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From Schubert Zhang <>
Subject Re: Cassandra's bad behavior on disk failure
Date Thu, 29 Apr 2010 04:23:02 GMT
On Wed, Apr 21, 2010 at 10:08 PM, Oleg Anastasjev <>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
> Good 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.
> [schubert] Usually, we should use RAID to avoud disk failure.
And add some system monitors to maintain/shutdown node.

> 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.
> [schubert] I think this is also a bad implementation of current Cassandra
on CommitLogSync.
The default config is <CommitLogSync>periodic</CommitLogSync>
The write commit-log will be responsed immediately, but only buffered in
memory, and will be synced to disk periodically according

In your case, the buffer will be immoderately increased, and use many and
many heap.

In 0.6.x, you can use batch CommitLogSync to alleviate this issue.

I think the right design should be:
It should use a threshold to avoid too much buffered commitlog. The
buffer-time and buffer-size should also be the sync trigger:
When the commit-log buffer size threshold is reached, sync.
When the commit-log buffer time is reached, sync.

> 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 ?

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