reproduce :

1.choose a megaraid sas series raidcard, for example:
LSI Logic / Symbios Logic MegaRAID　SAS 2008 [Falcon] (rev 03)

2. configure the raid card as HBA card (all disks are jbod mode)
We assume /dev/sdb is attached to this raid card

3.run process:
fd = open("/dev/sdb", O_RDWR);
while (1) {
    sleep (10);
}

4. run "udevadm monitor" watch udev message in a new shell

5.unplug disk sdb and then insert 

We can see new udev message about disk's hotswap

6.redo process 5
There will be no udev message only if you reboot the server