[lvm-devel] master - systemd: depend on systemd-udev-settle unit in activation unit

[Bill Nottingham]

Peter Rajnoha (prajnoha at redhat.com) said: 
> Yes, you're right... for older systems where this module is still
> present, we should still use the scsi-wait-scan.
> 
> However, this is now (at least in Fedora) maintained under
> our own initscripts which is not present in all distros I guess.
> I'm just trying to point out that it would be better to have
> this "fedora-storage-wait" unit in systemd/udev upstream directly
> and give it a common name like "storage-wait" which other
> storage-related upstream projects could use for reference in
> their systemd units if we want to keep it for backward compatibility.
> 
> But since the module is now gone, I'm inclined to using systemd-udev-settle
> instead and if an update/rebase is needed in packages in older
> distro releases, just providing a simple one line patch attached
> directly to the build (making use of the original fedora-storage-wait.service).

The problem with the module being gone (as I understand it) is
that systemd-udev-settle doesn't replace it. Before, the sequence was:

- udev starts
- udev loads stuff
- udev-settle, waits until all the loading of stuff is processed
- scsi_wait_scan loads
- ... waits for background scans to finish ...
- can continue with LVM or whatever activation

The removal of scsi_wait_scan doesn't actually mean any of the bus scans are
done when you wait on systemd-udev-settle, so making that change won't
really solve the issue you need. If I'm understanding it right, I think
in a world without scsi-wait-scan, you're likely going to need to enable
lvmetad and do incremental activation to have thigns work properly.

Bill