How Linux Disk Encryption works
First, we fill the disk with random data so that we don’t give away any clues should the disk fall into the wrong hands. The disk is then encrypted using LUKS tools, and each time the server is started a secure passphrase or pre-generated key is used to unlock the master key, which is then used to open the LUKS container.
The container is mapped through to the operating system via the device mapper crypt support, dm-crypt, and there you have it- a device that appears to be regular, as cryptographic operations are transparent to the filesytem via the Linux kernel. The default cipher for LUKS is aes-cbc-essiv:sha256, RedHat by default uses aes-xts-plain64:sha256.