Re: subvols and parents - how?

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

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On Wed, 2015-12-09 at 10:53 +0000, Duncan wrote:
> If you use the recipe (subvol create, cp with reflink) it suggests
> there, 
> you'll end up with the reflinked copy in a subvol.
> 
> You can then mount that subvol over top of the existing dir, and
> *new* 
> file opens will access the new subvol, tho existing open files will
> of 
> course continue to access the files/reflinks to which they have a 
> reference, "underneath" the new mount.
Sure, which would mean however that a downtime is still necessary.



> For some services it's possible to signal them to reload their
> files.  
> Where this is possible, you can do the overmount trick and then
> signal 
> them to reload, and they should keep running, otherwise undisturbed
> (altho 
> any changes between the reflink and processing the signal will still
> go 
> to the existing open files, I don't believe there's a way around
> that).
Yep,.. but as you say,... it doesn't really help to avoid the
downtime,... rather lead to data corruption (not on the filesystem
level, of course, but within the application).


> But AFAIK that's the closest it gets, and nothing more along that
> line is 
> planned.
I've been so free to add that idea to the project ideas:
https://btrfs.wiki.kernel.org/index.php/Project_ideas#.28Atomically.29_convert_directories_into_subvolumes_and_vice_versa

any developer or more experienced user than me is of course free to
remove that again, if it seems not so important or isn't possible to be
implemented.


> In general, keep in mind that subvolumes work in most respects very
> much 
> like normal directories do, except where they don't. =:^)
:-P :-P 


Cheers,
Chris.

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