how efficient is the block diagonal format?

[Alejandro Garzon]

Hi, first of all thanks for your previous help. In section 3.1.1 of the user's
manual it is said "Additional formats (such as block compressed row and block
diagonal storage, which are generally much more efficient for problems with
multiple degrees of freedom per node) are discussed below." I want to know if
the efficiency refers to the assembly only or if more important it refers also
to how the KSPSolve function will perform with this format. I did a trial and it
seems KSPSolve performs at around the same speed with both the default and the
block diagonal formats (is even a bit faster with the default format). Is this
comparison highly problem dependent? Or could I be missing to use some option?
My problem is a 1D pde with 2 variables (two degrees of freedom per node(?))
first and second derivatives terms and a block on the main diagonal mixing the
two variables. Thanks.

Alejandro.