[PATCH v7 0/5] readmirror feature (sysfs and in-memory only approach; with new read_policy device)

[PATCH v7 0/5] readmirror feature (sysfs and in-memory only approach; with new read_policy device)

[Anand Jain]

v7:
Fix switch's fall through warning. Changle logs updates where necessary.

v6:
Patch 4/5 - If there is no change in device's read prefer then don't log
Patch 4/5 - Add pid to the logs
Patch 5/5 - If there isn't read preferred device in the chunk don't reset
read policy to default, instead just use stripe 0. As this is in
the read path it avoids going through the device list to find
read preferred device. So inline to this drop to check if there
is read preferred device before setting read policy to device.

__ Original email: __

v5:
Worked on review comments as received in its previous version.
Please refer to individual patches for the specific changes.
Introduces the new read_policy 'device'.

v4:
Rename readmirror attribute to read_policy. Drop separate kobj for
readmirror instead create read_policy attribute in fsid kobj.
merge v2:2/3 and v2:3/3 into v4:2/2. Patch titles have changed.
 
v3:
v2:
Mainly fixes the fs_devices::readmirror declaration type from atomic_t
to u8. (Thanks Josef).

v1:
As of now we use only %pid method to read stripped mirrored data. So
application's process id determines the stripe id to be read. This type
of routing typically helps in a system with many small independent
applications tying to read random data. On the other hand the %pid
based read IO distribution policy is inefficient if there is a single
application trying to read large data as because the overall disk
bandwidth would remains under utilized.

One type of readmirror policy isn't good enough and other choices are
routing the IO based on device's waitqueue or manual when we have a
read-preferred device or a readmirror policy based on the target storage
caching. So this patch-set introduces a framework where we could add more
readmirror policies.

This policy is a filesystem wide policy as of now, and though the
readmirror policy at the subvolume level is a novel approach as it
provides maximum flexibility in the data center, but as of now its not
practical to implement such a granularity as you can't really ensure
reflinked extents will be read from the stripe of its desire and so
there will be more limitations and it can be assessed separately.

The approach in this patch-set is sys interface with in-memory policy.
And does not add any new readmirror type in this set, which can be add
once we are ok with the framework. Also the default policy remains %pid.

Previous works:
----------------------------------------------------------------------
There were few RFCs [1] before, mainly to figure out storage
(or in memory only) for the readmirror policy and the interface needed.

[1]
https://www.mail-archive.com/linux-btrfs@vger.kernel.org/msg86368.html

https://lore.kernel.org/linux-btrfs/20190826090438.7044-1-anand.jain@oracle.com/

https://lore.kernel.org/linux-btrfs/5fcf9c23-89b5-b167-1f80-a0f4ac107d0b@oracle.com/

https://patchwork.kernel.org/cover/10859213/

Mount -o:
In the first trial it was attempted to use the mount -o option to carry
the readmirror policy, this is good for debugging which can make sure
even the mount thread metadata tree blocks are read from the disk desired.
It was very effective in testing radi1/raid10 write-holes.

Extended attribute:
As extended attribute is associated with the inode, to implement this
there is bit of extended attribute abuse or else makes it mandatory to
mount the rootid 5. Its messy unless readmirror policy is applied at the
subvol level which is not possible as of now. 

An item type:
The proposed patch was to create an item to hold the readmirror policy,
it makes sense when compared to the abusive extended attribute approach
but introduces a new item and so no backward compatibility.
-----------------------------------------------------------------------

Anand Jain (5):
  btrfs: add btrfs_strmatch helper
  btrfs: create read policy framework
  btrfs: create read policy sysfs attribute, pid
  btrfs: introduce new device-state read_preferred
  btrfs: introduce new read_policy device

 fs/btrfs/sysfs.c   | 128 +++++++++++++++++++++++++++++++++++++++++++++
 fs/btrfs/volumes.c |  39 +++++++++++++-
 fs/btrfs/volumes.h |  16 ++++++
 3 files changed, 182 insertions(+), 1 deletion(-)

