Swapping to a large sparse file

[jd1008]

Can it be done?

So far, swapon says:
swapon: /var/swapfile: skipping - it appears to have holes.

I was hoping that the kernel's swapper would allow the filesystem to 
allocate
real blocks to the sparse file when they are needed rather than check
up front to see if they already exist.

Maybe the devs can have a look and see if they can modify the swapper to
allow sparse swap files??