I'm experiencing the same behavior with a lvm pv on top of a md mirror:
Nov 2 22:03:13 zero systemd-udevd[515]: seq 4167
'/devices/virtual/block/md4' is taking a long time
pvscan
PV /dev/sda VG BACKUP0 lvm2 [9.10 TiB / 98.00 GiB free]
PV /dev/md4 VG MAIN0 lvm2 [9.10 TiB / 273.93 GiB free]
Total: 2 [18.19 TiB] / in use: 2 [18.19 TiB] / in no VG: 0 [0 ]
cat /proc/mdstat
Personalities : [raid1] [linear] [multipath] [raid0] [raid6] [raid5]
[raid4] [raid10]
md4 : active raid1 sdc4[1] sdb4[0]
9765778432 blocks super 1.2 [2/2] [UU]
bitmap: 4/73 pages [16KB], 65536KB chunk
I first thought, the delay has something to do with the network setup,
as it appears during the 'Raise network interfaces' message when systemd
counts about two and a half minute before continuing, but I think that's
more a coincidence.
