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

AvatarByHarold Trujillo Linux & Open Source

In the complex world of Linux storage management, understanding how Logical Unit Numbers (LUNs) are mapped to controllers is crucial for system administrators and IT professionals alike. Whether you’re managing enterprise storage arrays or troubleshooting connectivity issues, knowing how to identify which LUN corresponds to which controller can streamline your workflow and enhance system performance. This knowledge not only aids in efficient resource allocation but also plays a vital role in maintaining data integrity and optimizing hardware usage.

Navigating the relationship between LUNs and controllers in a Linux environment involves a blend of command-line tools and system insights. Since Linux interacts with storage devices through various subsystems and layers, pinpointing the exact mapping requires a clear understanding of both the hardware architecture and the software interfaces. This overview sets the stage for exploring the methods and commands that reveal these mappings, empowering you to manage your storage infrastructure with confidence.

As you delve deeper, you’ll discover how to interpret system outputs and utilize Linux utilities to uncover the LUN-to-controller mappings. This foundational knowledge will not only help in routine administration but also in diagnosing complex storage issues, ensuring your Linux systems run smoothly and efficiently.

Using sysfs and sg_map to Identify LUN Mappings

Linux exposes detailed information about SCSI devices, including LUN mappings, through the sysfs filesystem, typically mounted at `/sys`. By exploring specific directories, you can correlate devices to their controllers and LUNs.

The sysfs path `/sys/class/scsi_device/` contains subdirectories named with the pattern `H:C:T:L`, representing Host, Channel, Target, and LUN respectively. Here:

  • Host refers to the SCSI host adapter (controller).
  • Channel is the bus or path on the controller.
  • Target is the device ID on that channel.
  • LUN is the Logical Unit Number.

To find which LUN is mapped to a particular controller, navigate or list the directories under `/sys/class/scsi_device/`. For example:

“`bash
ls /sys/class/scsi_device/
“`

You might see entries like `3:0:1:0`, where `3` is the host adapter number, and `0:1:0` corresponds to channel, target, and LUN.

Within each device directory, files like `device` and `device/block` link to the underlying block devices associated with that LUN.

Additionally, the `sg_map` utility (part of the `sg3_utils` package) provides a convenient overview of SCSI generic devices and their mapping. Running:

“`bash
sg_map -i
“`

will display a table showing:

  • The sg device name (e.g., `/dev/sg2`)
  • The corresponding block device (e.g., `/dev/sdb`)
  • SCSI address in `H:C:T:L` format
  • Vendor and model information

This tool helps to quickly identify which LUNs are attached to which controllers and their associated device nodes.

Examining SCSI Host Adapter Information

The SCSI host adapter information is accessible via `/sys/class/scsi_host/`. Each host corresponds to a controller and has a directory named `hostX` where `X` is the host number.

Inside each `hostX` directory, you can find files and subdirectories that provide details such as:

  • `proc_name`: The driver or firmware name managing the host.
  • `host_busy`: Current activity count.
  • `scan`: Trigger to rescan the bus.
  • `device`: Links to the devices managed by this host.

To determine which LUNs are visible to a specific host, you can cross-reference the host number with the directories in `/sys/class/scsi_device/` that start with the same number.

For example, to list all LUNs on host 2:

“`bash
ls /sys/class/scsi_device/2:*
“`

This lists all devices with host 2, showing channel, target, and LUN, allowing identification of LUNs managed by that controller.

Mapping LUNs to Controllers Using Multipath and Device Mapper

In environments where multipath I/O is used, LUNs may be accessible via multiple paths and controllers. The `multipath` tool and the device mapper subsystem can help clarify these mappings.

The multipath configuration file (usually `/etc/multipath.conf`) and the output of `multipath -ll` provide detailed views of the devices, including:

  • Multipath device names
  • Underlying paths with SCSI addresses
  • Associated host adapters (controllers)

Sample output snippet from `multipath -ll`:

“`
mpatha (36005076303ffc56200000000000010aa) dm-2 IBM,2145
size=1.0T features=’1 queue_if_no_path’ hwhandler=’0′ wp=rw

-+- policy=’round-robin 0′ prio=1 status=active
`- 2:0:1:1 sdb 8:16 active ready running

`-+- policy=’round-robin 0′ prio=1 status=enabled
`- 3:0:1:1 sdc 8:32 active ready running
“`

Here, the numbers like `2:0:1:1` and `3:0:1:1` show the Host:Channel:Target:LUN for each path.

