> Wouldn't it be better to first check the overall status of the array?
> And once the array's parity has been correctly written, you can free
> the memory used to hold this bitmap.  It means that you're doing two
> checks, not just one, while you're actually doing the on-demand
> re-writing of the parity; but when you're not fixing parity, it ought
> to save you memory, and probably time, too, when you think about
> keeping that whole bitmap in the CPU's cache...
> 
> if(array_is_dirty)
>     if(this_block_is_dirty)
>         rewrite_parity();

if there's already a function pointer at the right place in the I/O
path, you can do the check with zero overhead -- you start off with it
pointing to the "dirty, slow" path and once parity is cleaned up
re-point it ot the "clean, fast" path.

					- Bill