On 2019/5/17 下午5:43, David Sterba wrote:
> The list of profiles in btrfs_chunk_max_errors lists DUP as a profile
> DUP able to tolerate 1 device missing. Though this profile is special
> with 2 copies, it still needs the device, unlike the others.
> 
> Looking at the history of changes, thre's no clear reason why DUP is
> there, functions were refactored and blocks of code merged to one
> helper.
> 
> d20983b40e828 Btrfs: fix writing data into the seed filesystem
>   - factor code to a helper
> 
> de11cc12df173 Btrfs: don't pre-allocate btrfs bio
>   - unrelated change, DUP still in the list with max errors 1
> 
> a236aed14ccb0 Btrfs: Deal with failed writes in mirrored configurations
>   - introduced the max errors, leaves DUP and RAID1 in the same group
> 
> CC: Qu Wenruo <wqu@suse.com>
> Signed-off-by: David Sterba <dsterba@suse.com

Reviewed-by: Qu Wenruo <wqu@suse.com>

Just some extra hint for the tolerance of DUP profile.

In case of DUP, either all stripes are missing, or all stripes exist.

So no matter whether the tolerance is 0 or 1, it will always work.
But indeed, setting it to 0 is more accurate.

Thanks,
Qu
> ---
>  fs/btrfs/volumes.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index 1c2a6e4b39da..8508f6028c8d 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -5328,8 +5328,7 @@ static inline int btrfs_chunk_max_errors(struct map_lookup *map)
>  
>  	if (map->type & (BTRFS_BLOCK_GROUP_RAID1 |
>  			 BTRFS_BLOCK_GROUP_RAID10 |
> -			 BTRFS_BLOCK_GROUP_RAID5 |
> -			 BTRFS_BLOCK_GROUP_DUP)) {
> +			 BTRFS_BLOCK_GROUP_RAID5)) {
>  		max_errors = 1;
>  	} else if (map->type & BTRFS_BLOCK_GROUP_RAID6) {
>  		max_errors = 2;
>