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Discovering new SCSI devices (LUNs) and multipath devices

LUNs on the storage cluster appear to the Linux host as SCSI devices, which are I/O paths that DM-Multipath aggregates into a new device, called a multipath device. The host does not automatically discover new SCSI devices (LUNs) that you add to your system. You must manually rescan them to discover them.

Before you begin

You must have a copy of the rescan script, which is in the sg3_utils package that is part of your Linux OS distribution. If you do not have sg3_utils, it can be installed with yum install sg3_utils package.

  1. Discover new SCSI devices (LUNs) and create the corresponding multipath devices for the LUNs: /usr/bin/rescan-scsi-bus.sh
  2. Verify DM-Multipath configuration:multipath -ll

    Example

    This displays the following type of output, listing the recommended settings for each NetApp C-Mode LUN:

    3600a0980324666546e2b443251655177 dm-2 NETAPP,LUN C-Mode
    size=10G features='4 queue_if_no_path pg_init_retries 50 retain_attached_hw_handle' hwhandler='1 alua' wp=rw
    |-+- policy='round-robin 0' prio=50 status=active
    | |- 0:0:1:0 sdb 8:16  active ready  running
    | |- 0:0:0:0 sda 8:0   active ready  running
    | |- 1:0:0:0 sde 8:64  active ready  running
    | `- 1:0:1:0 sdf 8:80  active ready  running
    `-+- policy='round-robin 0' prio=10 status=enabled
      |- 0:0:3:0 sdd 8:48  active ready  running
      |- 1:0:3:0 sdh 8:112 active ready  running
      |- 0:0:2:0 sdc 8:32  active ready  running
      `- 1:0:2:0 sdg 8:96  active ready  running