Cassandra take a snapshot after a column family update

[Roberto Bentivoglio]

Hi all,
I've executed from cassandra-cli on a production server (0.7.0) the
following instruction:

update column family CFName with gc_grace=0;

I'm really surprise that Cassandra take a snapshots of the data for every
update that I executed.
Is this a normal behaviour?

Thanks,
Roberto Bentivoglio