Description of problem:

When a system has tons of disks attached (~7000), user logins are slow due to "systemd --user" instance reading all the disks.

A customer reported 11 seconds to log in:
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------
$ time ssh system date
Tue Jul 6 06:39:59 GMT 2021

real 0m11.493s
user 0m0.012s
sys 0m0.004s
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------

Executing strace, we can see that all block devices (/sys/devices/virtual/block/<device>) are opened and stat'ed, which takes a lot of time in the end.

IMO "systemd --user" needs to be optimized to either not retrieve this information or get it from "systemd --system" upon start.

Version-Release number of selected component (if applicable):

systemd-239-41.el8_3.2.x86_64

How reproducible:

Always on customer site having VxVM volumes.
Cannot reproduce locally using either iscsi or loopback devices