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From Mohammed Guller <>
Subject no change observed in read latency after switching from EBS to SSD storage
Date Wed, 17 Sep 2014 00:35:45 GMT
Hi -

We are running Cassandra 2.0.5 on AWS on m3.large instances. These instances were using EBS
for storage (I know it is not recommended). We replaced the EBS storage with SSDs. However,
we didn't see any change in read latency. A query that took 10 seconds when data was stored
on EBS still takes 10 seconds even after we moved the data directory to SSD. It is a large
query returning 200,000 CQL rows from a single partition. We are reading 3 columns from each
row and the combined data in these three columns for each row is around 100 bytes. In other
words, the raw data returned by the query is approximately 20MB.

I was expecting at least 5-10 times reduction in read latency going from EBS to SSD, so I
am puzzled why we are not seeing any change in performance.

Does anyone have insight as to why we don't see any performance impact on the reads going
from EBS to SSD?


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