If I map a journaled file system using dm-crypt/LUKS, does it still provide its usual transactional guarantees?
As far as I know it does (but I may be wrong), but please note that these “guarantees” are far weaker than they appear to be. For example, you may not get a hard flush to disk surface even on a call to fsync. In addition, the HDD itself may do independent write reordering. Some other things can go wrong as well. The filesystem developers are aware of these problems and typically can make it work anyways. That said, dm-crypt/LUKS should not make things worse. Personally, I have several instances of ext3 on dm-crypt and have not noticed any specific problems. Update: I did run into frequent small freezes (1-2 sec) when putting a vmware image on ext3 over dm-crypt. This does indicate that the transactional guarantees are in place, but at a cost. When I went back to ext2, the problem went away. This also seems to have gotten better with kernel 2.6.36 and the reworking of filesystem flush locking. Kernel 2.6.38 is expected to have more improvements here.
