Persistent storage device names in Linux

Recently I purchased two external USB hard-drives, one from LACIE and another one from IOMEGA, that I plugged into one of my Linux workstations. I wanted these drives to be auto-mounted during boot, so I created a couple of entries in /etc/fstab. The problem is that, somehow, the Linux kernel is not always able to enumerate devices on a bus in the same order between reboots. Thus, sometimes the LACIE hard disk gets /dev/sdb and IOMEGA gets /dev/sdc, and other times it is the other way around. In fact, it gets even worse as my machine as a built-in 6×1 card-reader, so I also have /dev/sd{d,e,f,g}.

Looking at /etc/udev/rules.d, I found that Ubuntu has a nice 65-persistent-disk.rules file with very smart and useful udev entries that create symlinks under /dev/disk by disk ID, disk label, device path and device UUID:

  • /dev/disk/by-id contains one symlink per physical device and partition. The symlink name is built using the device’s ID.
  • /dev/disk/by-label contains one symlink per partition. The symlink name is built using the partition volume label.
  • /dev/disk/by-path contains one symlink per physical device and parition. The symlink name is built using the device bus name.
  • /dev/disk/by-uuid contains one symlink per partition. The symlink name is built using the partition UUID.

For example:

$ tree /dev/disk
/dev/disk
|-- by-id
| |-- scsi-1ATA_WDC_WD1600JS-75NCB2_WD-WCANM44 -> ../../sda
| |-- scsi-1ATA_WDC_WD1600JS-75NCB2_WD-WCANM44-part1 -> ../../sda1
| |-- scsi-1ATA_WDC_WD1600JS-75NCB2_WD-WCANM44-part2 -> ../../sda2
| |-- scsi-1ATA_WDC_WD1600JS-75NCB2_WD-WCANM44-part5 -> ../../sda5
| |-- scsi-1ATA_WDC_WD1600JS-75NCB2_WD-WCANM44-part6 -> ../../sda6
| |-- usb-Generic_Flash_HS-CF_26020128B005 -> ../../sdd
| `-- usb-Generic_Flash_HS-COMBO_26020128B005 -> ../../sde
|-- by-label
| `-- DellUtility -> ../../sda1
|-- by-path
| |-- pci-0000:00:1f.1-scsi-0:0:0:0 -> ../../scd0
| |-- pci-0000:00:1f.2-scsi-0:0:0:0 -> ../../sda
| |-- pci-0000:00:1f.2-scsi-0:0:0:0-part1 -> ../../sda1
| |-- pci-0000:00:1f.2-scsi-0:0:0:0-part2 -> ../../sda2
| |-- pci-0000:00:1f.2-scsi-0:0:0:0-part5 -> ../../sda5
| |-- pci-0000:00:1f.2-scsi-0:0:0:0-part6 -> ../../sda6
| |-- usb-26020128B005:0:0:0 -> ../../sdd
| `-- usb-26020128B005:0:0:1 -> ../../sde
`-- by-uuid
|-- 07D6-0701 -> ../../sda1
|-- 29982b9e-63a4-4d3f-8b88-ebc9d000c09f -> ../../sda5
`-- 6aa8745b-dbe2-4386-9697-cc0c2dee27d4 -> ../../sda6

Basically, what I did is replacing the entries I previously created in /et/fstab using absolute device names, like /dev/sdb1, with /dev/disk/by-label/<VOLUME> instead.

Name stability and predictability across reboots for devices is quite a nice feature in Ubuntu Linux. And this can be easily ported to other Linux distributions simply by copying /etc/udev/rules.d/65-persistent-disk.rules.

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6 thoughts on “Persistent storage device names in Linux

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