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From "Jacob, Arun" <>
Subject Re: Cassandra Node Requirements
Date Thu, 25 Aug 2011 00:53:31 GMT
Thanks for the links and the answers. The vagueness of my initial questions reflects the fact
that I'm trying to configure for a general case — I will clarify below:

I need to account for a variety of use cases.
(1) they will be both read and write heavy.  I was assuming that SSDs would be really good
to handle the heavy read load, but with the amount of data I need to store, SSDs arent economical.
(2) I should have clarified, the main use case has  95% of writes going to a single column
family. The other CFs are going to be much smaller in relation to the primary CF, which will
be sized to sizes below. Given that this is the case, are my assumptions about storage correct
for that use case?
(3) In  that use case, the majority of reads will actually come from the most recently inserted
7% of the data. In other use cases, reads will be random. Another use case uses Solandra,
and I am assuming that use case results in random reads.

Assuming 250-360TB storage, need for the primary use case, I'm still trying to determine 
how many nodes  I need to stand up to service that much data. What is a reasonable amount
of storage per node? You mentioned memory to storage ratio: I'm assuming that ratio trends
higher with the more random reads you do. Could you provide an example ratio for a heavy read
use case?


-- Arun

From: Edward Capriolo <<>>
Reply-To: "<>" <<>>
Date: Wed, 24 Aug 2011 14:54:56 -0700
To: "<>" <<>>
Subject: Re: Cassandra Node Requirements

On Wed, Aug 24, 2011 at 2:54 PM, Jacob, Arun <<>>
I'm trying to determine a node configuration for Cassandra. From what I've been able to determine
from reading around:

 1.  we need to cap data size at 50% of total node storage capacity for compaction
 2.  with RF=3, that means that I need to effectively assume that I have 1/6th of total storage
 3.  SSDs are preferred, but of course  reduce storage capacity
 4.  using standard storage means you bump up your RAM to keep as much in memory as possible.

Right now we are looking at storage requirements of 42 – 60TB, assuming a baseline of 3TB/day
and expiring data after 14-20 days (depending on use case),  I would assume based on above
that we need 252- 360TB total storage max.

My questions:

 1.  is 8TB (meaning 1.33 actual TB storage/node) a reasonable per node storage size for Cassandra?
I don’t want to use SSDs due to reduced storage capacity -- I don't want to buy 100s of
nodes to support that reduced storage capacity of SSDs.  Given that I will be using standard
drives, what is a reasonable/effective per node storage capacity?
 2.  other than splitting the commit log onto a separate drive, is there any other drive allocation
I should be doing?
 3.  Assuming I'm not using SSDs, what would be a good memory size for a node? I've heard
anything from 32-48 GB, but need more guidance.

Anything else that anyone has run into? What are common configurations being used by others?

Thanks in advance,

-- Arun

I would suggest checking out:

1. we need to cap data size at 50% of total node storage capacity for compaction

False. You need 50% the capacity of your largest column family free with some other room for
overhead. This changes all your numbers.

3. SSDs are preferred, but of course  reduce storage capacity

Avoid generalizations. Many use cases may get little benefit from SSD disks.

4. using standard storage means you bump up your RAM to keep as much in memory as possible.

In most cases you want to maintain some RAM / Hard disk ratio. SSD setups still likely need
sizable RAM.

Your 3 questions are hard to answer because what hardware you need workload dependent. If
really depends on active set, what percent of the data is active at any time. It also depends
on your latency requirements, if you are modeling something like the way-back machine, that
has different usage profile then a stock ticker application, that is again different from
the usage patterns of an email system.

Generally people come to Cassandra because they are looking for low latency access to read
and write data. This is hard to achieve on 8TB of disk. The size of the bloom filters and
index files are themselves substantial with 8TB of data. You will also require a large amount
of RAM on this disk to minimize disk seeks (or a super like SSD raid-0 (does this sound like
a bad idea to you? It does to me :))

The only way to answer the question of how much hardware your need is with load testing. The
Yahoo Cloud Serving Benchmark can help you fill up a node and test it with different load
patterns to see how it performs.

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