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From "J. Ryan Earl (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-6364) Cassandra should exit or otherwise handle when the commit volume dies
Date Wed, 20 Nov 2013 20:10:35 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-6364?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13828057#comment-13828057
] 

J. Ryan Earl commented on CASSANDRA-6364:
-----------------------------------------

[~iamaleksey] So again, that's bad behavior, since the node will essentially crater and be
unable to handle read requests in a timely manner or at all, not because the data isn't there
to be read, but due to GC death as uncommitted writes pile up on the heap and the JVM spends
all of its time doing garbage collection.  Furthermore, it affects reads and writes not just
to said node, but on any connection that uses said node as its coordinator.  At a minimum,
there should be different failure policies for commit and data volumes.  The scope or description
of the ticket can be changed to that effect, maybe there is a corner case where people only
read from Cassandra such that "best_effort" makes sense on the commit volume, but it's really
hard to see any plausible use-case where that would be desired.

Cassandra needs to be able to do "best_effort" on the data volumes, where it makes no sense
for a node to die when one of a JBOD of data disks fails, while gracefully and immediately
exiting on commit disk failures, which will guarantee the node will become unresponsive in
a short of amount of time under write load.



> Cassandra should exit or otherwise handle when the commit volume dies
> ---------------------------------------------------------------------
>
>                 Key: CASSANDRA-6364
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-6364
>             Project: Cassandra
>          Issue Type: Improvement
>         Environment: JBOD, single dedicated commit disk
>            Reporter: J. Ryan Earl
>
> We're doing fault testing on a pre-production Cassandra cluster.  One of the tests was
to simulation failure of the commit volume/disk, which in our case is on a dedicated disk.
 We expected failure of the commit volume to be handled somehow, but what we found was that
no action was taken by Cassandra when the commit volume fail.  We simulated this simply by
pulling the physical disk that backed the commit volume, which resulted in filesystem I/O
errors on the mount point.
> What then happened was that the Cassandra Heap filled up to the point that it was spending
90% of its time doing garbage collection.  No errors were logged in regards to the failed
commit volume.  Gossip on other nodes in the cluster eventually flagged the node as down.
 Gossip on the local node showed itself as up, and all other nodes as down.
> The most serious problem was that connections to the coordinator on this node became
very slow due to the on-going GC, as I assume uncommitted writes piled up on the JVM heap.
 What we believe should have happened is that Cassandra should have caught the I/O error and
exited with a useful log message, or otherwise done some sort of useful cleanup.  Otherwise
the node goes into a sort of Zombie state, spending most of its time in GC, and thus slowing
down any transactions that happen to use the coordinator on said node.
> A limit on in-memory, unflushed writes before refusing requests may also work.  Point
being, something should be done to handle the commit volume dying as doing nothing results
in affecting the entire cluster.  I should note, we are using: disk_failure_policy: best_effort



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