Trying to understand and learn how to use btrfs

I have been the past days trying to get my head around how btrfs works
I have been trying tools like mkinitcpio-btrfs (very poorly documented) and now, if i list the subvolumes i have in certain volume (/) i see i have 4 subvolumes that have been created while playing around.
If i try to delete them with "sudo btrfs subvolume delete __active", for example, I get a "ERROR: Error accessing '__active'
What am I doing wrong?
Also, I cannot get the whole idea about the difference between snapshots and subvolumes, i mean, a snapshot should be a directory that saves the changes made on the fs, s, if you want to roll back those changes, you just have to make btrfs "forget" the changes stored in that directory and move along, but I cannot take the idea of the subvolume thing.....
As btrfs is quite experimental and the wikis are not noob-proof still, I'd appreciate if someone gave me a hand trying to understand these concept...
For the moment, I'm just using it on a test computer and on a personal laptop with no fear of data loss,.
Any help is welcome
Thanks!
Last edited by jasonwryan (2013-07-19 23:13:27)

I honestly think it is probably a better idea to not use mkinitpcio-btrfs. As mentioned above, it is poorly documented, and for me it has never worked right (if at all). It is an unofficial AUR package, and unfortunately our wiki still seems to give the false impression that using this package is the way to user btrfs with Arch.
The way I have my system set up is that in subvolid=0 (the root of the btrfs filesystem) I have a rootfs subvolume and a home subvolume (there are others, but these are what primarily make up my system). So in my fstab, I basically have two nearly identical lines, but one has no subvol specified and is mounted at /, and the other has 'subvol=home' mounted at /home.
So in order to make it so that I can change the root filesystem as I please, instead of having the / fstab entry specify the subvolume, I put it in the kernel command line. That is, I have 'rootflags=subvol=rootfs' in the kernel command line. So if I want to change it, I simply change the path to one of the snapshots.
Just remember that if you are one who likes a custom kernel, it is likely that you will have to have an initamfs no matter what you compile into your kernel. For one thing, the kernel has no mechanism for scanning for multiple device btrfs filesystems. But also, I have read that the kernel itself cannot handle the rootflags kernel command line argument.
Oracle Linux does something interesting with their default setup. They are not a rolling release, so this probably wouldn't work so well in Arch Linux, but they actually install the root filesystem (I think it is actually done to subvolid=0) and then after installation of the packages, a snapshot of the root filesystem is made, and the system is setup to boot off of that snapshot. So it is almost like having an overlayfs on openwrt. There is always a copy of the original system, and and changes that are being made are being done "on top" of the original. So in the event of an emergency, yo can always get back to the original working state.
If you put your root filesystem on something other than the root of the btrfs filesystem (which you should, as it makes the whole setup much more flexible), then you should also set up a mountpoint somewhere to give administrator access to the filesystem from subvolid=0. For example, I have an autofs mountpoint at /var/lib/btrfs-root. chose that spot because /var/lib is where devtools puts the clean chroot. So it seemed as reasonable a place as any.
You should go to the btrfs wiki, and peruse through the stuff there... not our wiki, but the actual btrfs one, as our wiki is pretty sparse. There is not all that much content there (not like the Arch wiki), but it does cover the features pretty well. I mean, there is certainly quite a lot for being information on only a single filesystem, but it shouldn't take you too long to get through it. There are a few links to articles about midway down the front page. What really gave me a better grasp of getting started with btrfs were the ones titled "How I Got Started with the Btrfs Filesystem for Oracle Linux" and "How I Use the Advanced Capabilities of Btrfs".

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at java.net.URLClassLoader...
there are several more lines but as I was typing them out I thought that maybe the code is not written to run it in the command window. The errors seem to be around the ClassLoader. Is that correct? On the SUN example page when you launch the program a Java Web Start runs. So does this mean that the code is designed to only run in WebStart?

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