Union Mounts

Mac OS X lets you use "union" mounts. Imagine you have a directory with files in it, and you then mount some device on that directory. Ordinarily, the original files would no longer be available, but a union mount leaves them visible: you can see both the files from the device you mounted and the files that were originally in the directory you mounted it on.

Yes, you can do this on Linux also: see Unionfs -Union mounts for Linux

This mixing raises interesting problems - for example what happens if there are identically named files? You only get to see the one in the "topmost" mount - everything below is invisible.

Let's play. First, you need some disk images:

 hdiutil create /tmp/dir1 -volname dir1 -size 512k -fs MS-DOS
 hdiutil create /tmp/dir2 -volname dir2 -size 512k -fs MS-DOS
 
 

I used MS-DOS as the file system type just to make things simple. You might need larger .dmg's if you put real files in the directories; I just use empty files to show the concept.

Next you run hdid with nomount to get an attach point:

 $ hdid -nomount /tmp/dir1.dmg        
 /dev/disk2 
 $ hdid -nomount /tmp/dir2.dmg 
 /dev/disk3         
 
 

and then you can mount /dev/disk3 or /dev/disk2. This is similar to using the loopback device from Unix and Linux.

You probably need to be root for the next part just because the ms-dos driver isn't ordinarily loaded:

 $ mkdir /tmp/testing
 $ sudo su -
 root# mount -t msdos /dev/disk2 /tmp/testing
 kextload: /System/Library/Extensions/msdosfs.kext loaded successfully
 root# cd /tmp/testing
 root# touch a b c d
 mount -t msdos /dev/disk3 /tmp/testing
 root# cd /tmp/testing
 root# ls
 root# touch d e f g 
 root# umount /tmp/testing
 
 

You can exit sudo after that if you want.

We're finally ready for the union mount:

 $ mount -t msdos  -o union /dev/disk2 /tmp/testing
 $ mount -t msdos  -o union /dev/disk3 /tmp/testing
 $ ls /tmp/testing
 ls /tmp/testing 
 a       b       c       d       e       f       g
 
 

Seven files from both file systems are available. We don't see the "underneath" file named "d". Let's remove some things:

 $ rm /tmp/testing/a /tmp/testing/d /tmp/testing/f
 $ $ ls /tmp/testing
 b       c       d       e       g
 
 

Looks like "d" is still there, but that's the "underneath" one. So which "d" got wiped out? Let's see:

 $ mount -t msdos /dev/disk3 /tmp/testing
 $ ls /tmp/testing
 e       g
 $ umount /dev/disk3
 $ mount -t msdos /dev/disk2 /tmp/testing
 $ ls /tmp/testing
 b       c       d
 
 

As expected, it was the "top" one - the last one mounted.

Although I can't find mention of it in Mac OS X, union mounts can also have a concept of "whiteouts". If we had that option above, the "lower" of the "d" files would not have been visible after we deleted the upper - a "whiteout" would have kept us from seeing it.

Leopard Update

The following information was provided by Jay Levitt (http://www.jay.fm).:

$ hdid -nomount /tmp/dir1.dmg       /dev/disk2
 

$ hdiutil attach -nomount /tmp/dir1.dmg
/dev/disk2              FDisk_partition_scheme            /dev/disk2s1            DOS_FAT_32
 

root# mount -t msdos /dev/disk2 /tmp/testing
 

mount_msdos: Unsupported sector size (0)
 

root# mount -t msdos /dev/disk2s1 /tmp/testing
 

root# umount /tmp/testing
 

umount: unmount(/private/tmp/testing): Resource busy
 

root# cd
root# hdiutil unmount /tmp/testing
"/tmp/testing" unmounted successfully.
root# hdiutil unmount /tmp/testing
"/tmp/testing" unmounted successfully.
 

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Fri Apr 11 22:38:22 2008:   TonyLawrence


Jay Levitt provided some updated Leopard info. Thanks Jay!



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