Re: A question about subvolumes

[Goffredo Baroncelli <kreijack@libero.it>]

On 07/04/2014 04:38 PM, Bob Williams wrote:
> I have a disc formatted as btrfs, on which is mounted /home.
> 
> /home/bob is a regular directory.
> 
> /home/bob/Documents is a btrfs subvolume
> 
> /home is btrfs root
> 
> If I do
> 
> # mv /home/bob /home/bob_original
> # btrfs subvolume create /home/bob
> # mv /home/bob_original/* /home/bob/
> # rm /home/bob_original
> 
> will the original subvolume /home/bob/Documents survive this
> operation, and will it now exist as a subvolume under the new
> subvolume /home/bob?

Yes. 

A subvolume is a way to partition a btrfs file-system. You can think a subvolume like a filesystem. 
For objects like files and directory a move command between *different* subvolumes is equal to a copy+remove. If fact is like you are moving data between different file-systems.

Instead moving a subvolume  in *its btrfs filesystem* is a cheap operation (is like moving a link); this is true even if you move a subvolume between different subvolumes.

To increase the speed when you move files between subvolumes (of the *same* btrfs filesystem), you could do a "cp --reflink" + "rm" instead of a "mv"; eg

	# mv subvolume-A/* subvolume-B/

is equal to

	# cp -rf  subvolume-A/* subvolume-B/
	# rm -rf subvolume-A/*

but 
	# cp --reflink -R  subvolume-A/* subvolume-B/
	# rm -rf subvolume-A/

is faster  because "cp --reflink" shared the data between source and destination. This means that you are doing a copy (and a delete) of metadata only.




> 
> I realise it's best to create subvolumes progressively from the top of
> the filesystem tree, but this system originated as an ext4fs which was
> migrated to btrfs, and some sensible things got missed in all the
> excitement. ;-)
> 
> Bob
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