[PATCH 0/2] btrfs: read_policy type device

[PATCH 0/2] btrfs: read_policy type device

[Anand Jain]

This patch set provides a read_policy type device and was part of the
other read_policies before [1].

[1]
  Re: [PATCH v4 0/3, full-cover-letter] btrfs: read_policy types latency, device and round-robin
  https://lore.kernel.org/linux-btrfs/20210120123437.OVx7ybGaVfmOdZxtpp43qcB_ORHQQs5OzPSzr3ZUGbo@z/T/

I am sending them separately as they help to test the integrity of the
mirrored RAID devices.

No change from the previous V4 except for the rebase conflict fixes.

Anand Jain (2):
  btrfs: introduce new device-state read_preferred
  btrfs: introduce new read_policy device

 fs/btrfs/sysfs.c   | 55 +++++++++++++++++++++++++++++++++++++++++++++-
 fs/btrfs/volumes.c | 23 +++++++++++++++++++
 fs/btrfs/volumes.h |  3 +++
 3 files changed, 80 insertions(+), 1 deletion(-)

-- 
2.33.1

[PATCH 1/2] btrfs: introduce new device-state read_preferred

[Anand Jain]

This is a preparatory patch and introduces a new device flag
'read_preferred', RW-able using sysfs interface.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/btrfs/sysfs.c   | 53 ++++++++++++++++++++++++++++++++++++++++++++++
 fs/btrfs/volumes.h |  1 +
 2 files changed, 54 insertions(+)

diff --git a/fs/btrfs/sysfs.c b/fs/btrfs/sysfs.c
index d5d0717fd09a..ca9812cabece 100644
--- a/fs/btrfs/sysfs.c
+++ b/fs/btrfs/sysfs.c
@@ -1785,6 +1785,58 @@ static ssize_t btrfs_devinfo_error_stats_show(struct kobject *kobj,
 }
 BTRFS_ATTR(devid, error_stats, btrfs_devinfo_error_stats_show);
 
+static ssize_t btrfs_devinfo_read_pref_show(struct kobject *kobj,
+					    struct kobj_attribute *a, char *buf)
+{
+	int val;
+	struct btrfs_device *device = container_of(kobj, struct btrfs_device,
+						   devid_kobj);
+
+	val = !!test_bit(BTRFS_DEV_STATE_READ_PREFERRED, &device->dev_state);
+
+	return snprintf(buf, PAGE_SIZE, "%d\n", val);
+}
+
+static ssize_t btrfs_devinfo_read_pref_store(struct kobject *kobj,
+					     struct kobj_attribute *a,
+					     const char *buf, size_t len)
+{
+	int ret;
+	unsigned long val;
+	struct btrfs_device *device;
+
+	ret = kstrtoul(skip_spaces(buf), 0, &val);
+	if (ret)
+		return ret;
+
+	if (val != 0 && val != 1)
+		return -EINVAL;
+
+	/*
+	 * lock is not required, the btrfs_device struct can't be freed while
+	 * its kobject btrfs_device::devid_kobj is still open.
+	 */
+	device = container_of(kobj, struct btrfs_device, devid_kobj);
+
+	if (val &&
+	    !test_bit(BTRFS_DEV_STATE_READ_PREFERRED, &device->dev_state)) {
+		set_bit(BTRFS_DEV_STATE_READ_PREFERRED, &device->dev_state);
+		btrfs_info(device->fs_devices->fs_info,
+			   "set read preferred on devid %llu (%d)",
+			   device->devid, task_pid_nr(current));
+	} else if (!val &&
+		   test_bit(BTRFS_DEV_STATE_READ_PREFERRED, &device->dev_state)) {
+		clear_bit(BTRFS_DEV_STATE_READ_PREFERRED, &device->dev_state);
+		btrfs_info(device->fs_devices->fs_info,
+			   "reset read preferred on devid %llu (%d)",
+			   device->devid, task_pid_nr(current));
+	}
+
+	return len;
+}
+BTRFS_ATTR_RW(devid, read_preferred, btrfs_devinfo_read_pref_show,
+	      btrfs_devinfo_read_pref_store);
+
 /*
  * Information about one device.
  *
@@ -1798,6 +1850,7 @@ static struct attribute *devid_attrs[] = {
 	BTRFS_ATTR_PTR(devid, replace_target),
 	BTRFS_ATTR_PTR(devid, scrub_speed_max),
 	BTRFS_ATTR_PTR(devid, writeable),
+	BTRFS_ATTR_PTR(devid, read_preferred),
 	NULL
 };
 ATTRIBUTE_GROUPS(devid);
diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
index 5639961b3626..f04a177136b5 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -85,6 +85,7 @@ struct btrfs_io_geometry {
 #define BTRFS_DEV_STATE_REPLACE_TGT	(3)
 #define BTRFS_DEV_STATE_FLUSH_SENT	(4)
 #define BTRFS_DEV_STATE_NO_READA	(5)
+#define BTRFS_DEV_STATE_READ_PREFERRED	(6)
 
 struct btrfs_zoned_device_info;
 
-- 
2.33.1

[PATCH 2/2] btrfs: introduce new read_policy device

[Anand Jain]

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