Re: [PATCH 1/6] btrfs: add flags to give an hint to the chunk allocator

[Boris Burkov <boris@bur.io>]

On Fri, Dec 17, 2021 at 07:47:17PM +0100, Goffredo Baroncelli wrote:
> From: Goffredo Baroncelli <kreijack@inwind.it>
> 
> Add the following flags to give an hint about which chunk should be
> allocated in which a disk:
> 
> - BTRFS_DEV_ALLOCATION_HINT_PREFERRED_DATA
>   preferred data chunk, but metadata chunk allowed
> - BTRFS_DEV_ALLOCATION_HINT_PREFERRED_METADATA
>   preferred metadata chunk, but data chunk allowed
> - BTRFS_DEV_ALLOCATION_HINT_METADATA_ONLY
>   only metadata chunk allowed
> - BTRFS_DEV_ALLOCATION_HINT_DATA_ONLY
>   only data chunk allowed

Weighing in on the naming discussion:

I think DATA_ONLY > ONLY_DATA, with my best argument for this subjective
opinion being that it follows the example of read-only.

Therefore, I think you should go with DATA_ONLY, DATA_PREFERRED,
METADATA_ONLY, METADATA_PREFERRED. Definitely put ONLY/PREFERRED on the
same side of DATA/METADATA for all four, regardless of which you choose.

Looks good otherwise.

> 
> Signed-off-by: Goffredo Baroncelli <kreijack@inwid.it>
> ---
>  include/uapi/linux/btrfs_tree.h | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/include/uapi/linux/btrfs_tree.h b/include/uapi/linux/btrfs_tree.h
> index 5416f1f1a77a..55da906c2eac 100644
> --- a/include/uapi/linux/btrfs_tree.h
> +++ b/include/uapi/linux/btrfs_tree.h
> @@ -386,6 +386,22 @@ struct btrfs_key {
>  	__u64 offset;
>  } __attribute__ ((__packed__));
>  
> +/* dev_item.type */
> +
> +/* btrfs chunk allocation hint */
> +#define BTRFS_DEV_ALLOCATION_HINT_BIT_COUNT	2
> +/* btrfs chunk allocation hint mask */
> +#define BTRFS_DEV_ALLOCATION_HINT_MASK	\
> +	((1 << BTRFS_DEV_ALLOCATION_HINT_BIT_COUNT) -1)
> +/* preferred data chunk, but metadata chunk allowed */
> +#define BTRFS_DEV_ALLOCATION_HINT_PREFERRED_DATA	(0ULL)
> +/* preferred metadata chunk, but data chunk allowed */
> +#define BTRFS_DEV_ALLOCATION_HINT_PREFERRED_METADATA	(1ULL)
> +/* only metadata chunk are allowed */
> +#define BTRFS_DEV_ALLOCATION_HINT_METADATA_ONLY		(2ULL)
> +/* only data chunk allowed */
> +#define BTRFS_DEV_ALLOCATION_HINT_DATA_ONLY		(3ULL)
> +
>  struct btrfs_dev_item {
>  	/* the internal btrfs device id */
>  	__le64 devid;
> -- 
> 2.34.1
>