RHEL: Scan and configure new SAN (fibre channel) LUNs

# Tested on RHEL 5 & 6

# Identify host adapters on the system

systool -c fc_host

   Class = "fc_host"

     Class Device = "host3"
       Device = "host3"

     Class Device = "host4"
       Device = "host4"

ls /sys/class/fc_host
   host3  host4

# Rescan the hbas
# ------------------------------------------------------------------------------------------

echo "- - -" > /sys/class/scsi_host/host3/scan
echo "- - -" > /sys/class/scsi_host/host4/scan

# If this commands don't get to make new LUNs visible to system, you can try a "issue_lip"
# rescan.

# Take into account that rescanning via "issue_lip" will reset the HBAs what may cause
# a data loss if I/O operations are going on.
Use this type of scan only when necessary.
# In addition, as per Red Hat recommendations, the following restrictions must be observed
# when scanning storage interconnects:
# All I/O on the effected interconnects must be paused and flushed before executing the
# procedure, and the results of the scan checked before I/O is resumed.
# As with removing a device, interconnect scanning is not recommended when the system is
# under memory pressure. To determine the level of memory pressure, run the command
# "vmstat 1 100"; interconnect scanning is not recommended if free memory is less than 5%
# of the total memory in more than 10 samples per 100. It is also not recommended if
# swapping is active (non-zero 'si' and 'so' columns in the "vmstat" output). The command
# "free" can also display the total memory.

# I recommend to use "issue_lip" rescan only in case of servers that are discovering fibre
# SAN disk for first time:

# echo "1" > /sys/class/fc_host/host3/issue_lip
# echo "1" > /sys/class/fc_host/host4/issue_lip
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