##Overview When Linux boots it will enumerate devices on the SCSI bus as they are presented to the kernel. This ordering can change due to several factors including device order initialization or the timing of the devices appearing after a LIP on a the SAN. Using the udev facility you can create symlinks to give a persistent name to these devices.

##udev and its Role in the Linux Boot Process udev is the daemon that receives information from the kernel about a device when added to the kernl. Devices can be added at boot time and dynamically after boot, such as USB devices or removable hard drives. For instance, on most machines you can look in /sys/block and see your block devices, hard drives most likely. SCSI tape drives appear in /sys/class/scsi_tape. Some information about each device is also passed to the udev system. Here is how to interrogate a device node to find out more information (commands run on a CentOS 6.5 machine):

[root@deer ~]# udevadm info --name=/dev/st0 --query=all
P: /devices/pci0000:00/0000:00:02.0/0000:09:00.0/0000:0a:00.0/0000:0b:00.1/host3/rport-3:0-0/target3:0:0/3:0:0:0/scsi_tape/st0
N: st0
S: char/9:0
S: tape/by-id/scsi-3500110a0008e226f
S: tape/by-path/pci-0000:0b:00.1-fc-0x500110a0008e2270-lun-0
S: tapes/hp-lto4-2
E: DEVPATH=/devices/pci0000:00/0000:00:02.0/0000:09:00.0/0000:0a:00.0/0000:0b:00.1/host3/rport-3:0-0/target3:0:0/3:0:0:0/scsi_tape/st0
E: DEVNAME=/dev/st0
E: SUBSYSTEM=scsi_tape
E: BSG_DEV=/dev/bsg/3:0:0:0
E: ID_VENDOR_ENC=HP\x20\x20\x20\x20\x20\x20
E: ID_MODEL=Ultrium_4-SCSI
E: ID_MODEL_ENC=Ultrium\x204-SCSI\x20\x20
E: ID_TYPE=tape
E: ID_SERIAL_RAW=3500110a0008e226f
E: ID_SERIAL=3500110a0008e226f
E: ID_SERIAL_SHORT=500110a0008e226f
E: ID_WWN=0x500110a0008e226f
E: ID_WWN_WITH_EXTENSION=0x500110a0008e226f
E: ID_BUS=scsi
E: ID_PATH=pci-0000:0b:00.1-fc-0x500110a0008e2270-lun-0
E: DEVLINKS=/dev/char/9:0 /dev/tape/by-id/scsi-3500110a0008e226f /dev/tape/by-path/pci-0000:0b:00.1-fc-0x500110a0008e2270-lun-0 /dev/tapes/hp-lto4-2

Each line starts with a letter. N is the kernel name, S are symlinks, E are environmental variables and P are the PCI paths. Record the ID_SERIAL_SHORT output for use in the rules.

###Adding udev Rules On CentOS (and RHEL) 6.5 the rules are located in /etc/udev/rules.d. The rules are numbered and each file is read in order starting with the smallest. 72-tape-persistent-names.rules was created for the tape drives. In this file the following lines were added:

KERNEL=="st*", ENV(ID_SERIAL_SHORT}=="3500110a0008e226f", SYMLINK+="tapes/hp-lto4-2", MODE="0666"

The ID_SERIAL_SHORT was used from the udevadm output and a symlink will be created in /dev/tapes.
This is the second LTO4 drive in a library which is why it was named hp-lto4-2. Additionally the permissions were opened up so any user on the system can write to the tape drive which may not be necessary depending on use case.