[PATCH 1/4] btrfs: add tier score to device

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

We use a single score value to define the tier level of a device.
Different score means different tier, and bigger is faster.
    DAX device(dax=1)
    SSD device(rotational=0)
    HDD device(rotational=1)
TODO/FIXME: FIXME: detect bus(DIMM, NVMe, SCSI, SATA, Virtio, ...)
TODO/FIXME: user-assigned property(refactoring the coming 'read_preferred' property?) to
            set to the max score for some not-well-supported case.

In most case, only 1 or 2 tiers are used at the same time, so we group them into
top tier and other tier(s).

Signed-off-by: wangyugui <wangyugui@e16-tech.com>
---
 fs/btrfs/volumes.c | 18 ++++++++++++++++++
 fs/btrfs/volumes.h |  5 +++++
 2 files changed, 23 insertions(+)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 58b9c41..efffcbc 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -608,6 +608,22 @@ static int btrfs_free_stale_devices(const char *path,
 	return ret;
 }
 
+/*
+ * Get the tier score to the device, bigger score is faster.
+ * FIXME: detect bus(DIMM, NVMe, SCSI, SATA, Virtio, ...)
+ * FIXME: detect media inside(SLC/MLC of SSD, SMR/PMR of HDD, ...)
+ * FIXME: user-assigned property to set to max score for some complex case.
+ */
+static void dev_get_tier_score(struct btrfs_device *device, struct request_queue *q)
+{
+	if (blk_queue_dax(q))
+		device->tier_score = 50;
+	else if (blk_queue_nonrot(q))
+		device->tier_score = 10;
+	else
+		device->tier_score = 0;
+}
+
 /*
  * This is only used on mount, and we are protected from competing things
  * messing with our fs_devices by the uuid_mutex, thus we do not need the
@@ -660,6 +676,7 @@ static int btrfs_open_one_device(struct btrfs_fs_devices *fs_devices,
 	}
 
 	q = bdev_get_queue(bdev);
+	dev_get_tier_score(device,q);
 	if (!blk_queue_nonrot(q))
 		fs_devices->rotating = true;
 
@@ -2590,6 +2607,7 @@ int btrfs_init_new_device(struct btrfs_fs_info *fs_info, const char *device_path
 
 	atomic64_add(device->total_bytes, &fs_info->free_chunk_space);
 
+	dev_get_tier_score(device,q);
 	if (!blk_queue_nonrot(q))
 		fs_devices->rotating = true;
 
diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
index bf27ac0..cf426ec 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -138,6 +138,11 @@ struct btrfs_device {
 	struct completion kobj_unregister;
 	/* For sysfs/FSID/devinfo/devid/ */
 	struct kobject devid_kobj;
+
+	/* Storage tier score, bigger score is faster.
+	 * In most case, only 1 or 2 tiers are used at the same time, so we group them
+	 * into top tier and other tier(s). */
+	u8 tier_score;
 };
 
 /*
-- 
2.29.1