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From Benjamin Roth <>
Subject Re: Disc size for cluster
Date Thu, 26 Jan 2017 09:11:57 GMT

This is basically right, but:
1. How do you know the 3TB storage will be 3TB on cassandra? This depends
how the data is serialized, compressed and how often it changes and it
depends on your compaction settings
2. 50% free space on STCS is only required if you do a full compaction of a
single CF that takes all the space. Normally you need as much free space as
the target SSTable of a compaction will take. If you split your data across
more CFs, its unlikely you really hit this value.

.. probably you should do some tests. But in the end it is always good to
have some headroom. I personally would scale out if free space is < 30% but
that always depends on your model.

2017-01-26 9:56 GMT+01:00 Raphael Vogel <>:

> Hi
> Just want to validate my estimation for a C* cluster which should have
> around 3 TB of usable storage.
> Assuming a RF of 3 and SizeTiered Compaction Strategy.
> Is it correct, that SizeTiered Compaction Strategy needs (in the worst
> case) 50% free disc space during compaction?
> So this would then result in a cluster of 3TB x 3 x 2 == 18 TB of raw
> storage?
> Thanks and Regards
> Raphael Vogel

Benjamin Roth

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