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From Alprema <alpr...@alprema.com>
Subject Read performance
Date Thu, 07 May 2015 08:27:14 GMT
Hi,

I am writing an application that will periodically read big amounts of data
from Cassandra and I am experiencing odd performances.

My column family is a classic time series one, with series ID and Day as
partition key and a timestamp as clustering key, the value being a double.

The query I run gets all the values for a given time series for a given day
(so about 86400 points):

SELECT "UtcDate", "Value"FROM "Metric_OneSec"WHERE "MetricId" =
12215ece-6544-4fcf-a15d-4f9e9ce1567eAND "Day" = '2015-05-05
00:00:00+0000'LIMIT 86400;


This takes about 450ms to run and when I trace the query I see that it
takes about 110ms to read the data from disk and 224ms to send the data
from the responsible node to the coordinator (full trace in attachment).

I did a quick estimation of the requested data (correct me if I'm wrong):
86400 * (column name + column value + timestamp + ttl)
= 86400 * (8 + 8 + 8 + 8?)
= 2.6Mb

Let's say about 3Mb with misc. overhead, so these timings seem pretty slow
to me for a modern SSD and a 1Gb/s NIC.

Do those timings seem normal? Am I missing something?

Thank you,

Kévin

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