Re: subvols and parents - how?

[Christoph Anton Mitterer <calestyo@scientia.net>]

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On Tue, 2015-11-24 at 23:30 +0000, Hugo Mills wrote:
>    Yes, that makes sense.
Feel free to shamelessly use my idea (as well as the one to call btrfs'
RAID1 replica2 or something else)
:-O


>    With a recent mv
root@heisenberg:/mnt# mv --version
mv (GNU coreutils) 8.23

=> not recent enough...


> but I think you'll find that the UUID of the subvols changes. (At
> least, I hope it does. If it doesn't, then my mental model of what
> the FS is doing is *really* screwed up).
Well... see below:

root@heisenberg:~# truncate  -s 2G image
root@heisenberg:~# losetup -f image 
root@heisenberg:~# mkfs.btrfs /dev/loop0 
btrfs-progs v4.3
See http://btrfs.wiki.kernel.org for more information.

Performing full device TRIM (2.00GiB) ...
Label:              (null)
UUID:               10e1a55c-448a-4f37-ae5c-6a7801a7f202
Node size:          16384
Sector size:        4096
Filesystem size:    2.00GiB
Block group profiles:
  Data:             single            8.00MiB
  Metadata:         DUP             110.38MiB
  System:           DUP              12.00MiB
SSD detected:       no
Incompat features:  extref, skinny-metadata
Number of devices:  1
Devices:
   ID        SIZE  PATH
    1     2.00GiB  /dev/loop0

root@heisenberg:~# mount /dev/loop0 /mnt/
root@heisenberg:/mnt# btrfs subvolume create root
Create subvolume './root'
root@heisenberg:/mnt# btrfs subvolume create below-top
Create subvolume './below-top'
root@heisenberg:/mnt# cd root/
root@heisenberg:/mnt/root# btrfs subvolume create below-root
Create subvolume './below-root'
root@heisenberg:/mnt# btrfs subvolume list /mnt/ -pacguqt
ID	gen	cgen	parent	top level	parent_uuid	uuid	path	
--	---	----	------	---------	-----------	----	----	
257	9	7	5	5		-	8fbf521e-77f9-0d49-9891-87767f98c655	root
258	8	8	5	5		-	b49131e9-4207-aa42-8195-c50de5f06136	below-top
259	9	9	257	257		-	20c042be-ead8-204a-a684-94c1a770e739	<FS_TREE>/root/below-root

root@heisenberg:/mnt# mv root/below-root/ tmp
root@heisenberg:/mnt# mv below-top/ root/
root@heisenberg:/mnt# mv tmp/ below-root
root@heisenberg:/mnt# btrfs subvolume list /mnt/ -pacguqt
ID	gen	cgen	parent	top level	parent_uuid	uuid	path	
--	---	----	------	---------	-----------	----	----	
257	9	7	5	5		-	8fbf521e-77f9-0d49-9891-87767f98c655	root
258	8	8	257	257		-	b49131e9-4207-aa42-8195-c50de5f06136	<FS_TREE>/root/below-top
259	9	9	5	5		-	20c042be-ead8-204a-a684-94c1a770e739	below-root
root@heisenberg:/mnt# 


So the UUIDs seem to stay the same (or are these other UUIDs?)

Hope I haven't ruined your day now ;-)


Cheers,
Chris.

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