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[This is on Debian 12]

To fully access my DVD drive, I need to also know its SCSI generic path (/dev/sgX) in addition to its expected CDROM-/dev/srX path.

The specific device is important, though, and I need to access the device even if the system re-shuffles the /dev/srXs and /dev/sgXs.

sudo lsscsi -g [3:0:0:0] cd/dvd HL-DT-ST BD-RE WH16NS60 1.05 /dev/sr0 /dev/sg3 
sudo blkid /dev/sr0 /dev/sr0: UUID="035bbeef20202020" LABEL="WALDEINSAMKEIT" BLOCK_SIZE="2048" TYPE="udf" sudo blkid /dev/sg3 <no output>

/dev/sr0 is set with a /dev/disk/by-uuid/035bbeef20202020 but if /dev/sg3 doesn't actually have a UUID (I'm guessing it's not seen as a block device), I could go for the full controller/system path, per this Linus rant I found https://yarchive.net/comp/linux/scsi_ids.html

sudo udevadm info -q path -n /dev/sr0 /devices/pci0000:00/0000:00:17.0/ata4/host3/target3:0:0/3:0:0:0/block/sr0 sudo udevadm info -q path -n /dev/sg3 /devices/pci0000:00/0000:00:17.0/ata4/host3/target3:0:0/3:0:0:0/scsi_generic/sg3

This does not seem like a solution either, as the path includes sr0 or sg3 which will change order. /sys/devices/pci0000:00/0000:00:17.0/ata4/host3/target3:0:0/3:0:0:0 seems to be a directory the device is referencing but I can't find anything more than the folders block/sr0 and scsi_generic/sg3 which again revert to the original problem of sr0 and sg3 changing if the system cables are reordered or something.

How can I get a consistent identifier for /dev/sg3? I have searched hard, but I must be missing a command.

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  • I don't think there are other "candidate" block devices in /devices/pci0000:00/0000:00:17.0/ata4/host3/target3:0:0/3:0:0:0/block/ nor in /devices/pci0000:00/0000:00:17.0/ata4/host3/target3:0:0/3:0:0:0/scsi_generic/. Can you check that there's really more than one device in there that you could mistake for the right one? Commented Nov 19, 2024 at 22:37
  • The issue with using /devices/pci0000:00/0000:00:17.0/ata4/host3/target3:0:0/3:0:0:0/scsi_generic/sg3 is more that it would change to sg2 and then wherever I have the call for sg3 would not work. As a pro, you are right it does avoid another device being accessed as /dev/sg3 inadvertently, but it doesn't provide the reliability /dev/disk/by-uuid/035bbeef20202020 does - which is what I'm hoping for. Commented Nov 20, 2024 at 1:31
  • I don't see the problem, to be quite honest – if there's only one option in that directory, you cannot go wrong. Just open the device that is in the directory. Where's the problem? Commented Nov 20, 2024 at 9:29
  • In the case of DVD drives, the "UUID" 035bbeef20202020 is not associated with the drive, but with the currently inserted disc. A DVD drive with no disc inserted has no UUID at all. Commented Nov 20, 2024 at 11:34
  • I did not test the UUID without a disc in or with another disc in so did not notice the UUID is per-disc! Almost got bamboozled big with that - thanks for the catch Commented Nov 22, 2024 at 2:29

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The specific device is important, though, and I need to access the device even if the system re-shuffles the /dev/srXs and /dev/sgXs.

The "UUID" you are hoping to use is associated with the disc currently in the drive, not with the drive itself.

To really solve that problem, you'll need a pair of udev rules.

First, run udevadm info -q all -a -n /dev/sr0 | grep -e vendor -e model and make a note of the ATTRS{vendor} and ATTRS{model} strings.

Do the same with udevadm info -q all -a -n /dev/sg3 | grep -e vendor -e model.

You might see something like this in both cases:

ATTRS{model}=="BD-RE WH16NS60 " ATTRS{vendor}=="HL-DT-ST" ATTRS{subsystem_vendor}=="0x1234" ATTRS{vendor}=="0x8086"

You should ignore any lines with ATTRS{subsystem_vendor} and the lines where ATTRS{vendor} contains only hexadecimal numbers, as they'll won't refer to the DVD drive, but the upstream controller(s) on the PCI(e) bus.

A serial number would be a better unique identifier, but unfortunately the udev attributes for the generic SCSI device will not include a readily-accessible serial number attribute.

Note that the strings may include trailing whitespace, so be sure to copy the entire strings within the double quotes exactly. For SCSI-like devices, the maximum(/standard?) length of the vendor string seems to be 8 characters, and the model string seems to be 16 characters respectively.

Once you know the vendor and model strings, you will be able to construct a pair of udev rules like this:

SUBSYSTEM=="scsi_generic", ATTRS{vendor}=="HL-DT-ST", ATTRS{model}=="BD-RE WH16NS60 ", SYMLINK+="consistent_generic" SUBSYSTEM=="block", DRIVERS=="sr", ATTRS{vendor}=="HL-DT-ST", ATTRS{model}=="BD-RE WH16NS60 ", SYMLINK+="consistent_block"

(You can replace consistent_generic and consistent_block with any names you want.)

Place them into e.g. /etc/udev/rules.d/80-local-consistent-dvd.rules, then reboot.

Now you will have symbolic links /dev/consistent_block and /dev/consistent_generic (or however you named them), which will always point to the DVD drive that has the specified attributes.

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