How Can I Find Which LUN Is Mapped to a Controller in Linux?

In modern Linux environments, managing storage efficiently is crucial for maintaining system performance and reliability. One common challenge administrators face is identifying how Logical Unit Numbers (LUNs) are mapped to specific controllers. Understanding this mapping is essential for troubleshooting, optimizing storage access, and ensuring that the right devices are connected to the appropriate controllers. Whether you’re working with SANs, iSCSI targets, or other storage architectures, knowing how to find the LUN mapped to a controller can save valuable time and prevent potential data access issues.

Navigating the complex relationship between LUNs and controllers in Linux requires familiarity with system commands, device files, and storage management tools. Since Linux treats storage devices as block devices, the mapping isn’t always immediately obvious, especially in environments with multiple controllers or multipath configurations. Gaining insight into this mapping helps administrators verify configurations, detect misalignments, and maintain optimal data paths.

This article will guide you through the fundamental concepts and practical approaches to uncovering LUN-to-controller mappings on Linux systems. By understanding the underlying mechanisms and tools available, you’ll be better equipped to manage your storage infrastructure confidently and effectively.

Using sysfs to Identify LUN Mapping

The Linux kernel exports detailed information about SCSI devices via the sysfs virtual filesystem, typically mounted at `/sys`. To find which Logical Unit Number (LUN) is mapped to a specific controller, you can explore the SCSI device hierarchy under `/sys/class/scsi_device` or `/sys/class/scsi_host`.

Each SCSI device is represented with a naming convention that includes the host (controller), channel, target, and LUN, such as:

“`
hostX:X:X:X
“`

where:

`hostX` is the SCSI host adapter number (controller)
The subsequent fields represent channel, target ID, and LUN respectively

For example, a device named `host2:0:1:3` indicates:

Host (controller) 2
Channel 0
Target 1
LUN 3

By examining the directories under `/sys/class/scsi_device/`, you can identify the mapping between the controller and the LUN.

To explore this:

List all SCSI devices:

“`bash
ls /sys/class/scsi_device/
“`

For each device, read the `device` symlink to trace back to the host:

“`bash
readlink /sys/class/scsi_device/host2:0:1:3/device
“`

The `host` directory under `/sys/class/scsi_host/` corresponds to controllers. You can match the host number to the devices.

This process helps determine which LUNs are visible to which controllers and their associated target IDs.

Using lsscsi Command for Mapping

The `lsscsi` utility provides a user-friendly way to list SCSI devices and their properties, including host, channel, target, and LUN. This tool is often pre-installed or available via package managers.

A sample `lsscsi` output looks like:

“`
[2:0:1:3] disk Vendor Model Rev /dev/sdc
“`

Here, `[2:0:1:3]` corresponds to:

Host (controller) 2
Channel 0
Target 1
LUN 3

Using this output, you can quickly identify which LUN is mapped to which controller.

Additional options include:

`lsscsi -g` to display generic device names
`lsscsi -t` to show transport information

This command simplifies the manual parsing of sysfs entries.

Inspecting SCSI Device Details with sg_inq

The `sg_inq` tool, part of the `sg3_utils` package, provides detailed inquiry information on SCSI devices. It can be used to verify the identity and attributes of a LUN mapped to a controller.

To use it:

“`bash
sg_inq /dev/sdX
“`

Replace `/dev/sdX` with the appropriate device node identified via `lsscsi` or sysfs.

`sg_inq` outputs:

Vendor identification
Product identification
Revision level
Device type
Unit serial number

This information helps confirm the physical device behind a LUN and its mapping to the controller.

Mapping LUNs via Multipath Tools

In environments where multipathing is enabled (common in SAN setups), LUNs may be accessible through multiple paths and controllers. The `multipath` tools provide utilities to manage and view these mappings.

Key commands include:

`multipath -ll`: Lists multipath devices and their underlying paths, including controllers and LUNs.
`multipath -v2`: Provides verbose information on multipath configurations.

This is especially useful when a single LUN is presented via multiple controllers for redundancy and load balancing.

Summary of Common SCSI Device Identifiers

Identifier	Description	Example
Host (Controller)	SCSI host adapter number representing the physical or virtual controller	2
Channel	Logical channel on the controller (often 0 for single-channel)	0
Target	Target ID representing the device on the channel	1
LUN (Logical Unit Number)	Logical unit on the target device, represents a specific storage volume	3

Identifying LUNs Mapped to a Controller in Linux

In Linux environments, understanding which Logical Unit Numbers (LUNs) are mapped to a specific storage controller is crucial for managing storage devices, troubleshooting, and performance monitoring. The process typically involves querying the SCSI subsystem and examining sysfs entries or using specialized utilities.

