Re: [PATCH 1/3] btrfs: add readmirror type framework

[Josef Bacik <josef@toxicpanda.com>]

On 12/20/19 12:06 AM, Anand Jain wrote:
> As of now we use %pid method to read stripped mirrored data. So
> application's process id determines the stripe id to be read. This type
> of routing typically helps in a system with many small independent
> applications tying to read random data. On the other hand the %pid
> based read IO distribution policy is inefficient if there is a single
> application trying to read large data and the overall disk bandwidth
> remains under utilized.
> 
> So this patch introduces a framework where we could add more readmirror
> policies, such as routing the IO based on device's waitqueue or manual
> when we have a read-preferred device or a policy based on the target
> storage caching.
> 
> Signed-off-by: Anand Jain <anand.jain@oracle.com>
> ---
>   fs/btrfs/volumes.c | 16 +++++++++++++++-
>   fs/btrfs/volumes.h |  8 ++++++++
>   2 files changed, 23 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index c95e47aa84f8..0c6caae29248 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -1162,6 +1162,8 @@ static int open_fs_devices(struct btrfs_fs_devices *fs_devices,
>   	fs_devices->opened = 1;
>   	fs_devices->latest_bdev = latest_dev->bdev;
>   	fs_devices->total_rw_bytes = 0;
> +	/* Set the default readmirror policy */
> +	atomic_set(&fs_devices->readmirror, BTRFS_READMIRROR_DEFAULT);
There's no reason for this to be atomic, it's just a behavior change, if you 
really want to be super safe use READ_ONCE/WRITE_ONCE and have readmirror be 
your enum.  Thanks,

Josef