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From "Benedict (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-7066) Simplify (and unify) cleanup of compaction leftovers
Date Fri, 20 Feb 2015 16:35:11 GMT


Benedict commented on CASSANDRA-7066:

So, I'm pretty sure this is still buggy. It also doesn't integrate well with the macro compaction
actions. I'd like to propose a slight change to how we write sstables that would integrate
better with when we make changes, and that would also eliminate TMP and TMPLINK files: basically
write a mini transaction log file prior to any modification of the liveset, that outlines
what we are doing, then delete it once we're done. So:

* if we're flushing a memtable we write that the new file is "in progress"
* if we're compacting multiple files into one, we write that the new file(s) are "in progress",
then when they're done, we write a new log file saying we're swapping these files (as a checkpoint),
then clear the "in progress" log file and write that we're "deleting" the old files, followed
by immediately promoting the new ones and deleting our "swapping" log entry

On startup we just read every log file in any CF directory, and take the appropriate action:
any "in progress" or "deleting" files are deleted; if there is a "swapping" log entry, we
just pick up where we left off, deleting the old.

This would dovetail well with the changes I plan for CASSANDRA-8568. [~marcuse] [~JoshuaMcKenzie]
[~yukim] [~carlyeks] thoughts?

> Simplify (and unify) cleanup of compaction leftovers
> ----------------------------------------------------
>                 Key: CASSANDRA-7066
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Benedict
>            Priority: Minor
>              Labels: compaction
>             Fix For: 3.0
> 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or conversely not cleanup files that have been superceded); and 2) it's only used for a regular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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