Lower levels should have higher priority in LeveledCompactionStrategy
---------------------------------------------------------------------
Key: CASSANDRA-3854
URL: https://issues.apache.org/jira/browse/CASSANDRA-3854
Project: Cassandra
Issue Type: Improvement
Components: Core
Affects Versions: 1.1
Reporter: Stu Hood
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 will:
# 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 threshold).
----
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.)
--
This message is automatically generated by JIRA.
If you think it was sent incorrectly, please contact your JIRA administrators:
https://issues.apache.org/jira/secure/ContactAdministrators!default.jspa
For more information on JIRA, see: http://www.atlassian.com/software/jira
