Re: Query about proposed dedup patches and behaviours

["Austin S. Hemmelgarn" <ahferroin7@gmail.com>]

On 2016-01-14 11:13, James Hogarth wrote:
> Hi,
>
> The duperemove[1] tool is in the process for packaging for Fedora at
> present but I was wondering what future this may have with the 4.5
> dedup patches being proposed.
>
> WIll the btrfs command have the ability to out-of-line dedup files
> similar to duperemove (thus negating the need for it) or will this
> only control in-line dedup with a tool like duperemove still being
> required for periodic only (or restricted path) dedup?
Unless I'm horribly misreading the code, the regular btrfs-progs will 
not be adding the ability to do out-of-band deduplication.  It may at 
some point add a shortcut for the required ioctl to be used from 
scripts, but that's probably unlikely.
>
> To avoid memory usage bloat if the btrfs command can order dedup  of X
> files on the path correctly can it be passed a path to carry the hash
> map in some form (similar to how dupeemeove can use sqlite for this)
> or is this another use case for the external tool?
This shouldn't be an issue for in-line deduplication, as that's handled 
in the kernel.
>
> Finally what's the present situation with regards to defragmentation
> and deduplication? Is it safe to turn on autodefrag now when using
> snapshots and duperemove? What should the behaviour be with the
> proposed 4.5 dedup patches if both inline dedup and autodefrag are
> enabled as mount options?
I'm not entirely certain how deduplication would interact with any form 
of defragmentation.  I'm pretty certain though that autodefrag does 
properly handle snapshots, such that the reflinks aren't broken, and 
it's the original copy that gets any shared extents defragmented into it.