udev

cloud_user@emu8971c:~$ udevadm info --name=/dev/xvdb1 --attribute-walk

Udevadm info starts with the device specified by the devpath and then
walks up the chain of parent devices. It prints for every device
found, all possible attributes in the udev rules key format.
A rule to match, can be composed by the attributes of the device
and the attributes from one single parent device.

  looking at device '/devices/vbd-51728/block/xvdb/xvdb1':
    KERNEL=="xvdb1"
    SUBSYSTEM=="block"
    DRIVER==""
    ATTR{alignment_offset}=="0"
    ATTR{discard_alignment}=="0"
    ATTR{inflight}=="       0        0"
    ATTR{partition}=="1"
    ATTR{ro}=="0"
    ATTR{size}=="4192256"
    ATTR{start}=="2048"
    ATTR{stat}=="     154        0    10352      424        0        0        0        0        0      356      424"



cloud_user@emu8971c:~$ sudo cat /etc/udev/rules.d/81-persistent-dev.rules
ATTR{size}=="4192256", ATTR{partition}=="1", SUBSYSTEM=="block", KERNEL=="xvdb1", SYMLINK+="/dev/test_link"

cloud_user@emu8971c:~$ udevadm control --reload-rules

cloud_user@emu8971c:~$ sudo /etc/init.d/udev restart

cloud_user@emu8971c:~$ udevadm test -a -p $(udevadm info -q path -n /dev/xvdb1)
calling: test
version 229
This program is for debugging only, it does not run any program
specified by a RUN key. It may show incorrect results, because
some values may be different, or not available at a simulation run.

