[
https://issues.apache.org/jira/browse/CASSANDRA-6696?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13976688#comment-13976688
]
Benedict commented on CASSANDRA-6696:
-------------------------------------
The problem here is packing vnodes fairly across the disks: either we need to
ensure that all vnodes are of roughly equal size (very difficult), or we
probably need to have a dynamic allocation strategy, and the problem with
_that_ is that when the token range gets redistributed by node
additions/removals, the whole cluster suddenly needs to start kicking off
rebalancing of their local disks.
We could support splitting the token range into M distinct chunks, where M is
preferably some multiple of the number of disks, and split the total token
range into M chunks, then allocate each chunk to a disk in round-robin fashion.
This then remains deterministic, and it is I think easier to guarantee an even
distribution within a given token range than it is to guarantee all vnodes are
of equal size, whilst still supporting a dynamic cluster size. Even here,
though, realistically I think we need the number of chunks to be quite a bit
smaller than the number of vnodes to guarantee anything approaching balance of
these chunks.
> 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.
>
--
This message was sent by Atlassian JIRA
(v6.2#6252)