-- 
2.23.0

[PATCH v7 1/5] btrfs: add btrfs_strmatch helper

[Anand Jain]

Add a generic helper to match the golden-string in the given-string,
and ignore the leading and trailing whitespaces if any.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Suggested-by: David Sterba <dsterba@suse.com>
---
v5: born

 fs/btrfs/sysfs.c | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/fs/btrfs/sysfs.c b/fs/btrfs/sysfs.c
index 93cf76118a04..7bb68cef98ab 100644
--- a/fs/btrfs/sysfs.c
+++ b/fs/btrfs/sysfs.c
@@ -809,6 +809,29 @@ static ssize_t btrfs_checksum_show(struct kobject *kobj,
 
 BTRFS_ATTR(, checksum, btrfs_checksum_show);
 
+/*
+ * Match the %golden in the %given. Ignore the leading and trailing whitespaces
+ * if any.
+ */
+static int btrfs_strmatch(const char *given, const char *golden)
+{
+	size_t len = strlen(golden);
+	char *stripped;
+
+	/* strip leading whitespace */
+	stripped = skip_spaces(given);
+
+	if (strncmp(stripped, golden, len) == 0) {
+		/* strip trailing whitespace */
+		if (strlen(skip_spaces(stripped + len)))
+			return -EINVAL;
+
+		return 0;
+	}
+
+	return -EINVAL;
+}
+
 static const struct attribute *btrfs_attrs[] = {
 	BTRFS_ATTR_PTR(, label),
 	BTRFS_ATTR_PTR(, nodesize),
-- 
2.23.0

[PATCH v7 2/5] btrfs: create read policy framework

[Anand Jain]

As of now we use %pid method to read stripped mirrored data, which means
process id determines the stripe id to be read. This type of routing
typically helps in a system with many small independent processes tying
to read random data. On the other hand the %pid based read IO policy is
inefficient because if there is a single process trying to read large
data the overall disk bandwidth remains under-utilized.

So this patch introduces read policy framework so that we could add more
read policies, such as IO routing based on device's wait-queue or manual
when we have a read-preferred device or a policy based on the target
storage caching.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
---
v7: Fix missing /* fall through */ in the switch
    Removed Reviewed-by: Josef Bacik <josef@toxicpanda.com>
v6:-
v5: Title renamed from:- btrfs: add read_policy framework
    Change log updated.
    Unnecessary comment dropped, added more where necessary.
    Optimize code in the switch remove duplicate code.
    Define BTRFS_READ_POLICY_DEFAULT dropped.
    Rename enum btrfs_read_policy_type to enum btrfs_read_policy.
    Rename BTRFS_READ_BY_PID to BTRFS_READ_POLICY_PID.
    (As its mainly renames. Reviewed-by retained).
v4: -
v3: Declare fs_devices::readmirror as enum btrfs_readmirror_policy_type
v2: Declare fs_devices::readmirror as u8 instead of atomic_t 
    A small change in comment and change log wordings.

 fs/btrfs/volumes.c | 15 ++++++++++++++-
 fs/btrfs/volumes.h | 14 ++++++++++++++
 2 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index d8a88866aaa3..9dd7e3687463 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -1205,6 +1205,7 @@ static int open_fs_devices(struct btrfs_fs_devices *fs_devices,
 	fs_devices->opened = 1;
 	fs_devices->latest_bdev = latest_dev->bdev;
 	fs_devices->total_rw_bytes = 0;
+	fs_devices->read_policy = BTRFS_READ_POLICY_PID;
 out:
 	return ret;
 }
@@ -5397,7 +5398,19 @@ static int find_live_mirror(struct btrfs_fs_info *fs_info,
 	else
 		num_stripes = map->num_stripes;
 
