[Xen-users] How to replicate a Xen user domain using BTRFS as the root filesystem.

[Austin S Hemmelgarn <ahferroin7@xxxxxxxxx>]

I would generally put something like this on the wiki, but I don't have 
an account and don't really have interest in getting an account.  I'm 
posting it here in the hopes that this will be useful to someone other 
than just me, and because it's something I've never seen suggested 
anywhere before (although I did post it to the BTRFS mailing list 
earlier today).
In essence, this makes use of BTRFS's send/receive feature to preform a 
safe online clone of an existing user domain with relatively minimal 
service degradation for that existing domain.  It goes as far as 
replicating block layout and inode numbers without needing to do a full 
block level copy of the domain's storage devices (which isn't safe with 
BTRFS anyway).
Using this methodology, I can have a new Gentoo PV domain running in 
about half an hour with all of my custom local configuration, whereas it 
takes me at least two and a half hours (and often much longer than that) 
when using the regular install process for Gentoo.
These steps assume you have a decent working knowledge of BTRFS and 
Linux in general, as well as access to both Domain-0 and the user domain 
you are copying (refereed to below as the 'source domain'.  I've only 
used this myself with Gentoo, but it should work for almost any Linux 
distro. I've also only done this with full PV domains, it's a lot more 
involved to do it with HVM domains the hotplug stuff for block devices 
in HVM domains can be tricky), although PVH domains should work with 
exactly the same steps as regular PV domains. Your source domain's root 
also needs to be in a subvolume on the root filesystem (and needs of 
course to be on a BTRFS filesystem), this won't work if it isn't.
1. Create your backend storage devices for the new domain.
2. Attach the storage device (or devices) that will hold the root 
filesystem for the new domain to the source domain.  Using xl, this 
translates to something similar to: 'xl block-attach source 
"/dev/vg/target-disk,raw,xvdz"'
3. From the source domain, use mkfs.btrfs to create a BTRFS filesystem 
on the target disk.
4. On the source domain, mount both the root filesystem (don't bind 
mount it, and make sure that you are mounting the top-level, not a 
subvolume) and the target filesystem.
5. Create a read-only snapshot of the root subvolume from the source 
filesystem.
6. Use btrfs send piped to btrfs receive to copy the snapshot from the 
source filesystem to the target filesystem.  This will likely take quite 
some time (on the low-end server equivalent hardware I have, it takes 
about 2o-25 minutes for a somewhat minimalistic Gentoo installation).
7. While btrfs send/receive is running, prepare the configuration file 
for the target domain (I usually just copy the config from the source 
domain, and then change only what I need to).
8. Once the send/receive operation is complete, use btrfs property set 
to change the snapshot on the target to be writable, then rename it to 
whatever you want the root subvolume to be called on the target system.
9. In the newly created root subvolume, change any system specific 
configuration to what is needed for the new system (at a bare minimum, 
you probably need to change the hostname and networking configuration, 
and should verify that /etc/fstab and /etc/localtime are correct for the 
target system).
10. Unmount the target filesystem in the source domain.
11. Use 'xl block-detach' to detach the target device from the source 
domain.
12. Use your regular tools to start your new domain, log in, and preform 
any final configuration needed.

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