Disk usage

[balanga]

Is there any way of determining disk usage of a given disk without having access to the file system? I'm trying to find out how much free space is available on a disk.

I guess I'm looking for something equivalent to du /dev/ada0...

 

[SirDice]

There's no way to know how much is used if you cannot access the filesystem. It's the filesystem that stores that information.

 

[balanga]

I thought there might be some way to read each sector on a disk and determine if it had been written to at some point...

 

[SirDice]

Sure, you could theoretically just read some blocks and interpret the data. But it'll be easier just to mount it.

 

[Beastie]

I thought there might be some way to read each sector on a disk and determine if it had been written to at some point...

Considering that, most of the time, removing a file won't zeroize sectors that were once allocated to that file, this method may give you highly erroneous results. For example, if I fill an entire disk with millions of files and then remove them all, analyzing the disk as a raw device and looking for blank sectors will show zero free space.
Only proper analysis of filesystem structures will give you valid results.

 

[SirDice]

For example, if I fill an entire disk with millions of files and then remove them all, analyzing the disk as a raw device and looking for blank sectors will show zero free space.

You typically read the FAT (File Allocation Table) or BAM (Block Availability Map), not the actual blocks.