Question

I’m working on a personal project to regularly (monthly-ish) traverse my hard disk and shred (overwrite with zeros) any blocks on the disk not currently allocated to any inode(s).

C seemed like the most logical language to do this in given the low-level nature of the project, but I am not sure how best to find the unused blocks in the filesystem. I’ve found some questions around S.O. and other places that are similar to this, but did not see any consensus on the best way to efficiently and effectively find these unused blocks.

df has come up in any questions even remotely similar to this, but I don’t believe it has the resolution necessary to specify exact block offsets unless I am missing something. Is there another utility I should look into or some other direction entirely?

Whatever solution I develop would need to be able to handle, at minimum, ext3 filesystems, and preferably ext4 also.

Answer 1

Linux currently support over a dozen different filesystems so the answer will depend on which one you choose.

However, they should all have an easy way of finding free blocks otherwise creating new files or extending current files would be a bit slow.

For example, ext2 has, at the start of each block group, a header containing, among other things, the free list for that block group. I don’t believe this has changed in ext4 even though there’s a lot of extra stuff in there.

You would probably be far better traversing the free blocks in those block headers rather than taking an arbitrary block and trying to figure out if it’s used or free.