-	preferred_mirror = first + current->pid % num_stripes;
+	switch (fs_info->fs_devices->read_policy) {
+	default:
+		/*
+		 * Shouldn't happen, just warn and use pid instead of failing.
+		 */
+		btrfs_warn_rl(fs_info,
+			      "unknown read_policy type %u, fallback to pid",
+			      fs_info->fs_devices->read_policy);
+		/* fall through */
+	case BTRFS_READ_POLICY_PID:
+		preferred_mirror = first + current->pid % num_stripes;
+		break;
+	}
 
 	if (dev_replace_is_ongoing &&
 	    fs_info->dev_replace.cont_reading_from_srcdev_mode ==
diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
index 7fa392f38262..1775d35706ab 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -207,6 +207,15 @@ BTRFS_DEVICE_GETSET_FUNCS(total_bytes);
 BTRFS_DEVICE_GETSET_FUNCS(disk_total_bytes);
 BTRFS_DEVICE_GETSET_FUNCS(bytes_used);
 
+/*
+ * Read policies for the mirrored block groups, read picks the stripe based
+ * on these policies.
+ */
+enum btrfs_read_policy {
+	BTRFS_READ_POLICY_PID,
+	BTRFS_NR_READ_POLICY,
+};
+
 struct btrfs_fs_devices {
 	u8 fsid[BTRFS_FSID_SIZE]; /* FS specific uuid */
 	u8 metadata_uuid[BTRFS_FSID_SIZE];
@@ -258,6 +267,11 @@ struct btrfs_fs_devices {
 	struct kobject *devices_kobj;
 	struct kobject *devinfo_kobj;
 	struct completion kobj_unregister;
+
+	/*
+	 * policy used to read the mirrored stripes
+	 */
+	enum btrfs_read_policy read_policy;
 };
 
 #define BTRFS_BIO_INLINE_CSUM_SIZE	64
-- 
2.23.0

[PATCH v7 3/5] btrfs: create read policy sysfs attribute, pid

[Anand Jain]

Add

 /sys/fs/btrfs/UUID/read_policy

attribute so that the read policy for the raid1 and raid10 chunks can be
tuned.

When this attribute is read, it shall show all available policies, with
active policy being with in [ ]. The read_policy attribute can be written
using one of the items listed in there.

For example:
  $cat /sys/fs/btrfs/UUID/read_policy
  [pid]
  $echo pid > /sys/fs/btrfs/UUID/read_policy

Signed-off-by: Anand Jain <anand.jain@oracle.com>
---
v5: 
  Title rename: old: btrfs: sysfs, add read_policy attribute
  Uses the btrfs_strmatch() helper (BTRFS_READ_POLICY_NAME_MAX dropped).
  Use the table for the policy names.
  Rename len to ret.
  Use a simple logic to prefix space in btrfs_read_policy_show()
  Reviewed-by: Josef Bacik <josef@toxicpanda.com> dropped.

v4:-
v3: rename [by_pid] to [pid]
v2: v2: check input len before strip and kstrdup

 fs/btrfs/sysfs.c | 49 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 49 insertions(+)

diff --git a/fs/btrfs/sysfs.c b/fs/btrfs/sysfs.c
index 7bb68cef98ab..c9a8850b186a 100644
--- a/fs/btrfs/sysfs.c
+++ b/fs/btrfs/sysfs.c
@@ -832,6 +832,54 @@ static int btrfs_strmatch(const char *given, const char *golden)
 	return -EINVAL;
 }
 
