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Date: Sat, 04 Dec 2010 02:27:36 +0100
From: Daniel Doubleday <daniel.doubleday@gmx.net>
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To: user@cassandra.apache.org
Subject: Re: Dont bogart that connection my friend
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Yes.

I thought that would make sense, no? I guessed that the quorum read 
forces the slowest of the 3 nodes to keep the pace of the faster ones. 
But it cant. No matter how small the performance diff is. So it will 
just fill up.

Also when saying 'practically dead' and 'never recovers' I meant for the 
time I kept the reads up. As soon as I stopped the scan it recovered. It 
just was not able to recover during the load because for that it would 
have to become faster that the other nodes and with full queues that 
just wouldn't happen.

By changing the node for every read I would hit the slower node every 
couple of reads. This forced the client to wait for the slower node.

I guess to change that behavior you would need to use something like 
dynamic snitch and ask only as many peer nodes as necessary to satisfy 
quorum and only ask other nodes when reads fail. But that would probably 
increase latency and cause whatever other problems. Since you probably 
don't want to run the cluster at a load at which the weakest node of a 
replication group can't keep up I don't think this is an issue at all.

Just wanted to prevent others shooting their own foot as I did.

On 03.12.10 23:36, Jonathan Ellis wrote:
> Am I understanding correctly that you had all connections going to one
> cassandra node, which caused one of the *other* nodes to die, and
> spreading the connections around the cluster fixed it?
>
> On Fri, Dec 3, 2010 at 4:00 AM, Daniel Doubleday
> <daniel.doubleday@gmx.net>  wrote:
>> Hi all
>>
>> I have found an anti pattern the other day which I wanted to share, although its pretty special case.
>>
>> Special case because our production cluster is somewhat strange: 3 servers, rf = 3. We do consistent reads/writes with quorum.
>>
>> I did a long running read series (loads of reads as fast as I can) with one connection. Since all queries could be handled by that node the overall latency is determined by its own and the fastest second node (cause the quorum is satisfied with 2 reads). What will happen than is that after a couple of minutes one of the other two nodes will go in 100% io wait and will drop most of its read messages. Leaving it practically dead while the other 2 nodes keep responding at an average of ~10ms. The node that died was only a little slower ~13ms average but it will inevitably queue up messages. Average response time increases to timeout (10 secs) flat. It never recovers.
>>
>> It happened all the time. And it wasn't the same node that would die.
>>
>> The solution was that I return the connection to the pool and get a new one for every read to balance the load on the client side.
>>
>> Obviously this will not happen in a cluster where the percentage of all rows on one node is enough. But the same thing will probably happen if you scan by continuos tokens (meaning that you will read from the same node a long time).
>>
>> Cheers,
>>
>> Daniel Doubleday
>> smeet.com, Berlin
>
>