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From "Benedict (JIRA)" <>
Subject [jira] [Created] (CASSANDRA-9834) Merge Hints/CommitLog/BatchLog
Date Thu, 16 Jul 2015 18:00:04 GMT
Benedict created CASSANDRA-9834:

             Summary: Merge Hints/CommitLog/BatchLog
                 Key: CASSANDRA-9834
             Project: Cassandra
          Issue Type: Improvement
          Components: Core
            Reporter: Benedict
             Fix For: 3.x

As discussed briefly on CASSANDRA-6230, it should be quite possible to construct a single
log that can serve as commit log, hints log and batch log. The basic idea would be to write
sequentially, marking messages as members of one or more logical logs. We have a separate
efficient (possibly embedded) ledger for invalidation of log records. As entire log segments
become invalidated, we simply delete them; the rest we accumulate until we hit a high watermark,
and have segments that are at least half empty, at which point we begin rewriting the emptiest.

This absolutely bounds our worst case sequential IO at 2x that used by _just_ the commit log,
with normal operation under sufficiently high watermark having zero overhead. The upper bound
for space utilisation is the smaller of 2x the actual amount of data stored, and our high
watermark. This gives us batch and hints for free, and eliminates OOMs from hints.

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