Hello all,

We do btrfs subvolume snapshots over time for backups.  I would like to 
traverse the files in the subvolumes and find the total unique chunk count 
to calculate total space for a set of subvolumes.

This sounds kind of like the beginning of what a deduplicator would do, 
but I just want to count the blocks, so no submission for deduplication.  
I started looking at bedup and other deduplicator code, but the answer to 
this question wasn't obvious (to me, anyway).

Questions:

Is there an ioctl (or some other way) to get the block device offset for a 
file (or file offset) so I can count the unique occurances?

What API documentation should I review?

Can you point me at the ioctl(s) that would handle this?


Thank you for your help!


--
Eric Wheeler
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