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From "Jonathan Ellis (Updated) (JIRA)" <>
Subject [jira] [Updated] (CASSANDRA-3854) Lower levels should have higher priority in LeveledCompactionStrategy
Date Thu, 09 Feb 2012 03:57:59 GMT


Jonathan Ellis updated CASSANDRA-3854:

             Priority: Minor  (was: Major)
    Affects Version/s:     (was: 1.1)
> Lower levels should have higher priority in LeveledCompactionStrategy
> ---------------------------------------------------------------------
>                 Key: CASSANDRA-3854
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>    Affects Versions: 1.0.0
>            Reporter: Stu Hood
>            Priority: Minor
>              Labels: compaction, leveled
> In LeveledCompactionStrategy, there is a comment explaining why compactions prioritize
the top levels:
> bq. So instead, we force compacting higher levels first.  This may not minimize the number
of reads done as quickly in the short term, but it minimizes the i/o needed to compact optimially
which gives us a long term win.
> The argument is that compacting the lower levels first causes data to be re-compacted
more frequently.
> But the result of compacting the top levels first is that each compaction pass does less
total work, because the number of overlapping files in a particular level is limited to the
number of files that can be generated by a single compaction pass.
> Consider the overload example from that comment:
> {quote}
> L0: 988 [ideal: 4]
> L1: 117 [ideal: 10]
> L2: 12  [ideal: 100]
> {quote}
> Assuming that there are initially no overlapping files in L2 or L1, the current implementation
> # Compact 32 files from L0 into L1 (which will cause 32 L0 files and 117 L1 files to
be compacted together)
> # Possibly compact L1 and L2 in order to remove overlapping files due to the L0 compaction
> # Repeat
> The problem with this strategy is that only every 3rd compaction will be working to drain
L0. Additionally, each compaction that occurs in L1 and L2 will be doing the minimum possible
work that can trigger a compaction (the equivalent of SizeTieredStrategy triggering the min
> ----
> If we agree that the goal is to ensure that L0 is drained (since a proliferation of tiny
overlapping files means worst case read behavior), then a better strategy would be to start
from the lower levels, and to change compaction of L0 to not include L1. With this strategy,
the steps would be:
> # Compact 32 files from L0 (causing them to be partitioned and placed in L1)
> # Repeat
> # Compact ?? files from LN into LN+1 (existing strategy)
> # Repeat
> With this approach, L0 is quickly partitioned, considerably shortening the time during
which 988 files need to be checked. Additionally, because each level is completely drained
before moving to the next, compactions occurring within a level are triggering the equivalent
of SizeTieredStrategy's max threshold (aka, always hitting a cap of 32 involved files.)

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