Union mounts for Linux.
There, wasn't that helpful?
Just kidding: So what's a union mount? 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, Mac has this also.
This mixing raises interesting problems - for example what happens if there are identically named files? I'm not going to go into the details of all this here because it takes a lot of space and it makes my head hurt, but this is where whiteouts come into play. The basic issue is that the "upper" layer is the last thing mounted, and although everything below is visible, if you want to delete something in the lower layer, you really can't; a "whiteout" in the upper layer makes it seem to be deleted.
Union mounts have been available in BSD and Mac OS 10, see https://www.fsl.cs.sunysb.edu/project-unionfs.html for Linux. Interestingly, unionfs allows you to create snapshots.
If you really want to know about whiteouts, go read
A common use is in Live Cd's: the read only cd file system is paired with a writeable file system on a hard drive.
Got something to add? Send me email.
More Articles by Tony Lawrence © 2011-03-20 Tony Lawrence
The people I distrust most are those who want to improve our lives but have only one course of action in mind. (Frank Herbert)
Sun Nov 19 10:43:37 2006: 2631 drag
Oh, UnionFS is nice. Lots of flexibility.
For instance I wanted to have a Linux install on a USB key. It would operate in similar fasion to 'live linux cdroms' but instead of having the top layer of the stack be in tmpfs it would be writing out to the usb pen drive.
By using Squashfs (read only compressed file system) I was able to make the equivelent of 1.3 or so gigs of disk space compress down to under 700 megs. I don't remember the actual amount. Using Unionfs I was able to make it seems fully writable and have this be persistant through reboots.
------------------------
Printer Friendly Version
unionfs: Tech Words of the Day Copyright © November 2004 Tony Lawrence
Have you tried Searching this site?
This is a Unix/Linux resource website. It contains technical articles about Unix, Linux and general computing related subjects, opinion, news, help files, how-to's, tutorials and more.
Contact us
Printer Friendly Version