Re: Cassandra's bad behavior on disk failure

[Jonathan Ellis <jb...@gmail.com>]

We have a ticket open for this:
https://issues.apache.org/jira/browse/CASSANDRA-809

Ideally I think we'd like to leave the node up to serve reads, if a
disk is erroring out on writes but still read-able.  In my experience
this is very common when a disk first begins to fail, as well as in
the "disk is full" case where there is nothing actually wrong with the
disk per se.

On Wed, Apr 21, 2010 at 9:08 AM, Oleg Anastasjev <ol...@gmail.com> wrote:
> Hello,
>
> I am testing how cassandra behaves on single node disk failures to know what to
> expect when things go bad.
> I had a cluster of 4 cassandra nodes, stress loaded it with client and made 2
> tests:
> 1. emulated disk failure of /data volume on read only stress test
> 2. emulated disk failure of /commitlog volumn on write intensive test
>
> 1. On read test with data volume down, a lot of
> "org.apache.thrift.TApplicationException: Internal error processing get_slice"
> was logged at client side. On cassandra server logged alot of IOExceptions
> reading every *.db file it has. Node continued to show as UP in ring.
>
> OK, the behavior is not ideal, but still can be worked around at client side,
> throwing out nodes as soon as TApplicationException is received from cassandra.
>
> 2. Much worse was with write test:
> No exception was seen at client, writes are going through normally, but
> PERIODIC-COMMIT-LOG-SYNCER failed to sync commit logs, heap of node quickly
> became full and node freezed in GC loop. Still, it continued to show as UP in
> ring.
>
> This, i believe, is bad, because no quick workaround could be done at client
> side (no exceptions are coming from failed node) and in real system will lead to
> dramatic slow down of the whole cluster, because clients, not knowing, that node
> is actually dead, will direct 1/4th of requests to it and timeout.
>
> I think that more correct behavior here could be halting cassandra server on any
> disk IO error, so clients can quickly detect this and failover to healthy
> servers.
>
> What do you think ?
>
> Did you guys experienced disk failure in production and how was it ?
>
>
>

Re: Cassandra's bad behavior on disk failure

[Oleg Anastasjev <ol...@gmail.com>]

> 
> Ideally I think we'd like to leave the node up to serve reads, if a
> disk is erroring out on writes but still read-able.  In my experience
> this is very common when a disk first begins to fail, as well as in
> the "disk is full" case where there is nothing actually wrong with the
> disk per se.

This depends on hardware/drivers in use as well as failling part. 
On some failures disk just disappears completely (controller failures, 
SAN links etc.).
And the easiest way to bring operations team attention to node is 
shutting it down - anyway ppl have something to do with it.
Furthermore, single node shutdown should be not very hurting to cluster's 
performance in production - everyone planning capacity in a way to survive 
single node failure.