in the bucket.
Default 1.5

26. STCS - cold_reads_to_omit
Maximum percentage of reads
SSTables ignored by STCS may
be responsible for.
Default 0

27. min_compaction_threshold
Compact buckets with at least
this many SSTables.
Default 4

28. max_compaction_threshold
Compact no more than this
many SSTables in a bucket.
Default 32

29. Compaction?
STCS
LCS
DTCS

30. LeveledCompactionStrategy
Based on LevelDB from the
Chromium team.
http://leveldb.org/

31. LeveledCompactionStrategy
Works well with overwrites
and tombstones.
Provides low read latency.

32. LeveledCompactionStrategy
“Uses twice the disk IO”

33. DataStax Blogs
“Leveled Compaction in Apache
Cassandra”
“When to Use Leveled
Compaction”

34. LCS - “It’s going to be all levels Jerry”
Level Number
of Files
0 Unlimited*
1 100
2 1000
3 10000

35. LCS in nodetool cfstats
Column Family: HappyPandaCF
SSTable count: 21
SSTables in each level: [1, 7, 13, 0, 0, 0, 0, 0, 0]
Column Family: SadPandaCF
SSTable count: 710
SSTables in each level: [1, 10, 117/100, 582, 0, 0,
0, 0, 0]

36. LCS - Starting out
level 0

37. LCS - New File in Level 1
level 0 level 1

38. LCS - Later, Compact L0 With Overlapping L1
level 0 level 1

39. LCS - Another File in L1
level 0 level 1

40. LCS - Level 1 Full, compact overlapping
level 0 level 1

41. LCS - New Files in Level 2
level 0 level 1 level 2

42. LCS - sstable_size_in_mb
Maximum* size of each
SSTable at all levels.
Default 160

43. Compaction?
STCS
LCS
DTCS

44. DateTieredCompactionStrategy
CASSANDRA-6602
In 2.0.11 and 2.1.1
“Experimental”

45. DTCS - Compact Newest Time Bucket
4 hours 20 hours 80 hours > 365 days

46. DTCS - New File in First Bucket
4 hours 20 hours 80 hours > 365 days

47. DTCS - Promoted to Later Bucket
4 hours 20 hours 80 hours > 365 days

48. DTCS - base_time_seconds
Target size.
Multiplied by
min_sstable_size.

49. DTCS - timestamp_resolution
What TimeUnit you are
using for your WriteTime.

50. DTCS - max_sstable_age_days
Do not compact SSTables
where the youngest
WRITETIME is older than this.

51. Thanks.

52. Aaron Morton
@aaronmorton
Co-Founder & Principal Consultant
www.thelastpickle.com
Licensed under a Creative Commons Attribution-NonCommercial 3.0 New Zealand License