Re: how many chunk trees and extent trees present

[David Sterba <dsterba@suse.cz>]

On Fri, Mar 04, 2016 at 10:58:15AM +0800, Anand Jain wrote:
> 
> 
> On 04/18/2015 01:29 AM, David Sterba wrote:
> > On Fri, Apr 17, 2015 at 09:19:11AM +0000, Hugo Mills wrote:
> >>> In, some article i read that future there will be more chunk tree/ extent
> >>> tree for single btrfs. Is this true.
> >>
> >>     I recall, many moons ago, Chris saying that there probably wouldn't
> >> be.
> >
> > More extent trees tied to a set of fs trees/subvolumes would be very
> > useful for certain usecases *cough*encryption*cough*.
> 
>   I didn't understand in full what's the idea here, but let met try..
>   would it not defeat the purpose of encryption which is not to let
>   disk have the un-encrypted data ? Looks like I am missing something
>   here.

Depends how the encryption is designed. The separate extent trees would
allow to have mixed data in the filesystem, encrypted or not.

I can start with a normal filesystem, and then create encrypted
subvolumes any time later.

The idea of multiple extent trees:

Currently we have only one, all subvolume share the extent tree, can do
reflinks freely. We can create a subvolume (S1) and ask for a separate
extent tree (E1). Now we can create snapshots of S1 that would share E1,
and reflink accross snapshots that share E1.

Why is this useful to encryption: all data _and_ metadata blocks tied to
E1 and the attached subvolums are encrypted, the plain text is not
accessible without the key.

But the separate extent trees are useful on itself.