How to Scan for New LUNs with Multipath on RHEL, AlmaLinux and Rocky Linux

Updated: 28 September 2026 · Applies to: device-mapper-multipath 0.8 and 0.9, sg3_utils 1.47 and 1.48 on AlmaLinux and Rocky Linux 9 and 10 and RHEL 9 and 10; multipath-tools on Ubuntu 24.04 and 26.04 and Debian 13

When a storage administrator assigns a new LUN (a disk from a SAN, over Fibre Channel or iSCSI) to a server, Linux does not always notice it by itself. You tell the kernel to scan for it, and then multipathd combines the paths to the new LUN into one device. No reboot is needed.

Before you start

• Install the tools: dnf install sg3_utils device-mapper-multipath (AlmaLinux, Rocky Linux, RHEL) or apt install sg3-utils multipath-tools (Ubuntu, Debian).
• Note the current devices, so you can spot the new one: lsblk and multipath -ll.

Steps

1. Scan for new LUNs on all SCSI hosts:

   rescan-scsi-bus.sh -a

   This script from sg3_utils scans every adapter. Without it, you can ask each host adapter directly:

   for h in /sys/class/scsi_host/host*; do echo "- - -" > $h/scan; done

2. For iSCSI LUNs, rescan the existing sessions instead:

   iscsiadm -m session --rescan

3. Check that the new disks appeared (one per path, for example sdc and sdd):

   lsblk
   dmesg | tail -20

4. Let multipathd pick up the new paths and show the result:

   multipath -r
   multipath -ll

   The new LUN appears as one mpath device (or with its WWID as the name) with its paths listed below it, under /dev/mapper/.
5. Use the multipath device, never one of the single paths, for file systems or LVM:

   pvcreate /dev/mapper/mpathc

Common problems

• No new disk after the scan: check with the storage team that the LUN is mapped to this server's initiator (WWPN for Fibre Channel, IQN for iSCSI: cat /etc/iscsi/initiatorname.iscsi).
• Disks appear but no multipath device: check /etc/multipath.conf for a blacklist that covers the new device, and that multipathd runs: systemctl status multipathd.
• A LUN was resized on the storage side: rescan with rescan-scsi-bus.sh -s, which finds resized disks and reloads their multipath devices. Check the new size with multipath -ll; if it has not changed, run multipathd resize map mpathc.

To remove a LUN safely, see how to remove a SAN disk or LUN from a Linux server.

Official documentation: Configuring device mapper multipath (Red Hat).