Re: [PATCH v2 1/3] btrfs: add readmirror type framework

[Josef Bacik <josef@toxicpanda.com>]

On 1/2/20 5:12 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 read IO 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 wait-queue 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>
> ---
> v2: Declare fs_devices::readmirror as u8 instead of atomic_t
>      A small change in comment and change log wordings.
> 
>   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..e26af766f2b9 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 */
> +	fs_devices->readmirror = BTRFS_READMIRROR_DEFAULT;
>   out:
>   	return ret;
>   }
> @@ -5300,7 +5302,19 @@ static int find_live_mirror(struct btrfs_fs_info *fs_info,
>   	else
>   		num_stripes = map->num_stripes;
>   
> -	preferred_mirror = first + current->pid % num_stripes;
> +	switch (fs_info->fs_devices->readmirror) {
> +	case BTRFS_READMIRROR_BY_PID:
> +		preferred_mirror = first + current->pid % num_stripes;
> +		break;
> +	default:
> +		/*
> +		 * Shouln't happen, just warn and use by_pid instead of failing.
> +		 */
> +		btrfs_warn_rl(fs_info,
> +			      "unknown readmirror type %u, fallback to by_pid",
> +			      fs_info->fs_devices->readmirror);
> +		preferred_mirror = first + current->pid % num_stripes;
> +	}
>   
>   	if (dev_replace_is_ongoing &&
>   	    fs_info->dev_replace.cont_reading_from_srcdev_mode ==
> diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
> index 68021d1ee216..f5f091f3c72b 100644
> --- a/fs/btrfs/volumes.h
> +++ b/fs/btrfs/volumes.h
> @@ -209,6 +209,12 @@ struct btrfs_device {
>   BTRFS_DEVICE_GETSET_FUNCS(disk_total_bytes);
>   BTRFS_DEVICE_GETSET_FUNCS(bytes_used);
>   
> +/* readmirror_policy types */
> +#define BTRFS_READMIRROR_DEFAULT	BTRFS_READMIRROR_BY_PID
> +enum btrfs_readmirror_policy_type {
> +	BTRFS_READMIRROR_BY_PID,
> +};
> +
>   struct btrfs_fs_devices {
>   	u8 fsid[BTRFS_FSID_SIZE]; /* FS specific uuid */
>   	u8 metadata_uuid[BTRFS_FSID_SIZE];
> @@ -260,6 +266,8 @@ struct btrfs_fs_devices {
>   	struct kobject *devices_kobj;
>   	struct kobject *devinfo_kobj;
>   	struct completion kobj_unregister;
> +
> +	u8 readmirror;

The only valid values for this are the enum, so make this

enum btrfs_readmirror_policy_type readmirror;

Thanks,

Josef