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From "Benedict (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-7066) Simplify (and unify) cleanup of compaction leftovers
Date Tue, 28 Jul 2015 16:43:05 GMT


Benedict commented on CASSANDRA-7066:

bq. Is that SOL as in "Now we can't tell which is new so we have to keep both and do redundant
compaction" or as in "Now the note can't start up?"

Well, SOL in that we have no way of guaranteeing C* will be able to notice this has happened
and do anything to avoid breaking things. If we see just one of the two files present, under
non-fs failure that means we were part way through cleaning up, and need to finish deleting
its contents. So if we lose the "new" file, say, then we will delete the old files on startup
none-the-wiser. The result being a partial replacement of the sstables (perhaps with nothing
at all). There's really nothing you can do to guarantee anything in the face of a filesystem

If the operator _knows this has happened_ before starting up they can delete all the log files,
sure, and then we'll keep everything.

> Simplify (and unify) cleanup of compaction leftovers
> ----------------------------------------------------
>                 Key: CASSANDRA-7066
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Benedict
>            Assignee: Stefania
>            Priority: Minor
>              Labels: benedict-to-commit, compaction
>             Fix For: 3.0 alpha 1
>         Attachments: 7066.txt
> Currently we manage a list of in-progress compactions in a system table, which we use
to cleanup incomplete compactions when we're done. The problem with this is that 1) it's a
bit clunky (and leaves us in positions where we can unnecessarily cleanup completed files,
or conversely not cleanup files that have been superceded); and 2) it's only used for a regular
compaction - no other compaction types are guarded in the same way, so can result in duplication
if we fail before deleting the replacements.
> I'd like to see each sstable store in its metadata its direct ancestors, and on startup
we simply delete any sstables that occur in the union of all ancestor sets. This way as soon
as we finish writing we're capable of cleaning up any leftovers, so we never get duplication.
It's also much easier to reason about.

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