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From aaron morton <>
Subject Re: Data overhead discussion in Cassandra
Date Sun, 17 Jul 2011 23:04:17 GMT
What RF are you using ? 

On disk each column has 15 bytes of overhead, plus the column name and the column value. So
for an 8 byte long and a 8 byte double there will be 16 bytes of data and 15 bytes of data.

The index file also contains the the row key, the MD5 token (for RP) and the row offset for
the data file. 


Aaron Morton
Freelance Cassandra Developer

On 15 Jul 2011, at 07:09, Sameer Farooqui wrote:

> We just set up a demo cluster with Cassandra 0.8.1 with 12 nodes and loaded 1.5 TB of
data into it. However, the actual space on disk being used by data files in Cassandra is 3
TB. We're using a standard column family with a million rows (key=string) and 35,040 columns
per key. The column name is a long and the column value is a double.
> I was just hoping to understand more about why the data overhead is so large. We're not
using expiring columns. Even considering indexing and bloom filters, it shouldn't have bloated
up the data size to 2x the original amount. Or should it have?
> How can we better anticipate the actual data usage on disk in the future?
> - Sameer

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