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From "Branimir Lambov (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-8496) Remove MemtablePostFlusher
Date Fri, 20 May 2016 09:21:12 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-8496?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15293060#comment-15293060
] 

Branimir Lambov commented on CASSANDRA-8496:
--------------------------------------------

I did not fully appreciate how much correctness relies on {{PostFlush}}. There are several
aspects to letting flushes continue after a stuck/failed one:
- Commit log must track unflushed intervals to be aware if there's a flushing "hole", to avoid
discarding the relevant segments. (1)
- Replay must be able to see such unflushed intervals, even after later flushes and compaction.
(2)
- Snapshotting should be able to tell if there are unflushed intervals, i.e. holes in the
snapshot. Should we error out if this is the case? (3)
- Truncation should be able to deal with a flush of old data completing after truncation.
(4)
- We should be able to re-attempt flushes to solve transient failures (e.g. space ran out
but was then made available). (5)

Bonus complication:
- There is potential replay order problem involving table metadata: if an incompatible change
to a table's metadata was flushed, but older data remained unflushed, commit log replay will
attempt to apply old data on new format which could fail, resulting in data loss on the node.
(6)

Anything I'm missing?

CASSANDRA-9669 made sstables track intervals of replay positions and added replay machinery
to replay unflushed intervals that may be earlier than the latest flush position. CASSANDRA-11828
expands on this to track sets of flushed intervals in commit log segments and sstables, which
finishes (1) and (2). I am currently looking to address (5) which would enable (3) and (4)
to see a successful flush round as a guarantee of lack of holes.

I am a little worried about (6). Is there anything I may be missing that could help with this
scenario?


> Remove MemtablePostFlusher
> --------------------------
>
>                 Key: CASSANDRA-8496
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-8496
>             Project: Cassandra
>          Issue Type: Improvement
>            Reporter: Benedict
>            Assignee: Branimir Lambov
>            Priority: Minor
>
> To improve clearing of the CL, prevent infinite growth, and ensure the prompt completion
of tasks waiting on flush in the case of transient errors, large flushes or slow disks, in
2.1 we could eliminate the post flusher altogether. 
> Since we now enforce that Memtables track contiguous ranges, a relatively small change
would permit Memtables to know the exact minimum as well as the currently known exact maximum.
The CL could easily track the total dirty range, knowing that it must be contiguous, by using
an AtomicLong instead of an AtomicInteger, and tracking both the min/max seen, not just the
max. The only slight complexity will come in for tracking the _clean_ range as this can now
be non-contiguous, if there are 3 memtable flushes covering the same CL segment, and one of
them completes later. To solve this we can use an interval tree since these operations are
infrequent, so the extra overhead is nominal. Once the interval tree completely overlaps the
dirty range, we mark the entire dirty range clean.



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