+static const char* const btrfs_read_policy_name[] = { "pid" };
+
+static ssize_t btrfs_read_policy_show(struct kobject *kobj,
+				      struct kobj_attribute *a, char *buf)
+{
+	int i;
+	ssize_t ret = 0;
+	struct btrfs_fs_devices *fs_devices = to_fs_devs(kobj);
+
+	for (i = 0; i < BTRFS_NR_READ_POLICY; i++) {
+		if (fs_devices->read_policy == i)
+			ret += snprintf(buf + ret, PAGE_SIZE - ret, "%s[%s]",
+					(ret == 0 ? "" : " "),
+					btrfs_read_policy_name[i]);
+		else
+			ret += snprintf(buf + ret, PAGE_SIZE - ret, "%s%s",
+					(ret == 0 ? "" : " "),
+					btrfs_read_policy_name[i]);
+	}
+
+	ret += snprintf(buf + ret, PAGE_SIZE - ret, "\n");
+
+	return ret;
+}
+
+static ssize_t btrfs_read_policy_store(struct kobject *kobj,
+				       struct kobj_attribute *a,
+				       const char *buf, size_t len)
+{
+	int i;
+	struct btrfs_fs_devices *fs_devices = to_fs_devs(kobj);
+
+	for (i = 0; i < BTRFS_NR_READ_POLICY; i++) {
+		if (btrfs_strmatch(buf, btrfs_read_policy_name[i]) == 0) {
+			if (i != fs_devices->read_policy) {
+				fs_devices->read_policy = i;
+				btrfs_info(fs_devices->fs_info,
+					   "read policy set to '%s'",
+					   btrfs_read_policy_name[i]);
+			}
+			return len;
+		}
+	}
+
+	return -EINVAL;
+}
+BTRFS_ATTR_RW(, read_policy, btrfs_read_policy_show, btrfs_read_policy_store);
+
 static const struct attribute *btrfs_attrs[] = {
 	BTRFS_ATTR_PTR(, label),
 	BTRFS_ATTR_PTR(, nodesize),
@@ -840,6 +888,7 @@ static const struct attribute *btrfs_attrs[] = {
 	BTRFS_ATTR_PTR(, quota_override),
 	BTRFS_ATTR_PTR(, metadata_uuid),
 	BTRFS_ATTR_PTR(, checksum),
+	BTRFS_ATTR_PTR(, read_policy),
 	NULL,
 };
 
-- 
2.23.0

[PATCH v7 4/5] btrfs: introduce new device-state read_preferred

[Anand Jain]

This is a preparatory patch and introduces a new device flag
'read_preferred', and is a generic flag which along with the read_policy
'device' in the following patch the user can route the read IO to the
device of choice.

This also provides a sysfs interface to set the device state as
read_preferred.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
---
v7: Change log updated.
v6: If there is no change in device's read prefer then don't log. 
    Add pid to the logs.
v5: born

 fs/btrfs/sysfs.c   | 55 ++++++++++++++++++++++++++++++++++++++++++++++
 fs/btrfs/volumes.h |  1 +
 2 files changed, 56 insertions(+)

diff --git a/fs/btrfs/sysfs.c b/fs/btrfs/sysfs.c
index c9a8850b186a..72daaedb7b04 100644
--- a/fs/btrfs/sysfs.c
+++ b/fs/btrfs/sysfs.c
@@ -1317,11 +1317,66 @@ static ssize_t btrfs_devinfo_writeable_show(struct kobject *kobj,
 }
 BTRFS_ATTR(devid, writeable, btrfs_devinfo_writeable_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);
+
 static struct attribute *devid_attrs[] = {
 	BTRFS_ATTR_PTR(devid, in_fs_metadata),
 	BTRFS_ATTR_PTR(devid, missing),
 	BTRFS_ATTR_PTR(devid, replace_target),
 	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 1775d35706ab..487a54c3140e 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -50,6 +50,7 @@ struct btrfs_io_geometry {
 #define BTRFS_DEV_STATE_MISSING		(2)
 #define BTRFS_DEV_STATE_REPLACE_TGT	(3)
 #define BTRFS_DEV_STATE_FLUSH_SENT	(4)
+#define BTRFS_DEV_STATE_READ_PREFERRED	(5)
 
 struct btrfs_device {
 	struct list_head dev_list; /* device_list_mutex */
-- 
2.23.0

[PATCH v7 5/5] 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 chunks of type raid1, raid10,
raid1c3 and raid1c4.

