Secure Hard Disk Erasure
Before setting up dm-crypt, one is supposed to overwrite the hard disk with random data. As it turns out, this is harder to do than initially expected. Using /dev/random or /dev/urandom as a data source to overwrite the disk will take ages (multiple days) for a hard disk drive of typical size. The same applies to using the wipe command.
Fortunately, there is an alternative available by means of the badblocks command:
badblocks -c 10240 -s -w -v -t random /dev/<device>
This takes approximately four hours per terabyte of data (YMMV).
Source: http://chakra-project.org/wiki/index.php/LUKS_for_dm-crypt