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From "Daniel Doubleday (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-3862) RowCache misses Updates
Date Mon, 06 Feb 2012 21:27:00 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-3862?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13201610#comment-13201610
] 

Daniel Doubleday commented on CASSANDRA-3862:
---------------------------------------------

Hmokay ... don't want to abuse Jira as an educational forum but maybe as a reward for the
bugreport :-) ... are you saying that a reader could see a memtable view where flushing memtables
are gone (flushed) and sstables don't contain the flushed memtables?

If that's the case than yes the cache would lose an update. But that what also imply that
a read could miss an update without caching being in place at all no?

Otherwise (and that's how I read the code) given that the memtable switch will only happen
after the merge the reader will read all updates because they are either in (flushing) memtables
or in sstables and the cache will be in fact valid.


                
> RowCache misses Updates
> -----------------------
>
>                 Key: CASSANDRA-3862
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-3862
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Core
>    Affects Versions: 1.0.7
>            Reporter: Daniel Doubleday
>         Attachments: include_memtables_in_rowcache_read.patch
>
>
> While performing stress tests to find any race problems for CASSANDRA-2864 I guess I
(re-)found one for the standard on-heap row cache.
> During my stress test I hava lots of threads running with some of them only reading other
writing and re-reading the value.
> This seems to happen:
> - Reader tries to read row A for the first time doing a getTopLevelColumns
> - Row A which is not in the cache yet is updated by Writer. The row is not eagerly read
during write (because we want fast writes) so the writer cannot perform a cache update
> - Reader puts the row in the cache which is now missing the update
> I already asked this some time ago on the mailing list but unfortunately didn't dig after
I got no answer since I assumed that I just missed something. In a way I still do but haven't
found any locking mechanism that makes sure that this should not happen.
> The problem can be reproduced with every run of my stress test. When I restart the server
the expected column is there. It's just missing from the cache.
> To test I have created a patch that merges memtables with the row cache. With the patch
the problem is gone.
> I can also reproduce in 0.8. Haven't checked 1.1 but I haven't found any relevant change
their either so I assume the same aplies there.

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