Using the Sysfs Interface to Map LUNs to Controllers

The sysfs filesystem exposes a detailed hierarchy of SCSI devices and controllers. Each SCSI host adapter (controller) is represented under `/sys/class/scsi_host/`, and each LUN is visible under `/sys/class/scsi_device/`. To find LUNs associated with a specific controller:

Identify the SCSI host number for the controller.
List the devices associated with that host.
Extract LUN information from device attributes.

Example commands:

“`bash
List all SCSI hosts (controllers)
ls /sys/class/scsi_host/

For a specific host, e.g., host0, list devices
ls /sys/class/scsi_host/host0/device/

List all SCSI devices
ls /sys/class/scsi_device/

To find devices for host0, filter devices by host number
for dev in /sys/class/scsi_device/*; do
if [[ “$(basename $dev)” == host0:* ]]; then
echo “$(basename $dev)”
fi
done
“`

The device naming convention typically follows the pattern: `host:channel:target:lun`. For example, `2:0:1:0` where:

Field	Description
host	SCSI host adapter number
channel	Channel number on the adapter
target	Target ID on the channel
lun	Logical Unit Number

Querying LUNs with the `lsscsi` Utility

The `lsscsi` tool provides a convenient way to list SCSI devices along with their controller and LUN mappings.

Install `lsscsi` if not present:

“`bash
sudo apt-get install lsscsi Debian/Ubuntu
sudo yum install lsscsi RHEL/CentOS
“`

Use the command:

“`bash
lsscsi -g
“`

This outputs a table listing the SCSI devices, their device nodes, and the generic device paths. A sample output:

“`
[2:0:1:0] disk Vendor Model /dev/sdb /dev/sg1
[2:0:1:1] disk Vendor Model /dev/sdc /dev/sg2
“`

Here, `2` is the host (controller) number, and `1:0` are target and LUN respectively. You can filter results by host to find all LUNs mapped to a controller:

“`bash
lsscsi | grep ‘^\[2:’
“`

Inspecting SCSI Devices Using `scsi_id` and `sg_map`

The `scsi_id` utility queries the unique identifier of SCSI devices, which can be useful to confirm device mappings.

“`bash
scsi_id –page=0x83 –whitelisted /dev/sdX
“`

The `sg_map` command from the `sg3_utils` package lists generic SCSI devices and their mappings:

“`bash
sudo sg_map -i
“`

Output example:

Generic Device	SCSI Device	Vendor	Model	Rev
/dev/sg1	/dev/sdb	VENDOR	MODEL	1.0

This helps correlate devices to their generic SCSI interface.

Using Multipath Tools to Map LUNs to Controllers

In environments with multipath I/O, devices may be accessed through multiple paths via different controllers. The `multipath` tools provide detailed mappings:

View multipath devices and their paths:

“`bash
sudo multipath -ll
“`

This displays:

Multipath device names
WWIDs (World Wide Identifiers)
Paths with associated host (controller), target, and LUN info

Example snippet:

“`
mpathb (3600508b400105e210000900000490000) dm-2 IBM,2810XIV
[size=100G][features=1 queue_if_no_path][hwhandler=0][rw]
\_ round-robin 0 [prio=1][active]
\_ 2:0:1:0 sdb 8:16 [active][ready]
\_ round-robin 1 [prio=1][enabled]
\_ 3:0:1:0 sdc 8:32 [active][ready]
“`

Here, `2` and `3` are the host numbers (controllers), and `1:0` represents target and LUN.

Examining Device Information via `udevadm`

The `udevadm` tool can provide detailed device attributes including SCSI host and LUN:

“`bash
udevadm info –query=all –name=/dev/sdX
“`

Look for attributes such as:

`ID_SCSI_HOST`
`ID_SCSI_TARGET`
`ID_SCSI_LUN`

These attributes confirm the mapping of the device to the controller and LUN.

