I've been reading up on tools like Truecrypt, loop-AES and Cryptoloop that use a "loop device" to create an encrypted disk volume within a file.
On the README page for loop-AES, it has this paragraph about journaling file systems:
Don't use a journaling file system on top of file backed loop device. Device backed loop device can be used with journaling file systems as device backed loops guarantee that writes reach disk platters in order required by journaling file system (write caching must be disabled on the disk drive, of course). With file backed loop devices, correct write ordering may extend only to page cache (which resides in RAM) of underlying file system. VM can write such pages to disk in any order it wishes, and thus break write order expectation of journaling file system.
Do you know what they mean by "on top of file backed loop device"? Are they referring to a situation where my disk is physically formatted with a journaling file system (like ext3, ext4, or XFS) and then within that file system I create a file which houses the virtual disk volume? Or are they describing the reverse, where the journaling file system is within the file that houses the virtual disk volume?
I have my Linux partitions formatted as ext4, so if they're referring to the first situation, I would have to resize partitions and create an ext2 or FAT partition to save the file that will house the Truecrypt/loop-AES/Cryptoloop volume.