Metadata size estimation

[Serguei Tzukanov]

Ok, another crazy idea.


Let's take simple case:
converting one filesystem to another with the same block size;
both filesystems does not do tail-packing and data-into-inode inlining;

Now the process.
Let's copy(!) file from primary fs to second as always...
But when it comes to allocate block on the secondary (and thus on the
primary) fs let underlying block driver point it to old block.
Thus we building new filesystem with metadata in it only.
Yes, we need to compare all the blocks we are copying.
It's not very hard to handle inlined data and tail-packing.
It seems to me that handling that way different block sizes also are
possible (with some care).

Shrinking easy too. 
Need to think about growing.

If it fails at some point then just umount loopback and delete it.

Side effect is that we accelerate overall conversion process.