Why to use udev rule to insure persistent naming/permission instead of mknod?

Question

I have seen some configurations using udev rule to insure the disk name and permissions in the disk. But recently I've find out a command called mknod, where it works like an alias to the block device, by specify the major:minor of the disk, we can "create an alias", something like it:

[root@dbnode1 disks]# lsblk
NAME                MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
sdd                   8:48   0  160M  0 disk

mknod /disks/QUORUML b 8 48

Now I'd like to know why to use udev rules instead of mknod, since mknod is very simple to use. My main point is insure persistent naming and permission.

Answer 1

The biggest reason is that udev rule is "automatic" -- every time a new device is connected (or discovered during boot), udev will process all the rules files so you don't need to do anything manually.

Also mknod creates a new block (or character) device file, but you already have one -- /dev/sdd so you don't need to create a new one, you only need a symlink to it (which is what udev does with the SYMLINK property).

If you don't want/need the "automation" part of udev and are ok with running a command manually every time you boot or connect the device, I suggest you still shouldn't use mknod but create a symlink instead.

If all you want is a nice (and stable) "path" for the device, I'd suggest setting label for the filesystem or partition and then you can use one of the default symlinks created by udev in /dev/disk/by-partlabel and /dev/disk/by-label.

Note that udev is also used by other system tools that work with storage so if you create the symlink with udev, these will know that it is a symlink to that device. This might not be true with manually created nodes (but for example lsblk is smart enough and if you run lsblk /disks/QUORUML it will show you /dev/sdd).

Answer 2

Why ? Reliability !

The major and minor device numbers are allocated when the driver detects a new device and, when a device is removed, these device numbers are freed and can be reused later.

The consequence of this being that the association between the device numbers and corresponding sd labels changes when the order of detection change.

This might be considered unusual on a small system with a unique disk, however just think of removable medias, hot pluggable disk arrays… cover this with parallelized detection… sdb, sdc,… the order is, strictly speaking : unpredictable.

This is the reason why it is undesirable to use the major and minor number range (or the associated sd names when referring to devices, such as in the /etc/fstab file). The wrong device could be mounted leading to big troubles.