[zrm]

A good way to do this is to create a swap file, both because then you can use it as a swap file until you need to delete it and because swap files are required to not be sparse.

[genewitch]

I'm not at a machine, linux doesn't zero the swap "file" does it? if i set vm.swappiness = 0, will your "trick" work if i never hit memory pressure?

[zrm]

If you create the file with 'mkswap --file' it allocates the blocks. Trying to use 'swapon' with an existing sparse file won't remove the holes for you but does notice them and then refuse to use it.