As system might contain ssd, nvme, iscsi or san lun, and which are all
a non-rotational device its 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, like for example,
 - In heterogeneous-disk volume it provides an ability to 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 mark the 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 target device.

As of now this is in-memory only feature.

Its point less to set the read-preferred flag on the missing device,
as IOs aren't submitted to the missing device.

If there are more than one read-preferred device in a chunk, the read IO
shall go to the stripe 0 (as of now, when qdepth 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>
---
v7: Change log updated.
v6:
. If there isn't read preferred device in the chunk don't reset
read policy to default, instead just use stripe 0. As this is in
the read path it avoids going through the device list to find
read preferred device. So inline to this drop to check if there
is read preferred device before setting read policy to device.
. Commit log updated. Adds more info about this new feature.
v5: born

 fs/btrfs/sysfs.c   |  3 ++-
 fs/btrfs/volumes.c | 24 ++++++++++++++++++++++++
 fs/btrfs/volumes.h |  1 +
 3 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/sysfs.c b/fs/btrfs/sysfs.c
index 72daaedb7b04..af53ed879dd6 100644
--- a/fs/btrfs/sysfs.c
+++ b/fs/btrfs/sysfs.c
@@ -832,7 +832,8 @@ static int btrfs_strmatch(const char *given, const char *golden)
 	return -EINVAL;
 }
 
-static const char* const btrfs_read_policy_name[] = { "pid" };
+/* Must follow the order as in enum btrfs_read_policy */
+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 9dd7e3687463..5e53380e1d8d 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -5380,6 +5380,26 @@ 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 num_stripe)
+{
+	int i;
+
+	/*
+	 * If there are more than one read preferred devices, then just pick the
+	 * first found read preferred device as of now. Once we have the Qdepth
+	 * based device selection, we could pick the least busy device among the
+	 * read preferred devices.
+	 */
+	for (i = 0; i < num_stripe; i++) {
+		if (test_bit(BTRFS_DEV_STATE_READ_PREFERRED,
+			     &map->stripes[i].dev->dev_state))
+			return i;
+        }
+
+	/* If there is no read preferred device then just use stripe 0 */
+	return 0;
+}
+
 static int find_live_mirror(struct btrfs_fs_info *fs_info,
 			    struct map_lookup *map, int first,
 			    int dev_replace_is_ongoing)
@@ -5399,6 +5419,10 @@ static int find_live_mirror(struct btrfs_fs_info *fs_info,
 		num_stripes = map->num_stripes;
 
 	switch (fs_info->fs_devices->read_policy) {
+	case BTRFS_READ_POLICY_DEVICE:
+		preferred_mirror = btrfs_find_read_preferred(map, num_stripes);
+		preferred_mirror = first + preferred_mirror;
+		break;
 	default:
 		/*
 		 * Shouldn't happen, just warn and use pid instead of failing.
diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
index 487a54c3140e..efa9635a4748 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -214,6 +214,7 @@ BTRFS_DEVICE_GETSET_FUNCS(bytes_used);
  */
 enum btrfs_read_policy {
 	BTRFS_READ_POLICY_PID,
+	BTRFS_READ_POLICY_DEVICE,
 	BTRFS_NR_READ_POLICY,
 };
 
-- 
2.23.0

Re: [PATCH v7 0/5] readmirror feature (sysfs and in-memory only approach; with new read_policy device)

[Anand Jain]

  Could I ping you on this?

