Introduction to Systemd

Systemd is a system and service manager for Linux operating systems. It is designed to be backwards compatible with SysV init scripts, and provides a number of features such as parallel startup of system services at boot time, on-demand activation of daemons, support for system state snapshots, or dependency-based service control logic.

How to Add / Remove Static route in a RHEL

List all the current static routes

# route -n

Add a route to the network 192.56.76.x via "eth1"

# route add -net 192.56.76.0 netmask 255.255.255.0 dev eth1

Add a route to a host 192.168.76.10 via "eth1"

How to Remove a LUN online

To remove a LUN on-line follow the below steps 

Identify the LUN that need to be removed

# lsscsi -g
[3:0:0:0]    storage HP       HSV300           1000  -         /dev/sg0
[3:0:0:1]    disk    HP       HSV300           1000  /dev/sda  /dev/sg1
[4:0:0:0]    disk    HP       HSV300           1000  /dev/sdb  /dev/sg2

Or use the Command sg_map ( sg_map is part of optional package sg3utils )

How to set LUN queue depths for Induvidal LUN

There may be having  situation like using luns from two different vendors which may recommend a different queue depths ,In such case you can change this value for individual devices dynamically if the device driver supports it.

# cat < new-depth-value > /sys/block/<sdxx>/device/queue_depth

If the device driver doesn't support you will get the below mentioned message

# echo 1024 > /sys/block/sdx/device/queue_depth
bash: /sys/block/sdx/device/queue_depth: Permission denied

kernel: scsi: host X channel Y id Z lun has a LUN larger than allowed by the host adapter

You gets the below error , because the LUN number  <xxx> is greater than lpfc module parameter lpfc_max_luns , because by default emulex controllers support a maximum of 255 LUNs by default. This may be increased to 65535 ,This issue can be also seen if you have a LUN ID that is greater than 255 even though you have a much fewer number of LUNs than 255.

kernel: scsi: host X channel Y id Z lun<xxx> has a LUN larger than allowed by the host adapter

To check the present lpfc_max_luns value

#cat /sys/module/lpfc/parameters/lpfc_max_luns

255

To change the same follow the below steps

1: Add the following to /etc/modprobe.conf file