[jira] [Commented] (CASSANDRA-6696) Drive replacement in JBOD can cause data to reappear.

["sankalp kohli (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CASSANDRA-6696?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13899297#comment-13899297 ] 

sankalp kohli commented on CASSANDRA-6696:
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With this, the whole disk_failure_policy stuff is broken. If you blacklist a drive, you can potentially bring data back to life. 

One of the fixes of this is one of my JIRA which I fixed long back. 
CASSANDRA-4784
If we divide each drive with ranges, then we are sure that the data along with the tombstone will get blacklisted. 
Example: Say a node is handling range 1-10 and 11-20. We can have drive A handle 1-10 and drive B handle 11-20. 
Thought this might have problems with load balancing. 

> 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: Bug
>            Reporter: sankalp kohli
>            Priority: Minor
>
> 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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