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From "Benedict (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-6696) Drive replacement in JBOD can cause data to reappear.
Date Tue, 22 Apr 2014 13:02:15 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-6696?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13976724#comment-13976724
] 

Benedict commented on CASSANDRA-6696:
-------------------------------------

bq. assuming that every vnode is of roughly equal side is a core part of consistent hashing.

Well the assumption is broken then. I can assure you vnodes are not of equal size, especially
not with our current allocation strategy, and getting them to be of equal size is kind of
tough. We may be able to improve that, though. 

I'm not sure how what I'm suggesting can't also provide most of these other benefits, however
we can bring the two approaches closer by simply saying all vnodes starting within the first
1/DISK portion of the token range are allocated to the first disk, and so on - and then they're
pretty similar. But the unequal size of vnodes means any compaction "tuning" will have limited
impact, and probably induce more random IO

> 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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