Re: [PATCH] btrfs: balance RAID1/RAID10 mirror selection

[Wang Yugui <wangyugui@e16-tech.com>]

Hi, Louis Jencka

Can we move 'atomic_t rr_counter' into 'struct btrfs_fs_info' to
support multiple mounted btrfs filesystem?

Although 'readmirror feature (read_policy sysfs)'  is talked about, 
round-robin is a replacement for BTRFS_READ_POLICY_PID in most case, 
we no longer need BTRFS_READ_POLICY_PID ?

Best Regards
Wang Yugui (wangyugui@e16-tech.com)
2020/10/23

> Balance RAID1/RAID10 mirror selection via plain round-robin scheduling. This should roughly double throughput for large reads.
> 
> Signed-off-by: Louis Jencka <louis@waffle.tech>
> ---
>  fs/btrfs/volumes.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index 58b9c419a..45c581d46 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -333,6 +333,9 @@ struct list_head * __attribute_const__ btrfs_get_fs_uuids(void)
>  	return &fs_uuids;
>  }
>  
> +/* Used for round-robin balancing RAID1/RAID10 reads. */
> +atomic_t rr_counter = ATOMIC_INIT(0);
> +
>  /*
>   * alloc_fs_devices - allocate struct btrfs_fs_devices
>   * @fsid:		if not NULL, copy the UUID to fs_devices::fsid
> @@ -5482,7 +5485,8 @@ static int find_live_mirror(struct btrfs_fs_info *fs_info,
>  	else
>  		num_stripes = map->num_stripes;
>  
> -	preferred_mirror = first + current->pid % num_stripes;
> +	preferred_mirror = first +
> +	    (unsigned)atomic_inc_return(&rr_counter) % num_stripes;
>  
>  	if (dev_replace_is_ongoing &&
>  	    fs_info->dev_replace.cont_reading_from_srcdev_mode ==