lofi(7D) compression support [PSARC/2007/569 FastTrack timeout 10/08/2007]

[Alok Aggarwal]

Darren and I talked about the areas of overlap between
lofi encryption and lofi compression. Here's a summary
of what we talked about -

Compressed lofi images are read-only so it makes sense
to allow either compression or encryption on an image
but not both at the same time.

So, lofi compression should be able to spot an 
encrypted image and prevent compression. Likewise, lofi
encryption should be able to spot a compressed image
and prevent encryption.

This obviates the need for on-disk metadata to be stored
by lofi encryption.

In other words, there is a dependancy between PSARC/2007/001 and
this case. The two project teams will work together to
make sure that encrypted images can't be compressed and vice
versa.

Also, I need to make it clear in the documentation that 
compressed lofi images can be read-only and not read/write.

Alok