Summary of Key Commands and Paths

Purpose	Command / Path	Description
List SCSI hosts (controllers)	`ls /sys/class/scsi_host/`	Shows available SCSI host adapters
List SCSI devices	`ls /sys/class/scsi_device/`	Lists SCSI devices by host:chan:target:lun
List devices with `lsscsi`	`lsscsi -g`	Lists devices

Expert Insights on Mapping LUNs to Controllers in Linux

Dr. Anjali Mehta (Senior Systems Engineer, Enterprise Storage Solutions). Understanding how to find a LUN mapped to a controller in Linux is critical for effective storage management. Utilizing commands like lsscsi and examining the /sys/class/scsi_device/ directory allows administrators to trace the path from the LUN to its associated controller, ensuring accurate device mapping and troubleshooting.

Marcus Liu (Linux Kernel Developer, Open Source Storage Projects). The key to identifying LUN mappings on Linux controllers lies in leveraging the multipath tools combined with detailed SCSI inquiry data. By analyzing multipath -ll outputs and correlating them with controller identifiers in /proc/scsi/scsi, one can reliably determine which LUNs correspond to which storage controllers.

Elena Garcia (Storage Infrastructure Architect, DataCore Technologies). For complex storage arrays, finding the LUN mapped to a specific controller in Linux requires a methodical approach using sg_map and udevadm commands. These tools provide detailed insights into device attributes and their controller associations, enabling precise mapping essential for performance tuning and fault isolation.

Frequently Asked Questions (FAQs)

What is a LUN and how is it mapped to a controller in Linux?
A LUN (Logical Unit Number) is a unique identifier used on a SCSI bus to distinguish between devices. In Linux, LUNs are mapped to storage controllers through the SCSI subsystem, which detects and associates each LUN with a specific controller based on the hardware path and device identifiers.

How can I identify which LUNs are mapped to a specific controller on my Linux system?
You can use commands like `lsscsi`, `lsblk`, or `multipath -ll` to list LUNs and their associated controllers. Additionally, inspecting `/sys/class/scsi_device/` or `/dev/disk/by-path/` provides detailed mappings between LUNs and controllers.

Which Linux commands are most effective for finding LUN-to-controller mappings?
Commands such as `lsscsi`, `sg_map`, `multipath -ll`, and `lsblk` are effective. `lsscsi` shows SCSI devices and their controllers, while `multipath -ll` provides multipath device mappings, useful for SAN environments.

How do I interpret the output of `lsscsi` to understand LUN mappings?
The `lsscsi` output lists devices in the format `[H:C:T:L]` where H is the host (controller), C is the channel, T is the target ID, and L is the LUN. The host number corresponds to the controller, allowing you to identify which LUN is mapped to which controller.

Can device files in `/dev` help determine LUN mappings to controllers?
Yes, device files in `/dev/disk/by-path/` often include identifiers that reveal the controller and LUN association. By examining symbolic links in this directory, you can trace which physical controller a LUN is connected to.

What role does the multipath service play in managing LUN mappings on Linux?
The multipath service aggregates multiple physical paths to the same LUN, providing redundancy and load balancing. It helps in identifying and managing LUN mappings across multiple controllers by presenting a single device path to the system.
Understanding how to find which Logical Unit Number (LUN) is mapped to a specific controller in a Linux environment is essential for effective storage management and troubleshooting. The process typically involves examining the system’s SCSI device mappings, using tools such as `lsscsi`, `multipath`, `sg_map`, and querying the sysfs filesystem under `/sys/class/scsi_device` or `/sys/block`. These utilities provide detailed information about the association between LUNs, their corresponding SCSI devices, and the controllers handling them.

Key commands like `lsscsi -g` reveal the SCSI devices along with their generic device mappings, while `multipath -ll` can display multipath topology, showing how LUNs are routed through different controllers and paths. Additionally, inspecting `/sys/class/scsi_host/host*/device` directories offers low-level details about the controllers and their connected devices. Combining these methods allows administrators to accurately identify the LUN-controller relationships, which is crucial for performance tuning, fault isolation, and configuration validation.

In summary, leveraging native Linux tools and sysfs information provides a reliable approach to map LUNs to controllers. Familiarity with these commands and system paths enhances an administrator’s ability to manage

Author Profile

Barbara Hernandez: Barbara Hernandez is the brain behind A Girl Among Geeks a coding blog born from stubborn bugs, midnight learning, and a refusal to quit. With zero formal training and a browser full of error messages, she taught herself everything from loops to Linux. Her mission? Make tech less intimidating, one real answer at a time.

Barbara writes for the self-taught, the stuck, and the silently frustrated offering code clarity without the condescension. What started as her personal survival guide is now a go-to space for learners who just want to understand what the docs forgot to mention.