Systemd/Microsoft Effort For A Global Counter On Block/Disk Changes Coming To Linux 5.15

[Developer12]

Imagine to have an usb disk named /dev/sdd. You unplug and replug it, how can an application know that /dev/sdd is the same as 10 minutes ago?
The number now increases for every disk registration, and it's unique since boot:

So it's more of a counter of "how many different disks have been sda since boot?"

It a counter strictly necessary for that? Wouldn't it be easier for both userspace and kernelspace to have a filehandle that expires on disk removal, as is the case for process filehandles when dealing with PID reuse?

[teknoraver]

So it's more of a counter of "how many different disks have been sda since boot?"

It a counter strictly necessary for that? Wouldn't it be easier for both userspace and kernelspace to have a filehandle that expires on disk removal, as is the case for process filehandles when dealing with PID reuse?

Yes, it's also a counter of how many disks have been seen.

The filehandle was not feasable, because only root can open a handle to most disks, and creating fds is more expensive anyway.
What we did, is just an incremental number to every disk registration, exposed to /sys

[reba]

Yes, it's also a counter of how many disks have been seen.

What would be a use case for this?

The filehandle was not feasable, because only root can open a handle to most disks, and creating fds is more expensive anyway.
What we did, is just an incremental number to every disk registration, exposed to /sys

Wouldn't have /dev/disk/by-uuid/... achieved the same and even more like:
- detecting if a disk has been removed from the system if the uuid has gone missing
- re-attaching the same disk on a different port (sdc -> sde) and still be able to use it transparently without a redetection and migration logic
- no explicit support for another field and additional logic for that counter to do stuff
- already etablished, nothing to add; to the kernel no less

[teknoraver]

If you want to have a look at (least one) bug solved by this counter, here is it:

https://github.com/systemd/systemd/issues/17469