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From "Nick Bailey (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-9195) commitlog replay only actually replays mutation every other time
Date Thu, 23 Apr 2015 16:16:40 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-9195?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14509286#comment-14509286
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Nick Bailey commented on CASSANDRA-9195:
----------------------------------------

bq. Because commitlog is acting correctly, it should not replay truncated data regardless
of where it comes from. In this particular case sstable loader is leaving the table in a bad
state; rather than try to live with it I prefer to correct it at the origin.

bq. The PITR process includes a truncation step. What I am doing here is giving PITR the option
to correctly continue after that step.

I don't think this is right. The attached reproduction step does include a truncation step
but that step is just there to clear the database before verifying that restore works. It
could be replaced with drop and recreate (and then you wouldn't see this bug). But in any
case, users may end up truncating and then regretting it and trying to restore from commitlogs.

I don't think sstableloader is the right place for this because restoring from a snapshot
is not required for point in time restore. You can simply archive every commitlog from the
start rather than ever taking snapshots if you want. Then if you go and replay those commitlogs
up to some time before the truncation C* should recognize that the replay is strictly before
any truncation took place and let things replay.

Also, I still think it seems like truncation records should exist in the commitlogs themselves,
or at the very least be a historical list of truncations.

> commitlog replay only actually replays mutation every other time
> ----------------------------------------------------------------
>
>                 Key: CASSANDRA-9195
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-9195
>             Project: Cassandra
>          Issue Type: Bug
>            Reporter: Jon Moses
>            Assignee: Branimir Lambov
>            Priority: Critical
>             Fix For: 2.1.5
>
>         Attachments: 9195-v2.1.patch, loader.py
>
>
> Version: Cassandra 2.1.4.374 | DSE 4.7.0
> The main issue here is that the restore-cycle only replays the mutations
> every other try.  On the first try, it will restore the snapshot as expected
> and the cassandra system load will show that it's reading the mutations, but
> they do not actually get replayed, and at the end you're left with only the
> snapshot data (2k records).
> If you re-run the restore-cycle again, the commitlogs are replayed as expected,
> and the data expected is present in the table (4k records, with a spot check of 
> record 4500, as it's in the commitlog but not the snapshot).
> Then if you run the cycle again, it will fail.  Then again, and it will work. The work/
> not work pattern continues.  Even re-running the commitlog replay a 2nd time, without
> reloading the snapshot doesn't work
> The load process is:
> * Modify commitlog segment to 1mb
> * Archive to directory
> * create keyspace/table
> * insert base data
> * initial snapshot
> * write more data
> * capture timestamp
> * write more data
> * final snapshot
> * copy commitlogs to 2nd location
> * modify cassandra-env to replay only specified keyspace
> * modify commitlog properties to restore from 2nd location, with noted timestamp
> The restore cycle is:
> * truncate table
> * sstableload snapshot
> * flush
> * output data status
> * restart to replay commitlogs
> * output data status
> ====
> See attached .py for a mostly automated reproduction scenario.  It expects DSE (and I
found it with DSE 4.7.0-1), rather than "actual" Cassandra, but it's not using any DSE specific
features.  The script looks for the configs in the DSE locations, but they're set at the top,
and there's only 2 places where dse is restarted.



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