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From Wei Zhu <wz1...@yahoo.com>
Subject Re: Truncate behaviour
Date Wed, 20 Mar 2013 02:55:04 GMT
There is setting in the cassandra.yaml file which controls that.


# Whether or not a snapshot is taken of the data before keyspace truncation
# or dropping of column families. The STRONGLY advised default of true 
# should be used to provide data safety. If you set this flag to false, you will
# lose data on truncation or drop.
auto_snapshot: true


----- Original Message -----
From: "VĂ­ctor Hugo Oliveira Molinar" <vhmolinar@gmail.com>
To: user@cassandra.apache.org
Sent: Tuesday, March 19, 2013 11:50:35 AM
Subject: Truncate behaviour

Hello guys! 
I'm researching the behaviour for truncate operations at cassandra. 


Reading the oficial wiki page( http://wiki.apache.org/cassandra/API ) we can understand it
as: 

"Removes all the rows from the given column family." 


And reading the DataStax page( http://www.datastax.com/docs/1.0/references/cql/TRUNCATE )
we can understand it as: 
" A TRUNCATE statement results in the immediate, irreversible removal of all data in the named
column family." 


But I think there is a missing and important point about truncate operations. 
At least at 1.2.0 version, whenever I run a truncate operation, C* automatically creates a
snapshot file of the column family, resulting in a fake free disk space. 

I'm intentionally mentioning 'fake free disk space' because I only figured it out when the
machine disk space was at high usage. 




- Is it a security C* behaviour of creating snapshots for each CF before truncate operation?

- In my scenario I need to purge my column family data every day. 
I thought that truncate could handle it based at the docs. But it doesnt. 
And since I dont want to manually delete those snapshots, I'd like to know if there is a safe
and practical way to perform a daily purge of this CF data. 



Thanks in advance!

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