Re: [PATCH v3] Btrfs: avoid losing data raid profile when deleting a device

[David Sterba <dsterba@suse.cz>]

On Wed, Nov 15, 2017 at 04:28:11PM -0700, Liu Bo wrote:
> We've avoided data losing raid profile when doing balance, but it
> turns out that deleting a device could also result in the same
> problem.
> 
> Say we have 3 disks, and they're created with '-d raid1' profile.
> 
> - We have chunk P (the only data chunk on the empty btrfs).
> 
> - Suppose that chunk P's two raid1 copies reside in disk A and disk B.
> 
> - Now, 'btrfs device remove disk B'
>          btrfs_rm_device()
> 	   -> btrfs_shrink_device()
> 	      -> btrfs_relocate_chunk() #relocate any chunk on disk B
> 	      	 			 to other places.
> 
> - Chunk P will be removed and a new chunk will be created to hold
>   those data, but as chunk P is the only one holding raid1 profile,
>   after it goes away, the new chunk will be created as single profile
>   which is our default profile.
> 
> This fixes the problem by creating an empty data chunk before
> relocating the data chunk.
> 
> Metadata/System chunk are supposed to have non-zero bytes all the time
> so their raid profile is preserved.
> 
> Reported-by: James Alandt <James.Alandt@wdc.com>
> Signed-off-by: Liu Bo <bo.li.liu@oracle.com>

Added to 4.16 queue, thanks.