support concurrent "tiered" compaction
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Key: CASSANDRA-1881
URL: https://issues.apache.org/jira/browse/CASSANDRA-1881
Project: Cassandra
Issue Type: Improvement
Components: Core
Reporter: Peter Schuller
Priority: Minor
(this has been discussed on the ML:s before; I am filing it now so that there
is a ticket to refer to on the wiki)
CASSANDRA-1876 is open to allow parallel compaction for the purpose of
throughput. However, that only addresses one aspect of why parallel compaction
is useful; the other half is ensuring that compaction is proceeding in a timely
fashion at each "size tier" (for lack of a better term).
Essentially, CASSANDRA-1876 is about CPU concurrency while this is about
functional concurrency. I propose that compaction be a process which performs
some amount of compaction work per second (I'm thinking ahead to future rate
limiting; that's another ticket to be filed). That work has to be spread out
over multiple compaction tiers in a way that is not coupled with CPU
concurrency.
Suggested solution is to have N number of concurrent compaction threads going
at any given moment (CASSANDRA-1876), but to have those compaction threads
perform work for a variable number of compaction jobs. Compactions would be
triggered according to similarly sized sstables as before, but each such
compaction would be a compaction "job" that is independent of any actual
compaction thread.
Compaction threads move between compaction jobs at a coarse granularity so that
synchronization overhead is irrelevant (for example it might go and look for
other work to do every memtable_throughput_in_mb megabytes). Smaller compaction
jobs take priority over larger jobs. This is intended to keep sstable counts
down, and always leave the larger jobs as the ones having to wait given that
they are not latency sensitive anyway due to their size.
The primary downside is that disk usage spikes would much more easily reach
"double cf size" levels when many compactions are running. This is probably
something that can be mitigated by CASSANDRA-1608 with its talk of limited
sstable sizes.
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