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Appendix A. Operating systems specifics 317
RedHat Enterprise Linux (RH-EL) multiple LUN support
RH-EL by default is not configured for multiple LUN support. It will only discover SCSI disks
addressed as LUN 0. The DS6000 provides the volumes to the host with a fixed Fibre
Channel address and varying LUN. Therefore RH-EL 3 will see only one DS6000 volume
(LUN 0), even if more are assigned to it.
Multiple LUN support can be added with an option to the SCSI midlayer Kernel module
scsi_mod. To have multiple LUN support added permanently at boot time of the system, add
the following line to the file /etc/modules.conf:
options scsi_mod max_scsi_luns=128
After saving the file, rebuild the module dependencies by running:
depmod -a
Now you have to rebuild the InitialRAMDisk, using the command:
mkinitrd <initrd-image> <kernel-version>
Issue mkinitrd -h for more help information. A reboot is required to make the changes
effective.
Fibre Channel disks discovered before internal SCSI disks
In some cases, when the Fibre Channel HBAs are added to a RedHat Enterprise Linux
system, they will be automatically configured in a way that they are activated at boot time,
before the built-in parallel SCSI controller that drives the system disks. This will lead to shifted
special device file names of the system disk and can result in the system being unable to boot
properly.
To prevent the FC HBA driver from being loaded before the driver for the internal SCSI HBA
you have to change the /etc/modules.conf file:
Locate the lines containing scsi_hostadapterx entries where x is a number.
Reorder these lines: first come the lines containing the name of the internal HBA driver
module, then the ones with the FC HBA module entry.
Renumber the lines: no number for the first entry, 1 for the second, 2 for the 3rd, and so
on.
After saving the file, rebuild the module dependencies by running:
depmod -a
Now you have to rebuild the InitialRAMDisk, using the command:
mkinitrd <initrd-image> <kernel-version>
Issue mkinitrd -h for more help information. If you reboot now, the SCSI and FC HBA drivers
will be loaded in the correct order.
Example A-12 shows how the /etc/modules.conf file should look with two Adaptec SCSI
controllers and two QLogic 2340 FC HBAs installed. It also contains the line that enables
multiple LUN support. Note that the module names will be different with different SCSI and
Fibre Channel adapters.
Example: A-12 Sample /etc/modules.conf
scsi_hostadapter aic7xxx
scsi_hostadapter1 aic7xxx
scsi_hostadapter2 qla2300