Many times, you need to know how a certain filesystem looked like at some point in time, and you want to be able to roll back changes that happened to it after that point. While there are multiple solutions to achieve this goal, certainly one of them is to use filesystem snapshots. One of the capabilities of unionfs is to offer the possibility of consistently freezing the status of the filesystem at any given point in time (snapshot). Read more…
Knoppix uses it to allow updating the boot CD or adding software…without remastering the disk.
Slick.
Would this functionality permit the creation of a Restore Point (a la Windows XP) type application, whereby a filesystem can be returned to a specific backup?
Or does this sort of gui app already exist for gnu/linux?
From the article it sounds just like a Restore Point from WinXP.
No, it is not a Restore Point from WXP (although I am not familiar with it, it just doesn’t sound like it:). UnionFS allows you to merge multiple directories into one — you can think of it as a directory level raid, if you like.
UnionFS, unfortunately, is still rather broken piece of software (although it does get better and better, I have to say) — if you use it, be ready that you may get some OOPSes. The best appliance indeed is LiveCDs and eg. web kiosks.
However, you indeed could probably use it for versioning — it would just need some wrapper around it, probably by making a new rw directory for each boot, merging the older ones into ro.
— iSteve, author of DeadCD, first LiveCD to use UnionFS on 2.6.
Would this functionality permit the creation of a Restore Point (a la Windows XP) type application, whereby a filesystem can be returned to a specific backup?
Here’s a good description;
“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.”
http://aplawrence.com/Words/2004_11_21.html
That allows you to do interesting things such as update a read-only device like a CD, or to “delete” a file without removing it (whiteouts). Read the rest of the link for a few other interesting ideas.
Union mounts are not exclusive to Linux…many other unix-like operating systems have had union file system support.
Or does this sort of gui app already exist for gnu/linux?
It’s not an app. It’s part of the operating system; it’s transparent to applications and other tools running under the OS. Any normal tool would be used to manage the data — gui or not. Some special modes can be used, but the defaults are usually fine.
Additional links;
http://www.fsl.cs.sunysb.edu/project-unionfs.html (mailing lists, other details)
http://www.unionfs.org (technical; few examples)
From the article it sounds just like a Restore Point from WinXP.
While you can use it like one, it’s not the same.
http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/snapshots…
I suppose you guys invented trolling, too
Have a look at Fossil/Venti, the technologies behind archival of a distributed FS and on-the-fly access to past snapshots:
http://www.cs.bell-labs.com/sys/doc/fs/fs.html
http://www.cs.bell-labs.com/sys/doc/venti/venti.html
And while you’re at there, read the whole thing about Plan9, it’s worth it. (Those guys altogether have created more things in the OS field that anybody else, they know what they’re doing…)
http://www.cs.bell-labs.com/sys/doc/index.html
(that is iff ext3cow is ported
to 2.6 and accepted in the kernel…)
The unionfs enabled the “maxi” DVD by Klaus Knopper wherein it is used to provide a “split” of the cloop compressed program data into two /KNOPPIX files. This enables the .iso to reach 4.0GB size on a Live DVD having 2GB on each of two compressed files.
Download is bittorent at this writing.
(that is iff ext3cow is ported
to 2.6 and accepted in the kernel…)
The two aren’t the same.
That said, ext3cow is quite nice;
http://www.ext3cow.com
Except it was only available in the unstable 5.x branch, and Solaris had fssnap first anyway…