Thursday, December 1, 2011

Linux: Lun detection

LUN detection procedures

This topic describes LUN detection procedures for the Linux host system.
If you have a Linux driver that does not automatically configure any LUNs other than LUN 0, you can manually configure the other LUNs, depending on the parameters and settings used for the SCSI mid-layer driver. Figure 1 shows an example of the /proc/scsi/scsi file for a Linux host that only configures the first LUN, LUN 0, on each host adapter port.
Figure 1. Example of a /proc/scsi/scsi file from a Linux host that only configures LUN 0

# cat proc/scsi/scsi
...
Host: scsi0 Channel: 00 Id: 00 Lun: 00
  Vendor: IBM-PSG  Model: DPSS-318350M  F  Rev: S9HA
  Type:   Direct-Access                    ANSI SCSI revision: 03
Host: scsi0 Channel: 00 Id: 15 Lun: 00
  Vendor: IBM      Model: TP4.6 V41b3      Rev: 4.1b
  Type:   Processor                        ANSI SCSI revision: 02
Host: scsi2 Channel: 00 Id: 00 Lun: 00
  Vendor: IBM      Model: 2105800          Rev: .294
  Type:   Direct-Access                    ANSI SCSI revision: 03
Host: scsi3 Channel: 00 Id: 00 Lun: 00
  Vendor: IBM      Model: 2105800          Rev: .294
  Type:   Direct-Access                    ANSI SCSI revision: 03
Host: scsi3 Channel: 00 Id: 01 Lun: 00
  Vendor: IBM      Model: 2105800          Rev: .294
  Type:   Direct-Access                    ANSI SCSI revision: 03

There are two ways to workaround the issue of only having LUN 0 configured:

   1. Create a script to manually add devices into /proc/scsi/scsi
   2. Detect LUNs automatically at system boot by modifying the initial ram-disk (initrd)

Create a script to echo the /proc filesystem

Use the scsi add-single-device command to consecutively configure all of the LUNs that are assigned to your host system. Write a script that repeats the scsi add-single-device command for each LUN on each ID for each host adapter. The script must scan all host adapter ports and identify all of the LUNs that are assigned to each port.

After you run the script, you can view all of the assigned LUNs in the /proc/scsi/scsi file.

Figure 2 shows an excerpt of an example /proc/scsi/scsi file for a Linux host after a script has configured every LUN.
Figure 2. Example of a /proc/scsi/scsi file for a Linux host with configured LUNs

# cat proc/scsi/scsi
...
Host: scsi3 Channel: 00 Id: 00 Lun: 00
  Vendor: IBM      Model: 2105800          Rev: .294
  Type:   Direct-Access                    ANSI SCSI revision: 03
Host: scsi3 Channel: 00 Id: 01 Lun: 00
  Vendor: IBM      Model: 2105800          Rev: .294
  Type:   Direct-Access                    ANSI SCSI revision: 03
Host: scsi3 Channel: 00 Id: 00 Lun: 01
  Vendor: IBM      Model: 2105800          Rev: .294
  Type:   Direct-Access                    ANSI SCSI revision: 03
Host: scsi3 Channel: 00 Id: 00 Lun: 02
  Vendor: IBM      Model: 2105800          Rev: .294
  Type:   Direct-Access                    ANSI SCSI revision: 03
Host: scsi3 Channel: 00 Id: 00 Lun: 03
  Vendor: IBM      Model: 2105800          Rev: .294
  Type:   Direct-Access                    ANSI SCSI revision: 03
Host: scsi3 Channel: 00 Id: 00 Lun: 04
  Vendor: IBM      Model: 2105800          Rev: .294
  Type:   Direct-Access                    ANSI SCSI revision: 03
...

Detect LUNs automatically at system boot
The second method of configuring LUNs for a Linux system with only LUN 0 configured involves setting the parameter for the SCSI mid-layer driver that controls how many LUNs are scanned during a SCSI bus scan. The following procedure works for both 2.4 and 2.6 kernels, but it assumes the SCSI mid-layer driver is compiled as a scsi_mod module that is loaded automatically at system boot time. For Linux 2.4 kernels, to set the maximum number of disk devices under Linux to properly detect all volumes, you need to set the max_scsi_luns option for the SCSI mid-layer driver. For example, if max_scsi_luns is set to 1 this limits SCSI bus scans to only LUN 0. This value should be set to the respective maximum number of disks the kernel can support, for example, 128 or 256. In Linux 2.6 kernels, the same procedure applies, except that the parameter has been renamed from max_scsi_luns to max_luns.

   1. Edit the /etc/modules.conf file.
   2. Add the following line:
          * options scsi_mod max_scsi_luns= (where is the total number of luns to probe.
   3. Save the file.
   4. Run the mkinitrd command to rebuild the ram-disk associated with the current kernel. You can use the following figures examples of what mkinitrd command to run for your operating system. refers to the ‘uname –r’ output which displays the currently running kernel level, for example:. 2.4.21-292-smp.
      For SUSE distributions, use the following command:

      cd /boot
      mkinitrd –k vmlinuz- -i initrd-

      For Red Hat distributions, use the following command:

      cd /boot
      mkinitrd –v initrd-.img

   5. Reboot the host.
   6. Verify that the boot files are correctly configured for the newly created initrd image in the /boot/grub/menu.lst file.