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From "Hiller, Dean" <>
Subject Re: High performance disk io
Date Wed, 22 May 2013 14:33:21 GMT
Well, if you just want to lower your I/O util %, you could always just add more nodes to the
cluster ;).


From: Igor <<>>
Reply-To: "<>" <<>>
Date: Wednesday, May 22, 2013 8:06 AM
To: "<>" <<>>
Subject: Re: High performance disk io


What level of read performance do you expect? We have limit 15 ms for 99 percentile with average
read latency near 0.9ms. For some CF 99 percentile actually equals to 2ms, for other - to
10ms, this depends on the data volume you read in each query.

Tuning read performance involved cleaning up data model, tuning cassandra.yaml, switching
from Hector to astyanax, tuning OS parameters.

On 05/22/2013 04:40 PM, Christopher Wirt wrote:

We’re looking at deploying a new ring where we want the best possible read performance.

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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