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From "Marcus Eriksson (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-6696) Drive replacement in JBOD can cause data to reappear.
Date Thu, 01 May 2014 17:37:19 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-6696?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13986799#comment-13986799
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Marcus Eriksson commented on CASSANDRA-6696:
--------------------------------------------

this special cases compaction a bit though, we could have sstables that overlap with other
sstables of similar size that we can't really compact together (which we probably shouldn't
since they overlap too little (CASSANDRA-6474)).

for LCS i guess we could align the vnode start/end to the sstables start/end. Ie, in level
1 (10 sstables) each sstable would contain ~100 vnodes, in level2 (100 sstables) ~10, and
in level3 (1000 sstables) 1 vnode. Then we could flush sstables mapping to the sstables in
level1 to only compact those together.


> Drive replacement in JBOD can cause data to reappear. 
> ------------------------------------------------------
>
>                 Key: CASSANDRA-6696
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-6696
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: sankalp kohli
>            Assignee: Marcus Eriksson
>             Fix For: 3.0
>
>
> In JBOD, when someone gets a bad drive, the bad drive is replaced with a new empty one
and repair is run. 
> This can cause deleted data to come back in some cases. Also this is true for corrupt
stables in which we delete the corrupt stable and run repair. 
> Here is an example:
> Say we have 3 nodes A,B and C and RF=3 and GC grace=10days. 
> row=sankalp col=sankalp is written 20 days back and successfully went to all three nodes.

> Then a delete/tombstone was written successfully for the same row column 15 days back.

> Since this tombstone is more than gc grace, it got compacted in Nodes A and B since it
got compacted with the actual data. So there is no trace of this row column in node A and
B.
> Now in node C, say the original data is in drive1 and tombstone is in drive2. Compaction
has not yet reclaimed the data and tombstone.  
> Drive2 becomes corrupt and was replaced with new empty drive. 
> Due to the replacement, the tombstone in now gone and row=sankalp col=sankalp has come
back to life. 
> Now after replacing the drive we run repair. This data will be propagated to all nodes.

> Note: This is still a problem even if we run repair every gc grace. 
>  



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