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From "Jonathan Ellis (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-2976) Severe bug on commit log replay
Date Sat, 30 Jul 2011 13:05:09 GMT


Jonathan Ellis commented on CASSANDRA-2976:

Looks like a non-problem to me -- I'm not sure why it bothers to sort at all (probably so
that if replay is aborted you end up with data from roughly the order it was received in)
but the conflict resolution is such that the same result will be arrived at no matter the
order in which updates are applied.

It's also worth pointing out that each CL segment is replayed independently, there is no "this
must be the last segment, skip the rest" logic anywhere.

> Severe bug on commit log replay
> -------------------------------
>                 Key: CASSANDRA-2976
>                 URL:
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Core
>    Affects Versions: 0.6.13
>            Reporter: Zhu Han
>   Original Estimate: 4h
>  Remaining Estimate: 4h
> For 0.6, the replay order of commit log is determined by modify time(mtime) of commit
log files. The roll and update of a new log segment can be finished under 1ms if the log device
is backed up by BBU or it is a SSD with super capacitor. So the last log segment and previous
segment can have the same mtime.
> While File#listFiles() does not guarantee the order of return files, after sorting by
mtime, it is possible that the new log segment are replayed before old log segment.  This
can causes data loss!
> Here is the output of log,  you can observe from the timestamp in the commit log file
names that new log segment are replayed before old segment...
>  INFO [main] 2011-07-30 01:21:07,569 (line 171) Replaying /var/lib/cassandra/commitlog/CommitLog-1311959800795.log,
/var/lib/cassandra/commitlog/CommitLog-1310661748573.log, /var/lib/cassandra/commitlog/CommitLog-1311838097776.log
>  INFO [main] 2011-07-30 01:21:07,571 (line 286) Finished reading /var/lib/cassandra/commitlog/CommitLog-1311959800795.log
>  INFO [main] 2011-07-30 01:21:15,813 (line 286) Finished reading /var/lib/cassandra/commitlog/CommitLog-1310661748573.log
>  INFO [main] 2011-07-30 01:21:15,865 (line 286) Finished reading /var/lib/cassandra/commitlog/CommitLog-1311838097776.log
>  INFO [main] 2011-07-30 01:21:15,926 (line 174) Log replay complete
> I have not checked 0.7.x and 0.8.x, I suppose they both have the same problem.

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