Encrypt an existing partition in Linux while preserving its data

Since this comes up near the top of google results, adding solution:

LUKS in place encryption via http://www.johannes-bauer.com/linux/luksipc/

This is trivial if you choose plain dm-crypt. It's risky - if it fails part-way through (power cut or whatever) then you're stuffed!

Ensure the raw device isn't mounted then create an encrypted device for it and use dd to copy from the raw device to the encrypted one:

$ cryptsetup open /dev/sda sda-crypt --type plain
$ dd if=/dev/sda of=/dev/mapper/sda-crypt bs=512

The plaintext data is read from /dev/sda and written to the device mapper, /dev/mapper/sda-crypt, which encrypts it and writes it back to /dev/sda, overwriting the plaintext data that was read.

It will likely take some time due to it reading and writing the entire disk.

There does not seem to be an solution to do that in place. Truecrypt offers the system encryption only for windows, dm-crypt overwrites partitions. Your best bet would be to move everything from that partition into a backup with cp -a, create an encrypted partition with luks/dm-crypt and move everything back.