[PATCH 3/4] btrfs: introduce new read_policy device

[Anand Jain <anand.jain@oracle.com>]

Read-policy type 'device' and device flag 'read-preferred':

The read-policy type device picks the device(s) flagged as
read-preferred for reading chunks of type raid1, raid10,
raid1c3 and raid1c4.

A system might contain SSD, nvme, iscsi or san lun, and which are all
a non-rotational device, so it is not a good idea to set the read-preferred
automatically. Instead device read-policy along with the read-preferred
flag provides an ability to do it manually. This advance tuning is
useful in more than one situation, for example,
 - In heterogeneous-disk volume, it provides an ability to manually choose
    the low latency disks for reading.
 - Useful for more accurate testing.
 - Avoid known problematic device from reading the chunk until it is
   replaced (by marking the other good devices as read-preferred).

Note:

If the read-policy type is set to 'device', but there isn't any device
which is flagged as read-preferred, then stripe 0 is used for reading.

The device replace won't migrate the read-preferred flag to the new
replace the target device.

As of now, this is an in-memory only feature.

It's pointless to set the read-preferred flag on the missing device,
as IOs aren't submitted to the missing device.

If there is more than one read-preferred device in a chunk, the read IO
shall go to the stripe 0 (as of now, when depth patches are integrated
we will use the least busy device among the read-preferred devices).

Usage example:

Consider a typical two disks raid1.

Configure devid1 for reading.

$ echo 1 > devinfo/1/read_preferred
$ cat devinfo/1/read_preferred; cat devinfo/2/read_preferred
1
0

$ pwd
/sys/fs/btrfs/12345678-1234-1234-1234-123456789abc

$ cat read_policy; echo device > ./read_policy; cat read_policy
[pid] device
pid [device]

Now read IOs are sent to devid 1 (sdb).

$ echo 3 > /proc/sys/vm/drop_caches; md5sum /btrfs/YkZI

$ iostat -zy 1 | egrep 'sdb|sdc' (from another terminal)
sdb              50.00     40048.00         0.00      40048          0

Change the read-preferred device from devid 1 to devid 2 (sdc).

$ echo 0 > ./devinfo/1/read_preferred; echo 1 > ./devinfo/2/read_preferred;

[ 3343.918658] BTRFS info (device sdb): reset read preferred on devid 1 (1334)
[ 3343.919876] BTRFS info (device sdb): set read preferred on devid 2 (1334)

Further read ios are sent to devid 2 (sdc).

$ echo 3 > /proc/sys/vm/drop_caches; md5sum /btrfs/YkZI

$ iostat -zy 1 | egrep 'sdb|sdc' (from another terminal)
sdc              49.00     40048.00         0.00      40048          0

Signed-off-by: Anand Jain <anand.jain@oracle.com>
---
 fs/btrfs/sysfs.c   |  3 ++-
 fs/btrfs/volumes.c | 22 ++++++++++++++++++++++
 fs/btrfs/volumes.h |  1 +
 3 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/sysfs.c b/fs/btrfs/sysfs.c
index 52b4c9bef673..d2a974e1a1c4 100644
--- a/fs/btrfs/sysfs.c
+++ b/fs/btrfs/sysfs.c
@@ -907,7 +907,8 @@ static bool btrfs_strmatch(const char *given, const char *golden)
 }
 
 /* Must follow the order as in enum btrfs_read_policy */
-static const char * const btrfs_read_policy_name[] = { "pid", "latency" };
+static const char * const btrfs_read_policy_name[] = { "pid", "latency",
+						       "device" };
 
 static ssize_t btrfs_read_policy_show(struct kobject *kobj,
 				      struct kobj_attribute *a, char *buf)
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 48587009b656..7ac675504051 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -5502,6 +5502,25 @@ static int btrfs_find_best_stripe(struct btrfs_fs_info *fs_info,
 	return best_stripe;
 }
 
+static int btrfs_find_read_preferred(struct map_lookup *map, int first, int num_stripe)
+{
+	int stripe_index;
+	int last = first + num_stripe;
+
+	/*
+	 * If there are more than one read preferred devices, then just pick the
+	 * first found read preferred device as of now.
+	 */
+	for (stripe_index = first; stripe_index < last; stripe_index++) {
+		if (test_bit(BTRFS_DEV_STATE_READ_PREFERRED,
+			     &map->stripes[stripe_index].dev->dev_state))
+			return stripe_index;
+        }
+
+	/* If there is no read preferred device then just use the first stripe */
+	return first;
+}
+
 static int find_live_mirror(struct btrfs_fs_info *fs_info,
 			    struct map_lookup *map, int first,
 			    int dev_replace_is_ongoing)
@@ -5536,6 +5555,9 @@ static int find_live_mirror(struct btrfs_fs_info *fs_info,
 		preferred_mirror = btrfs_find_best_stripe(fs_info, map, first,
 							  num_stripes);
 		break;
+	case BTRFS_READ_POLICY_DEVICE:
+		preferred_mirror = btrfs_find_read_preferred(map, first, num_stripes);
+		break;
 	}
 
 	if (dev_replace_is_ongoing &&
diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
index f1cbbb18f5ef..1448adb8993d 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -219,6 +219,7 @@ enum btrfs_chunk_allocation_policy {
 enum btrfs_read_policy {
 	BTRFS_READ_POLICY_PID,
 	BTRFS_READ_POLICY_LATENCY,
+	BTRFS_READ_POLICY_DEVICE,
 	BTRFS_NR_READ_POLICY,
 };
 
-- 
2.25.1