=== trie on-disk ===
tool version:          229
file size:         6841781 bytes
header size             80 bytes
strings            1755245 bytes
nodes              5086456 bytes
Load module index
timestamp of '/etc/systemd/network' changed
timestamp of '/lib/systemd/network' changed
Parsed configuration file /lib/systemd/network/99-default.link
Parsed configuration file /etc/systemd/network/50-cloud-init-eth0.link
Created link configuration context.
timestamp of '/etc/udev/rules.d' changed
Skipping overridden file: /lib/udev/rules.d/60-cdrom_id.rules.
Reading rules file: /lib/udev/rules.d/40-vm-hotadd.rules
Reading rules file: /lib/udev/rules.d/50-apport.rules
Reading rules file: /lib/udev/rules.d/50-firmware.rules
Reading rules file: /lib/udev/rules.d/50-udev-default.rules
Reading rules file: /lib/udev/rules.d/55-dm.rules
Reading rules file: /lib/udev/rules.d/56-lvm.rules
Reading rules file: /lib/udev/rules.d/60-block.rules
Skipping empty file: /etc/udev/rules.d/60-cdrom_id.rules
Reading rules file: /lib/udev/rules.d/60-drm.rules
Reading rules file: /lib/udev/rules.d/60-evdev.rules
Reading rules file: /lib/udev/rules.d/60-gnupg.rules
Reading rules file: /lib/udev/rules.d/60-open-vm-tools.rules
Reading rules file: /lib/udev/rules.d/60-persistent-alsa.rules
Reading rules file: /lib/udev/rules.d/60-persistent-input.rules
Reading rules file: /lib/udev/rules.d/60-persistent-storage-dm.rules
Reading rules file: /lib/udev/rules.d/60-persistent-storage-tape.rules
Reading rules file: /lib/udev/rules.d/60-persistent-storage.rules
Reading rules file: /lib/udev/rules.d/60-persistent-v4l.rules
Reading rules file: /lib/udev/rules.d/60-serial.rules
Reading rules file: /lib/udev/rules.d/60-vlan-network-interface.rules
Reading rules file: /lib/udev/rules.d/61-persistent-storage-android.rules
Reading rules file: /lib/udev/rules.d/63-md-raid-arrays.rules
Reading rules file: /lib/udev/rules.d/64-btrfs.rules
Reading rules file: /lib/udev/rules.d/64-md-raid-assembly.rules
Reading rules file: /lib/udev/rules.d/66-azure-ephemeral.rules
Reading rules file: /lib/udev/rules.d/66-snapd-autoimport.rules
Reading rules file: /lib/udev/rules.d/69-bcache.rules
Reading rules file: /lib/udev/rules.d/69-lvm-metad.rules
Reading rules file: /lib/udev/rules.d/70-debian-uaccess.rules
Reading rules file: /lib/udev/rules.d/70-iscsi-network-interface.rules
Reading rules file: /lib/udev/rules.d/70-mouse.rules
Reading rules file: /etc/udev/rules.d/70-persistent-net.rules
Reading rules file: /lib/udev/rules.d/70-power-switch.rules
Reading rules file: /lib/udev/rules.d/70-resolvconf-initramfs-copy.rules
Reading rules file: /lib/udev/rules.d/70-uaccess.rules
Reading rules file: /lib/udev/rules.d/71-power-switch-proliant.rules
Reading rules file: /lib/udev/rules.d/71-seat.rules
Reading rules file: /lib/udev/rules.d/73-seat-late.rules
Reading rules file: /lib/udev/rules.d/73-special-net-names.rules
Reading rules file: /lib/udev/rules.d/73-usb-net-by-mac.rules
Reading rules file: /lib/udev/rules.d/75-net-description.rules
Reading rules file: /lib/udev/rules.d/75-probe_mtd.rules
Reading rules file: /lib/udev/rules.d/78-graphics-card.rules
Reading rules file: /lib/udev/rules.d/78-sound-card.rules
Reading rules file: /lib/udev/rules.d/80-debian-compat.rules
Reading rules file: /lib/udev/rules.d/80-drivers.rules
Reading rules file: /lib/udev/rules.d/80-ifupdown.rules
Reading rules file: /lib/udev/rules.d/80-net-setup-link.rules
Reading rules file: /etc/udev/rules.d/81-persistent-dev.rules
Reading rules file: /lib/udev/rules.d/85-hdparm.rules
Reading rules file: /lib/udev/rules.d/85-keyboard-configuration.rules
Reading rules file: /lib/udev/rules.d/99-systemd.rules
Reading rules file: /lib/udev/rules.d/99-vmware-scsi-udev.rules
rules contain 49152 bytes tokens (4096 * 12 bytes), 14306 bytes strings
2178 strings (27343 bytes), 1451 de-duplicated (13765 bytes), 728 trie nodes used
value '[dmi/id]sys_vendor' is 'Xen'
value '[dmi/id]sys_vendor' is 'Xen'
IMPORT builtin 'blkid' /lib/udev/rules.d/60-persistent-storage.rules:76
Failure opening block device /dev/xvdb1: Permission denied
IMPORT builtin 'blkid' returned non-zero
IMPORT 'probe-bcache -o udev /dev/xvdb1' /lib/udev/rules.d/69-bcache.rules:16
starting 'probe-bcache -o udev /dev/xvdb1'
Process 'probe-bcache -o udev /dev/xvdb1' succeeded.
LINK '/dev/test_link' /etc/udev/rules.d/81-persistent-dev.rules:1
handling device node '/dev/xvdb1', devnum=b202:17, mode=0600, uid=0, gid=0
preserve already existing symlink '/dev/block/202:17' to '../xvdb1'
creating link '/dev//dev/test_link' to '/dev/xvdb1'
creating symlink '/dev//dev/test_link' to '../../xvdb1'
atomically replace '/dev//dev/test_link'
symlink '../../xvdb1' '/dev//dev/test_link.tmp-b202:17' failed: Permission denied
.ID_FS_TYPE_NEW=
ACTION=-p
DEVLINKS=/dev//dev/test_link
DEVNAME=/dev/xvdb1
DEVPATH=/devices/vbd-51728/block/xvdb/xvdb1
DEVTYPE=partition
ID_FS_TYPE=
ID_PART_TABLE_TYPE=dos
ID_PART_TABLE_UUID=9597e8c2
MAJOR=202
MINOR=17
SUBSYSTEM=block
TAGS=:systemd:
USEC_INITIALIZED=9707191285
Unload module index
Unloaded link configuration context.
cloud_user@emu8971c:~$