Hi!

Why don't you just use "du -B 4096 -sh /path/to/fs" vs.
"du -B 16384 -sh ..."?

Subtracting both results is the overhead of the one vs. the other.

But to answer your request for the formula, its:

blocks = (long)((file_size + block_size - 1) / block_size)
occupied_size = blocks * block_size

But this does not tell you the real on-disk occupation because it takes the 
logical file size of the files into account. But files may be sparse or 
compressed which sheds a complete different light on the problem.

I'd go with "du" command because it takes this into account.

Regards,
Kai

Sandra Schlichting [off-list ref] schrieb:

Hi all,

When having a non-btrfs fs and want to migrate to btrfs, it is hard to
know which sector size to choose in terms of disk utilization and
speed.

So I would like to make a script that scans the non-btrfs fs's
directory structure to find all file sizes and calculate how much
space these would take on btrfs 4k and 16k sector size.

Can anyone help me with a formulae to calculate this for a file?

It doesn't have to be exact, just enough to make a decision between 4k and
16k.

Best regards,
Sandra
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