Where do the files go if you mount a drive to a folder that already contains files?

Question

I just accidentally mounted a new drive to a folder that already contained files. I don't care about them and have them somewhere else, but that folder appears empty now. I'm curious what happened to the files. Are they simply deleted by Linux?

Answer 1

Just shadowed. Will be there again when unmounted. :) Though, if you're curious, you can reach them right now using mount --bind /Original/FS/Mount/Point /Somewhere/Else.

It's also worth noting Aufs, just in case. ;)

Answer 2

The newly mounted filesystem is like an overlay which hides part of the initial filesystem. There are trick to access the files, like the bind mount as described by poige. On Linux I personally like the fact that you can mount a filesystem more than once on different mount points. So you can perfectly well mount the root filesystem on / aswell as on /mnt. This comes in handy while debugging lost space / lost files / counting disk usage / ...

Gotcha's:

• The old files are still there and using up space. It is a common mistake to have 'missing' disk space in files hidden by a mount. Eg. when you accidentally wrote large files in a directory and at some time decided to mount a filesystem on it. Eg. accidentally starting a large database before the logging filesystem was properly mounted ...
• When a program has a file open which is subsequently hidden by a newly mounted filesystem, the program won't care about it and succesfully keeps using the 'hidden' file until it closes it. From that moment the file comes invisible to the process until the new filesystem is unmounted and the underlying directory shows up again.