Comment 7 for bug 1735839

Based on https://bugs.launchpad.net/curtin/+bug/1735839/comments/2

by-dname was intended to be "by device name" and ended up to be "by data name"

Curtin docs promise disk naming and partition naming:

https://curtin.readthedocs.io/en/latest/topics/storage.html
"If the name key is present, curtin will create a udev rule that makes a symbolic link to the disk with the given name value. This makes it easy to find disks on an installed system. The links are created in /dev/disk/by-dname/<name>. A link to each partition on the disk will also be created at /dev/disk/by-dname/<name>-part<number>..."

> "What we decided was that to work our way out of this issue, we should/would provide a /dev/disk/by-<something> that would *only* consider serial numbers or wwid as basis for its names."

I think "device name" vs "data name" usage should depend on an application and we need 2 symlink types:

* a "devicename" symlink should mean a wwn or serial number symlink for hw devices themselves, for virtual devices (partitions, bcache, md-raid etc.) - a superblock unique ID symlink;
* a "dataname" symlink should mean a GPT UUID/MBR disk identifier symlink for hw devices with a partition table, partition UUID symlink for partitions, superblock UUIDs for virtual devices.

There are several classes of block devices that I see:

1) hardware block devices with a unique factory identifier (wwn, EUI etc.);

lrwxrwxrwx 1 root root 14 Jul 25 16:28 nvme-eui.01000000010000005cd2e407a30d4f51 -> ../../nvme10n1
lrwxrwxrwx 1 root root 9 Jul 25 16:28 wwn-0x5000c500813becef -> ../../sda

2) hardware-level virtual block devices (created by RAID controllers) - WWN stability will depend on the raid controller's implementation but generally a RAID-controller superblock is written to every device in a RAID.

journalctl -k | grep -i lsi
JUN 08 16:38:57 ubuntu kernel: scsi 0:2:1:0: Direct-Access LSI MR9271-8i 3.46 PQ: 0 ANSI: 5

lspci | grep -i lsi
01:00.0 RAID bus controller: LSI Logic / Symbios Logic MegaRAID SAS 2208 [Thunderbolt] (rev 05)

tree /dev/disk/by-id/ | grep sda
├── scsi-3600605b006deb2f0ff00001f01ff0cdb -> ../../sda
└── wwn-0x600605b006deb2f0ff00001f01ff0cdb -> ../../sda

3) virtual block devices. Availability of persistent and unique identifiers depends on hypervisor support and persistent configuration that a hypervisor provides (libvirt domain xml files -> qemu args, VMDK file metadata etc.)

For QEMU, IDE and SCSI devices can utilize a wwn passed via qemu command line and virtio devices can utilize a serial number parameter.

https://git.qemu.org/?p=qemu.git;a=commit;h=95ebda85e09ed2b7f00deb2adbdafa5ccf5db948
https://git.qemu.org/?p=qemu.git;a=commitdiff;h=64cc22841e72d37d577416f5836155ecd0a9bfb6
https://git.qemu.org/?p=qemu.git;a=commitdiff;h=9a502563eef7d7c2c9120b237059426e229eefe9

https://libvirt.org/git/?p=libvirt.git;a=commitdiff;h=6e73850b01ee7d5816a803d684e9d669dad036f3

virtio:

#...
-drive file=/var/lib/libvirt/images/maas-1.qcow2,format=qcow2,if=none,id=drive-virtio-disk0,serial=a76f8801-92d3-4aff-9c96-a51a054517a0 # ...

cat /sys/class/block/vda/serial
a76f8801-92d3-4aff-9

(virtio)scsi:

virsh dumpxml maas | grep wwn
      <wwn>5000c50015ea71ad</wwn>

-device scsi-hd,bus=scsi0.0,channel=0,scsi-id=0,lun=0,drive=drive-scsi0-0-0-0,id=scsi0-0-0-0,bootindex=1,wwn=0x5000c50015ea71ad

├── wwn-0x5000c50015ea71ad -> ../../sda
├── wwn-0x5000c50015ea71ad-part1 -> ../../sda1
└── wwn-0x5000c50015ea71ad-part2 -> ../../sda2

4) data-based devices (partitions, bcache, md-raid, LUKS etc.) - superblocks/headers contain UUIDs and sometimes labels.