[PATCH 2/2] 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 stripes 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 advanced 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 replacement 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.

Usage example:

Consider a typical two disks raid1.

Configure devid1 for reading.

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

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

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

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

 $ echo 1 > ./devinfo/2/read_preferred

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

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

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

 $ 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>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/btrfs/sysfs.c   |  2 +-
 fs/btrfs/volumes.c | 23 +++++++++++++++++++++++
 fs/btrfs/volumes.h |  2 ++
 3 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/sysfs.c b/fs/btrfs/sysfs.c
index ca9812cabece..7c15fa1a8b33 100644
--- a/fs/btrfs/sysfs.c
+++ b/fs/btrfs/sysfs.c
@@ -1169,7 +1169,7 @@ static bool strmatch(const char *buffer, const char *string)
 	return false;
 }
 
-static const char * const btrfs_read_policy_name[] = { "pid" };
+static const char * const btrfs_read_policy_name[] = { "pid", "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 272901514b0c..cf358926b52a 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -5793,6 +5793,25 @@ int btrfs_is_parity_mirror(struct btrfs_fs_info *fs_info, u64 logical, u64 len)
 	return ret;
 }
 
+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)
@@ -5822,6 +5841,10 @@ static int find_live_mirror(struct btrfs_fs_info *fs_info,
 	case BTRFS_READ_POLICY_PID:
 		preferred_mirror = first + (current->pid % 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 f04a177136b5..e62252061606 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -260,6 +260,8 @@ enum btrfs_chunk_allocation_policy {
 enum btrfs_read_policy {
 	/* Use process PID to choose the stripe */
 	BTRFS_READ_POLICY_PID,
+	/* Use the device marked with READ_PREFERRED state */
+	BTRFS_READ_POLICY_DEVICE,
 	BTRFS_NR_READ_POLICY,
 };
 
-- 
2.33.1