Testing ext4 persistent preallocation patches for 64 bit features

I plan to test the persistent preallocation patches on a huge sparse
device, to know if >32 bit physical block numbers (upto 48bit) behave as
expected. I have following questions for this and will appreciate
suggestions here:

a) What should be the sparse device size which I should use for testing?
Should a size of > 8TB (say, 100 TB) be enough ?
The physical device (backing store device) size I can have is upto 70GB.

b) How do I test allocation of >32 bit physical block numbers ? I can
not fill > 8TB, since the physical storage available with me is just
70GB.

c) Do I need to put some hack in the filesystem code for above (to
allocate >32 bit physical block numbers) ?

Any further ideas on how to test this will help. Thanks!

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Regards,
Amit Arora