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From Tyler Hobbs <ty...@datastax.com>
Subject Re: Cassandra read throughput with little/no caching.
Date Mon, 31 Dec 2012 18:45:35 GMT
On Mon, Dec 31, 2012 at 11:24 AM, James Masson <james.masson@opigram.com>wrote:

>
> Well, it turns out the Read-Request Latency graph in Ops-Center is highly
> misleading.
>
> Using jconsole, the read-latency for the column family in question is
> actually normally around 800 microseconds, punctuated by occasional big
> spikes that drive up the averages.
>
> Towards the end of the batch process, the Opscenter reported average
> latency is up above 4000 microsecs, and forced compactions no longer help
> drive the latency down again.
>
> I'm going to stop relying on OpsCenter for data for performance analysis
> metrics, it just doesn't have the resolution.


James, it's worth pointing out that Read Request Latency in OpsCenter is
measuring at the coordinator level, so it includes the time spent sending
requests to replicas and waiting for a response.  There's another latency
metric that is per-column family named Local Read Latency; it sounds like
this is the equivalent number that you were looking at in jconsole.  This
metric basically just includes the time to read local
caches/memtables/sstables.

We are looking to rename one or both of the metrics for clarity; any input
here would be helpful. For example, we're considering "Coordinated Read
Request Latency" or "Client Read Request Latency" in place of just "Read
Request Latency".

-- 
Tyler Hobbs
DataStax <http://datastax.com/>

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