Kernels before sysfs devinfo (pre-5.13, e.g. Synology DSM 4.4) report
every Btrfs pool health as UNKNOWN and expose no member devices, even
though the same facts are available through BTRFS_IOC_FS_INFO and
BTRFS_IOC_DEV_INFO on the mountpoint.
When the devinfo directory does not exist, enumerate devids via DEV_INFO
after confirming the filesystem through FS_INFO: members that answer are
listed with their recorded capacity, a member reporting no device path is
missing (DEGRADED), freed devids (ENODEV gaps from remove/replace) are
skipped, and the kernel device count is cross-checked against the sysfs
devices links so an unattributed missing member still reads DEGRADED.
Any ioctl failure or count mismatch leaves health UNKNOWN rather than
reporting unverified ONLINE.
henrygd
Santhi PrakashandGitHub
