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From "Stu Hood (JIRA)" <>
Subject [jira] Commented: (CASSANDRA-1608) Redesigned Compaction
Date Tue, 12 Oct 2010 21:28:33 GMT


Stu Hood commented on CASSANDRA-1608:

One way to provide locality of reference for sstables would be to persist summaries of individual
rows which 'supersede' the content in sstables written before them. For example, if you have
five sstables containing key 'A', you would create a new sstable #6 containing all content
for 'A', and marked as superseding for 'A'. Then, since you have a full copy of data for 'A',
you no longer need to read from the other sstables.

But how would a reader know which sstables to check for a particular key? We have sstable
generation numbers, but they are currently only used as unique ids. Three approaches:
# The 'superseding' mark for a particular key could indicate which generations it superseded,
so we could prevent reads to superseded sstables
# If we refactored the system to read memtables/sstables in generation order, we could stop
looking for content for A when we reached an sstable that superseded older sstables
# When superseding a value, we could delete it from the bloom filters for the superseded sstables.
While hella cool, this solution  would be a big change:
** If you reached a threshold of emptiness in the bloom filter, you could perform single-sstable-compactions
filtered by values that still match
** Requires a bloom filter which supports deletes (ours don't yet)

> Redesigned Compaction
> ---------------------
>                 Key: CASSANDRA-1608
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Chris Goffinet
>             Fix For: 0.7.1
> After seeing the I/O issues in CASSANDRA-1470, I've been doing some more thinking on
this subject that I wanted to lay out.
> I propose we redo the concept of how compaction works in Cassandra. At the moment, compaction
is kicked off based on a write access pattern, not read access pattern. In most cases, you
want the opposite. You want to be able to track how well each SSTable is performing in the
system. If we were to keep statistics in-memory of each SSTable, prioritize them based on
most accessed, and bloom filter hit/miss ratios, we could intelligently group sstables that
are being read most often and schedule them for compaction. We could also schedule lower priority
maintenance on SSTable's not often accessed.
> I also propose we limit the size of each SSTable to a fix sized, that gives us the ability
to  better utilize our bloom filters in a predictable manner. At the moment after a certain
size, the bloom filters become less reliable. This would also allow us to group data most
accessed. Currently the size of an SSTable can grow to a point where large portions of the
data might not actually be accessed as often.

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