The Geek Diary

How to set “max_report_luns” and “max_luns” on CentOS/RHEL 6 to scan more than 512 LUNs

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The default limit of maximum luns that can be presented in a CentOS/RHEL 6 is 512. But in many cases, you may want to increase this limit. There is a parameter of the SCSI core module to control the number of multiple LUNs.

View Current Settings

To view the current setting of “max_luns” parameter, use the below command:

# systool -v -m scsi_mod
Module = "scsi_mod"

  Attributes:
    uevent              = 

  Parameters:
    default_dev_flags   = "0"
    eh_deadline         = "-1"
    inq_timeout         = "20"
    max_luns            = "512"
    scan                = "async"
    scsi_logging_level  = "0"
    use_blk_mq          = "Y"

You may also view the file /sys/module/scsi_mod/parameters/max_luns, to view the current settings:

# cat /sys/module/scsi_mod/parameters/max_luns
512

Change the max_report_luns parameter

1. In order to change the values for either max_report_luns or max_luns, append the following to your grub.conf ‘kernel‘ line:

# vi /boot/grub/grub.conf 
 kernel /vmlinuz-2.6.32-71.el6.x86_64 ro root=/dev/mapper/vg_rhel60-lv_root 
    rd_LVM_LV=vg_rhel60/lv_root rd_LVM_LV=vg_rhel60/lv_swap rd_NO_LUKS rd_NO_MD rd_NO_DM 
    LANG=en_US.UTF-8 SYSFONT=latarcyrheb-sun16 KEYBOARDTYPE=pc KEYTABLE=us crashkernel=auto 
    rhgb quiet scsi_mod.max_report_luns=[yourvalue] scsi_mod.max_luns=[yourvalue]

Here specify the number of max lunx you want the system to scan. You can specify a large number such as 2048 or 4096 etc.

2. Reboot the system. After the reboot, the additional LUNs should appear. The changes will be persistent across reboots.

# shutdown -r now

Verfiy

Once the system is up after reboot use any of the below commands to verify the settings once again.

# cat /sys/module/scsi_mod/parameters/max_luns
# cat /sys/module/scsi_mod/parameters/max_report_luns
# systool -v -m scsi_mod

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