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From "Christopher Wirt" <>
Subject High performance disk io
Date Wed, 22 May 2013 13:40:39 GMT


We're looking at deploying a new ring where we want the best possible read


We've setup a cluster with 6 nodes, replication level 3, 32Gb of memory, 8Gb
Heap, 800Mb keycache, each holding 40/50Gb of data on a 200Gb SSD and 500Gb
SATA for OS and commitlog

Three column families

ColFamily1 50% of the load and data

ColFamily2 35% of the load and data

ColFamily3 15% of the load and data


At the moment we are still seeing around 20% disk utilisation and
occasionally as high as 40/50% on some nodes at peak time.. we are
conducting some semi live testing.

CPU looks fine, memory is fine, keycache hit rate is about 80% (could be
better, so maybe we should be increasing the keycache size?)


Anyway, we're looking into what we can do to improve this.


One conversion we are having at the moment is around the SSD disk setup..


We are considering moving to have 3 smaller SSD drives and spreading the
data across those.


The possibilities are:

-We have a RAID0 of the smaller SSDs and hope that improves performance. 

Will this acutally yield better throughput?


-We mount the SSDs to different directories and define multiple data
directories in Cassandra.yaml.

Will not having a layer of RAID controller improve the throughput?


-We mount the SSDs to different columns family directories and have a single
data directory declared in Cassandra.yaml. 

Think this is quite attractive idea.

What are the drawbacks? System column families will be on the main SATA?


-We don't change anything and just keep upping our keycache.

-Anything you guys can think of.


Ideas and thoughts welcome. Thanks for your time and expertise. 





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