Question

I have a disk with some filesystem known by the Linux kernel. I need to get the physical block numbers for all blocks that are currently free on this filesystem. How can I do that?

Answer 1

Getting a free-block count:

In-kernel method:

Look at the .statfs superblock hook that filesystems implement. They return a kstatfs object which has a f_bfree parameter, a count of the free blocks. You should be able to call into this.

From userspace:

The df command is the simplest (which of course causes the kernel to call statfs – where else would it get the information? 🙂

Edit: Your original question conveyed getting just the free block count.

Getting block locations:

Getting the location of every free block and not just the total number is impossible in a filesystem-independent way with the current structure of the kernel (and it will probably remain this way). The VFS doesn’t impose any common structure on how the filesystem maintains its bitmap; bitmaps are kept track of in the filesystem-dependent part of the superblock.

So, if you want a solution that works across all filesystems, you are out of luck. If you know what the filesystems might be and its a small number, you can look at userspace fsck code for each of those and add functions to write out a bitmap in a format you expect, or you can modify the .statfs hooks to write out the bitmap.

Your method to allocate a large file that allocates all remaining free blocks doesn’t sound like a very good idea at all, especially if there are journals involved. It also won’t work for an active filesystem with files being written and deleted as you do this.