Thanks, Anand



On 21/2/20 2:15 PM, Anand Jain wrote:
> v7:
> Fix switch's fall through warning. Changle logs updates where necessary.
> 
> v6:
> Patch 4/5 - If there is no change in device's read prefer then don't log
> Patch 4/5 - Add pid to the logs
> Patch 5/5 - If there isn't read preferred device in the chunk don't reset
> read policy to default, instead just use stripe 0. As this is in
> the read path it avoids going through the device list to find
> read preferred device. So inline to this drop to check if there
> is read preferred device before setting read policy to device.
> 
> __ Original email: __
> 
> v5:
> Worked on review comments as received in its previous version.
> Please refer to individual patches for the specific changes.
> Introduces the new read_policy 'device'.
> 
> v4:
> Rename readmirror attribute to read_policy. Drop separate kobj for
> readmirror instead create read_policy attribute in fsid kobj.
> merge v2:2/3 and v2:3/3 into v4:2/2. Patch titles have changed.
>   
> v3:
> v2:
> Mainly fixes the fs_devices::readmirror declaration type from atomic_t
> to u8. (Thanks Josef).
> 
> v1:
> As of now we use only %pid method to read stripped mirrored data. So
> application's process id determines the stripe id to be read. This type
> of routing typically helps in a system with many small independent
> applications tying to read random data. On the other hand the %pid
> based read IO distribution policy is inefficient if there is a single
> application trying to read large data as because the overall disk
> bandwidth would remains under utilized.
> 
> One type of readmirror policy isn't good enough and other choices are
> routing the IO based on device's waitqueue or manual when we have a
> read-preferred device or a readmirror policy based on the target storage
> caching. So this patch-set introduces a framework where we could add more
> readmirror policies.
> 
> This policy is a filesystem wide policy as of now, and though the
> readmirror policy at the subvolume level is a novel approach as it
> provides maximum flexibility in the data center, but as of now its not
> practical to implement such a granularity as you can't really ensure
> reflinked extents will be read from the stripe of its desire and so
> there will be more limitations and it can be assessed separately.
> 
> The approach in this patch-set is sys interface with in-memory policy.
> And does not add any new readmirror type in this set, which can be add
> once we are ok with the framework. Also the default policy remains %pid.
> 
> Previous works:
> ----------------------------------------------------------------------
> There were few RFCs [1] before, mainly to figure out storage
> (or in memory only) for the readmirror policy and the interface needed.
> 
> [1]
> https://www.mail-archive.com/linux-btrfs@vger.kernel.org/msg86368.html
> 
> https://lore.kernel.org/linux-btrfs/20190826090438.7044-1-anand.jain@oracle.com/
> 
> https://lore.kernel.org/linux-btrfs/5fcf9c23-89b5-b167-1f80-a0f4ac107d0b@oracle.com/
> 
> https://patchwork.kernel.org/cover/10859213/
> 
> Mount -o:
> In the first trial it was attempted to use the mount -o option to carry
> the readmirror policy, this is good for debugging which can make sure
> even the mount thread metadata tree blocks are read from the disk desired.
> It was very effective in testing radi1/raid10 write-holes.
> 
> Extended attribute:
> As extended attribute is associated with the inode, to implement this
> there is bit of extended attribute abuse or else makes it mandatory to
> mount the rootid 5. Its messy unless readmirror policy is applied at the
> subvol level which is not possible as of now.
> 
> An item type:
> The proposed patch was to create an item to hold the readmirror policy,
> it makes sense when compared to the abusive extended attribute approach
> but introduces a new item and so no backward compatibility.
> -----------------------------------------------------------------------
> 
> Anand Jain (5):
>    btrfs: add btrfs_strmatch helper
>    btrfs: create read policy framework
>    btrfs: create read policy sysfs attribute, pid
>    btrfs: introduce new device-state read_preferred
>    btrfs: introduce new read_policy device
> 
>   fs/btrfs/sysfs.c   | 128 +++++++++++++++++++++++++++++++++++++++++++++
>   fs/btrfs/volumes.c |  39 +++++++++++++-
>   fs/btrfs/volumes.h |  16 ++++++
>   3 files changed, 182 insertions(+), 1 deletion(-)
>