[PATCH] Btrfs: enchanse raid1/10 balance heuristic for non rotating devices

[Timofey Titovets <nefelim4ag@gmail.com>]

Currently btrfs raid1/10 balancer blance requests to mirrors,
based on pid % num of mirrors.

Update logic and make it understood if underline device are non rotational.

If one of mirrors are non rotational, then all read requests will be moved to
non rotational device.

If both of mirrors are non rotational, calculate sum of
pending and in flight request for queue on that bdev and use
device with least queue leght.

P.S.
Inspired by md-raid1 read balancing

Signed-off-by: Timofey Titovets <nefelim4ag@gmail.com>
---
 fs/btrfs/volumes.c | 59 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 59 insertions(+)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 9a04245003ab..98bc2433a920 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -5216,13 +5216,30 @@ int btrfs_is_parity_mirror(struct btrfs_fs_info *fs_info, u64 logical, u64 len)
 	return ret;
 }
 
+static inline int bdev_get_queue_len(struct block_device *bdev)
+{
+	int sum = 0;
+	struct request_queue *rq = bdev_get_queue(bdev);
+
+	sum += rq->nr_rqs[BLK_RW_SYNC] + rq->nr_rqs[BLK_RW_ASYNC];
+	sum += rq->in_flight[BLK_RW_SYNC] + rq->in_flight[BLK_RW_ASYNC];
+
+	/*
+	 * Try prevent switch for every sneeze
+	 * By roundup output num by 2
+	 */
+	return ALIGN(sum, 2);
+}
+
 static int find_live_mirror(struct btrfs_fs_info *fs_info,
 			    struct map_lookup *map, int first, int num,
 			    int optimal, int dev_replace_is_ongoing)
 {
 	int i;
 	int tolerance;
+	struct block_device *bdev;
 	struct btrfs_device *srcdev;
+	bool all_bdev_nonrot = true;
 
 	if (dev_replace_is_ongoing &&
 	    fs_info->dev_replace.cont_reading_from_srcdev_mode ==
@@ -5231,6 +5248,48 @@ static int find_live_mirror(struct btrfs_fs_info *fs_info,
 	else
 		srcdev = NULL;
 
+	/*
+	 * Optimal expected to be pid % num
+	 * That's generaly ok for spinning rust drives
+	 * But if one of mirror are non rotating,
+	 * that bdev can show better performance
+	 *
+	 * if one of disks are non rotating:
+	 *  - set optimal to non rotating device
+	 * if both disk are non rotating
+	 *  - set optimal to bdev with least queue
+	 * If both disks are spinning rust:
+	 *  - leave old pid % nu,
+	 */
+	for (i = 0; i < num; i++) {
+		bdev = map->stripes[i].dev->bdev;
+		if (!bdev)
+			continue;
+		if (blk_queue_nonrot(bdev_get_queue(bdev)))
+			optimal = i;
+		else
+			all_bdev_nonrot = false;
+	}
+
+	if (all_bdev_nonrot) {
+		int qlen;
+		/* Forse following logic choise by init with some big number */
+		int optimal_dev_rq_count = 1 << 24;
+
+		for (i = 0; i < num; i++) {
+			bdev = map->stripes[i].dev->bdev;
+			if (!bdev)
+				continue;
+
+			qlen = bdev_get_queue_len(bdev);
+
+			if (qlen < optimal_dev_rq_count) {
+				optimal = i;
+				optimal_dev_rq_count = qlen;
+			}
+		}
+	}
+
 	/*
 	 * try to avoid the drive that is the source drive for a
 	 * dev-replace procedure, only choose it if no other non-missing
-- 
2.15.1