Comment 58 for bug 1729145

For older releases such as bionic we have one event:

UDEV [1304.304437] change /devices/virtual/block/bcache0 (block)
.ID_FS_TYPE_NEW=
ACTION=change
CACHED_LABEL=
CACHED_UUID=0192e0a7-9d8e-4771-9bb1-d730a2f66a9d
DEVLINKS=/dev/bcache/by-uuid/0192e0a7-9d8e-4771-9bb1-d730a2f66a9d
DEVNAME=/dev/bcache0
DEVPATH=/devices/virtual/block/bcache0
DEVTYPE=disk
DRIVER=bcache
ID_FS_TYPE=
MAJOR=251
MINOR=0
SEQNUM=4384
SUBSYSTEM=block
TAGS=:systemd:
USEC_INITIALIZED=1304237024

that creates the expected symlink.

---------

For focal, instead we get two events:

UDEV [2763.370729] change /devices/virtual/block/bcache0 (block)
ACTION=change
DEVPATH=/devices/virtual/block/bcache0
SUBSYSTEM=block
DRIVER=bcache
CACHED_UUID=1522715e-a390-40db-8d47-06fb728014e5
CACHED_LABEL=
DEVNAME=/dev/bcache0
DEVTYPE=disk
SEQNUM=5523
USEC_INITIALIZED=2763323504
.ID_FS_TYPE_NEW=
ID_FS_TYPE=
MAJOR=251
MINOR=0
DEVLINKS=/dev/bcache/by-uuid/1522715e-a390-40db-8d47-06fb728014e5
TAGS=:systemd:

KERNEL[2784.362374] change /devices/virtual/block/bcache0 (block)
ACTION=change
DEVPATH=/devices/virtual/block/bcache0
SUBSYSTEM=block
SYNTH_UUID=0
DEVNAME=/dev/bcache0
DEVTYPE=disk
SEQNUM=5524
MAJOR=251
MINOR=0

UDEV [2784.433618] change /devices/virtual/block/bcache0 (block)
ACTION=change
DEVPATH=/devices/virtual/block/bcache0
SUBSYSTEM=block
SYNTH_UUID=0
DEVNAME=/dev/bcache0
DEVTYPE=disk
SEQNUM=5524
USEC_INITIALIZED=2763323504
ID_FS_UUID=30b28bee-6a1e-423d-9d53-32c78ba5454a
ID_FS_UUID_ENC=30b28bee-6a1e-423d-9d53-32c78ba5454a
ID_FS_VERSION=1.0
ID_FS_TYPE=ext4
ID_FS_USAGE=filesystem
.ID_FS_TYPE_NEW=ext4
MAJOR=251
MINOR=0
DEVLINKS=/dev/disk/by-uuid/30b28bee-6a1e-423d-9d53-32c78ba5454a
TAGS=:systemd:

This performs:

pr 21 14:15:22 ubuntu-focal systemd-udevd[1870]: bcache0: Processing device (SEQNUM=5523, ACTION=change)
Apr 21 14:15:22 ubuntu-focal systemd-udevd[1870]: bcache0: Removing watch
Apr 21 14:15:22 ubuntu-focal systemd-udevd[1870]: value '[dmi/id]sys_vendor' is 'QEMU'
Apr 21 14:15:22 ubuntu-focal systemd-udevd[1870]: value '[dmi/id]sys_vendor' is 'QEMU'
Apr 21 14:15:22 ubuntu-focal systemd-udevd[1870]: bcache0: /usr/lib/udev/rules.d/60-persistent-storage.rules:109 Importing properties from results of builtin command 'blkid'
Apr 21 14:15:22 ubuntu-focal systemd-udevd[1870]: bcache0: Probe /dev/bcache0 with raid and offset=0
Apr 21 14:15:22 ubuntu-focal systemd-udevd[1870]: bcache0: /usr/lib/udev/rules.d/69-bcache.rules:16 Importing properties from results of 'probe-bcache -o udev /dev/bcache0'
Apr 21 14:15:22 ubuntu-focal systemd-udevd[1870]: bcache0: Starting 'probe-bcache -o udev /dev/bcache0'
Apr 21 14:15:22 ubuntu-focal systemd-udevd[1870]: Successfully forked off '(spawn)' as PID 1912.
Apr 21 14:15:22 ubuntu-focal systemd-udevd[1870]: bcache0: Process 'probe-bcache -o udev /dev/bcache0' succeeded.
Apr 21 14:15:22 ubuntu-focal systemd-udevd[1870]: bcache0: /usr/lib/udev/rules.d/69-bcache.rules:27 LINK 'bcache/by-uuid/1522715e-a390-40db-8d47-06fb728014e5'
Apr 21 14:15:22 ubuntu-focal systemd-udevd[1870]: bcache0: Handling device node '/dev/bcache0', devnum=b251:0
Apr 21 14:15:22 ubuntu-focal systemd-udevd[1870]: bcache0: Preserve already existing symlink '/dev/block/251:0' to '../bcache0'
Apr 21 14:15:22 ubuntu-focal systemd-udevd[1870]: bcache0: Creating symlink '/dev/bcache/by-uuid/1522715e-a390-40db-8d47-06fb728014e5' to '../../bcache0'
Apr 21 14:15:22 ubuntu-focal systemd-udevd[1870]: bcache0: sd-device: Created db file '/run/udev/data/b251:0' for '/devices/virtual/block/bcache0'
Apr 21 14:15:22 ubuntu-focal systemd-udevd[1870]: bcache0: Adding watch on '/dev/bcache0'
Apr 21 14:15:22 ubuntu-focal systemd-udevd[1870]: bcache0: sd-device: Created db file '/run/udev/data/b251:0' for '/devices/virtual/block/bcache0'
Apr 21 14:15:22 ubuntu-focal systemd-udevd[1870]: bcache0: Device (SEQNUM=5523, ACTION=change) processed
Apr 21 14:15:22 ubuntu-focal systemd-udevd[1870]: bcache0: sd-device-monitor: Passed 388 byte to netlink monitor

