[PATCH 1/3 v2 RFC] btrfs: total_devices vs num_devices

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

The intended usage of total_devices and num_devices
should be recorded in the comments so that these
two counters can be used correctly as originally
intended.

As of now there appears to be slight deviations/bugs
from the original intention, the bugs are apparent
that num_devices does not count seed devices where
as total_devices does, but total_devices does not
count the replacing devices where as num_devices does.

So in this situation the ioctl(BTRFS_IOC_DEVICES_READY)
will fails when there is seed FS. Next, applications
using ioctl(BTRFS_IOC_FS_INFO) and ioctl(BTRFS_IOC_DEV_INFO)
in conjunction to allocate the slots will also fail
as former depend on the num_devices where as latter
would match total_devices (except when replace is running).

This patch will help have the clarity on usage
of these two counter so that bugs related to this can
be fixed.

Signed-off-by: Anand Jain <Anand.Jain@oracle.com>
---
v2: edit commit

 fs/btrfs/volumes.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
index 91b7596..99c71aa 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -121,12 +121,18 @@ struct btrfs_fs_devices {
 	/* the device with this id has the most recent copy of the super */
 	u64 latest_devid;
 	u64 latest_trans;
+	/* num_devices contains run time count of devices which are part
+	 * of this FSID. (the FS devices + replacing devices + ?
+	 */
 	u64 num_devices;
 	u64 open_devices;
 	u64 rw_devices;
 	u64 missing_devices;
 	u64 total_rw_bytes;
 	u64 num_can_discard;
+	/* total_devices contains static recorded count of device which
+	 * are part of this FSID. (the FS devices + seed devices + ?
+	 */
 	u64 total_devices;
 	struct block_device *latest_bdev;
 
-- 
1.8.5.3