The document discusses compaction and splitting in Apache Accumulo distributed key-value stores. It explains that Accumulo tables are divided into non-overlapping ranges called tablets, and that compaction merges sorted files within a tablet into a single file to improve read performance. Splitting divides large tablets into two in order to balance workload. The document provides details on Accumulo's and HBase's compaction algorithms and how they determine when to compact and split tablets.

1. Compaction and
Splitting in Apache
Accumulo
Billie Rinaldi
billie@hortonworks.com
October 24, 2012
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2. What are compaction and splitting?
•Accumulo tables are divided into
non-overlapping key ranges called
tablets
•Compaction selects a set of sorted
files for a single tablet and rewrites
them into one file
•Splitting divides a tablet into two
tablets
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3. Tablet Overview
•When memory fills, new sorted files
are created by flushing
•Sorted files are combined together
into fewer sorted files
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4. How much data are you writing?
•If you never compact – O(N)
…
•If you always compact – O(N2)
…
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5. Accumulo Compaction Algorithm
•Compact a set of files when:
size of the
largest file
×
compaction
ratio ≤ sum of the
sizes of files
table.compaction.major.ratio
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6. In Action (r = 3, N = 1, W = 1)
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7. In Action (r = 3, N = 2, W = 2)
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8. In Action (r = 3, N = 3, W = 3)
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9. In Action (r = 3, N = 3, W = 6)
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10. In Action (r = 3, N = 4, W = 7)
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11. In Action (r = 3, N = 5, W = 8)
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12. In Action (r = 3, N = 6, W = 9)
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13. In Action (r = 3, N = 6, W = 12)
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14. In Action (r = 3, N = 7, W = 13)
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15. In Action (r = 3, N = 8, W = 14)
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16. In Action (r = 3, N = 9, W = 15)
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17. In Action (r = 3, N = 9, W = 24)
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18. In Action (r = 3, N = 27, W = 90*)
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19. Amount of data written
•W(rk) = (k+1)rk – (k-1)rk-1
•Thus, W(N) ≈ O(N log N)
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20. HBase Compaction Algorithm
•Compact a set of files when:
sum of the
size of the
largest file ≤ sizes of ×
compaction
ratio
smaller files
hbase.hstore.compaction.ratio
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21. HBase Compaction Algorithm
•Compact a set of files when:
sum of the
size of the
largest file ≤ sizes of × compaction
ratio
smaller files
1
HBase ratio = Accumulo
ratio –1
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22. Other Compaction-related Properties
•Accumulo
table.file.max
tserver.compaction.major.thread.files.open.max
tserver.compaction.major.delay
table.compaction.major.everything.idle
•Hbase
hbase.hstore.compactionThreshold
hbase.hstore.blockingStoreFiles
hbase.hstore.blockingWaitTime
hbase.hstore.compaction.min
hbase.hstore.compaction.max
hbase.hstore.compaction.min.size
hbase.hstore.compaction.max.size
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23. Accumulo Splitting
•Always check to see if a split is
needed before compacting
•If it is needed, split first
•File names stored in metadata table
split
threshold
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24. Accumulo Splitting Process
•Tablet closed, no new writes
•Three writes to the metadata table
–tablet made smaller & marked as splitting
–new tablet added
–original tablet's splitting marks removed
•Tablet server swaps new tablets for
old tablet in its online tablet list
•Master informed
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25. Accumulo Splitting Recovery
•Whenever a tablet is brought online,
the tablet server checks to see if it
has split marks.
•If so, it assumes the splitting
process was interrupted and
finishes making changes to the
metadata table.
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29. Questions?
dev@accumulo.apache.org
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