How to clear unused space with zeros ? (I am asking for tool for btrfs filesystem)
I'm looking for something smarter than
cat /dev/zero > /mnt/X/big_zero ; sync; rm /mnt/X/big_zero
Like FSArchiver is looking for "used space" and ignores unused, but opposite site.
Purpose: I'd like to compress partition images, so filling unused space with zeros is highly recommended.
Btw. For ext3,ext4 : Clear unused space with zeros (ext3,ext4) . Here I ask for btrfs
Clarification of "smarter algorithm": As I see that "smarter algorithm" leads to confusion, here I finally decided to add line about it. Dear reader, please think about filesystem as datastructure. For simplification purposes, let's imagine self balancing red-black tree holding map, that is used as association key->value array, where values are integers. Goal is to clear all negative values with zeros. "not smart" algorithm will iterate over it and add key:value pairs to new map (with eventual altering negative to zero). Smart algorithm will just detect places where negative values are stored and clear them in-place.
Best Answer
The correct and "smart" way to clear the free space is using:
Use this on a mounted filesystem.
It's important to note that there is no security advantage to this. If you wish to clear your disk for security, back up your files, zero the entire disk with a command like "shred" and then restore your files.
However the solution works fine if your disk is a thin provision on a sparse filesystem and you wish to recover the unused space.