Re: Creating a Btrfs subvolume

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On Wed, Dec 23, 2020 at 7:51 PM Chris Murphy <lists@xxxxxxxxxxxxxxxxx> wrote:
>
> mv will fail between destinations with different nodatacow settings,
> hence cp.

Sorry. This is wrong. mv also works.

mv tries three things in order: rename, reflink copy, traditional
copy. In the above case, it's the last one that works.

Rename fails because subvolumes are separate name spaces with their
own pile of inodes, similar to (in this case) separate file systems.
The reflink copy also fails because there is a cow nodatacow mismatch
between source and destination. It's not possible to have shared
extents that are both cow and nodatacow, so the file must be old
fashioned copied, so it has exclusive extents that are nodatacow. (The
copy task, once complete, is followed by an rm of the original file.
Hence it looks like a move except, yeah, it's slow because it's a big
VM file being copied.)


-- 
Chris Murphy
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