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From "Jonathan Ellis (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-1608) Redesigned Compaction
Date Wed, 24 Aug 2011 15:00:32 GMT


Jonathan Ellis commented on CASSANDRA-1608:

bq. I assumed that as I was doing operations on these SSTables in the referenced views I would
also need to use these referenced.

Right, but isn't that already the case pre-1608?

I'm just trying to understand if this fixes an existing bug, or if I'm missing how the usage

bq. Yes this is a potentially serious issue. This code gets called on every read.

Feels like DataTracker is the wrong granularity to be doing reference counting at.  I'd like
to see if we can push that into SSTableReader instead.  But let's do that in another ticket,
for our purposes here correctness will suffice and we can optimize later.

> Redesigned Compaction
> ---------------------
>                 Key: CASSANDRA-1608
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Chris Goffinet
>            Assignee: Benjamin Coverston
>         Attachments: 1608-22082011.txt, 1608-v2.txt, 1608-v4.txt
> 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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