followed by the extra action:

Apr 21 14:15:43 ubuntu-focal systemd-udevd[1916]: bcache0: Processing device (SEQNUM=5524, ACTION=change)
Apr 21 14:15:43 ubuntu-focal systemd-udevd[1916]: bcache0: Removing watch
Apr 21 14:15:43 ubuntu-focal systemd-udevd[1916]: value '[dmi/id]sys_vendor' is 'QEMU'
Apr 21 14:15:43 ubuntu-focal systemd-udevd[1916]: value '[dmi/id]sys_vendor' is 'QEMU'
Apr 21 14:15:43 ubuntu-focal systemd-udevd[1916]: bcache0: /usr/lib/udev/rules.d/60-persistent-storage.rules:109 Importing properties from results of builtin command 'blkid'
Apr 21 14:15:43 ubuntu-focal systemd-udevd[1916]: bcache0: Probe /dev/bcache0 with raid and offset=0
Apr 21 14:15:43 ubuntu-focal systemd-udevd[1916]: bcache0: /usr/lib/udev/rules.d/60-persistent-storage.rules:112 LINK 'disk/by-uuid/30b28bee-6a1e-423d-9d53-32c78ba5454a'
Apr 21 14:15:43 ubuntu-focal systemd-udevd[1916]: bcache0: Updating old name, '/dev/bcache/by-uuid/1522715e-a390-40db-8d47-06fb728014e5' no longer belonging to '/devices/virtual/block/bcache0'
Apr 21 14:15:43 ubuntu-focal systemd-udevd[1916]: bcache0: No reference left, removing '/dev/bcache/by-uuid/1522715e-a390-40db-8d47-06fb728014e5'
Apr 21 14:15:43 ubuntu-focal systemd-udevd[1916]: bcache0: Handling device node '/dev/bcache0', devnum=b251:0
Apr 21 14:15:43 ubuntu-focal systemd-udevd[1916]: bcache0: Preserve already existing symlink '/dev/block/251:0' to '../bcache0'
Apr 21 14:15:43 ubuntu-focal systemd-udevd[1916]: bcache0: Creating symlink '/dev/disk/by-uuid/30b28bee-6a1e-423d-9d53-32c78ba5454a' to '../../bcache0'
Apr 21 14:15:43 ubuntu-focal systemd-udevd[1916]: bcache0: sd-device: Created db file '/run/udev/data/b251:0' for '/devices/virtual/block/bcache0'
Apr 21 14:15:43 ubuntu-focal systemd-udevd[1916]: bcache0: Adding watch on '/dev/bcache0'
Apr 21 14:15:43 ubuntu-focal systemd-udevd[1916]: bcache0: sd-device: Created db file '/run/udev/data/b251:0' for '/devices/virtual/block/bcache0'
Apr 21 14:15:43 ubuntu-focal systemd-udevd[1916]: bcache0: Device (SEQNUM=5524, ACTION=change) processed
Apr 21 14:15:43 ubuntu-focal systemd-udevd[1916]: bcache0: sd-device-monitor: Passed 471 byte to netlink monitor

So it appears the symlink is being added to /dev/bcache/by-uuid/1522715e-a390-40db-8d47-06fb728014e5 and then the next udev action removes this symlink and adds dev/disk/by-uuid/30b28bee-6a1e-423d-9d53-32c78ba5454a instead.