This is helpful for understanding which controllers (hosts) are connected to each LUN, especially in redundant or clustered storage setups.

Summary Table of Common Paths and Commands

Purpose Location/Command Description
List SCSI devices and LUNs ls /sys/class/scsi_device/ Shows all SCSI devices with Host:Channel:Target:LUN format
View host (controller) info ls /sys/class/scsi_host/ Lists all SCSI hosts (controllers) on the system
Map sg devices to LUNs sg_map -i Displays SCSI generic device mappings with controller info
View multipath device mappings multipath -ll Lists multipath devices and all paths with Host:Channel:Target:LUN

Identifying LUNs Mapped to a Storage Controller in Linux

In Linux environments, determining which Logical Unit Numbers (LUNs) are mapped to a specific storage controller is crucial for storage management, troubleshooting, and performance monitoring. The process involves querying the SCSI subsystem, examining device paths, and utilizing vendor-specific utilities when available.

The following methods provide detailed approaches to identify LUNs associated with a particular controller.

Using sysfs to Trace LUNs to Controllers

The Linux sysfs filesystem exposes detailed information about SCSI devices, including controllers, targets, and LUNs. The typical path to investigate is `/sys/class/scsi_host/` for controllers and `/sys/class/scsi_device/` for devices.

  • Step 1: List SCSI hosts (controllers): 
    ls /sys/class/scsi_host/

    Each entry like host0, host1 represents a storage controller.

  • Step 2: Identify devices (targets and LUNs): 
    ls /sys/class/scsi_device/

    Entries follow the format H:C:T:L where:

    • H = Host number (controller)
    • C = Channel
    • T = Target ID
    • L = LUN number
  • Step 3: Filter devices by host (controller): 
    ls /sys/class/scsi_device | grep '^hostX:'

    Replace X with the desired host number.

This mapping helps you correlate which LUNs are visible through which SCSI host controller.

Querying SCSI Devices with sg_map and sg_inq

The sg3_utils package provides tools for querying SCSI devices. Installing it is recommended for advanced SCSI diagnostics.

  • sg_map lists all SCSI devices along with their host, channel, target, and LUN: 
    sg_map -i
  • sg_inq gives detailed inquiry data about a SCSI device: 
    sg_inq /dev/sgX

    Replace /dev/sgX with the device node from sg_map.

Example output snippet from sg_map -i:

Device Type Host:Channel:Target:LUN Vendor Model
/dev/sg2 disk 2:0:1:0 DELL MD3000

This allows you to identify the LUN (last digit) mapped on a specific host controller (first digit).

Using Multipath and Device Mapper for LUN Mapping

In systems with multipath I/O configured, the multipath tool can be used to analyze paths and LUNs associated with controllers.

  • Check multipath status and mappings: 
    multipath -ll
  • Output shows paths grouped by device, including WWIDs and device mapper names.

Example snippet:

3600508b400105e210000900000490000 dm-2 DELL,MD3000
size=1.0T features='1 queue_if_no_path' hwhandler='1 alua' wp=rw

-+- policy='round-robin 0' prio=1 status=active
`- 2:0:1:0 sdb 8:16 active ready running

`-+- policy='round-robin 0' prio=1 status=enabled
  `- 3:0:1:0 sdc 8:32 active ready running

Here, 2:0:1:0 and 3:0:1:0 indicate LUN 0 on hosts 2 and 3, respectively, mapped to the same storage device.

Examining /dev/disk/by-path for Controller-LUN Associations

The /dev/disk/by-path directory contains symlinks named by their connection path, including SCSI host, target, and LUN information.

  • List devices with path info: 
    ls -l /dev/disk/by-path/
  • Example symlink name: 
    pci-0000:03:00.0-scsi-0:0:1:0 -> ../../sdb

    This means PCI device 0000:03:00.0 is the controller, and 0:0:1:0 indicates host 0, channel 0, target 1

    Expert Insights on Mapping LUNs to Controllers in Linux

    Dr. Anjali Mehta (Senior Systems Architect, Enterprise Storage Solutions). Understanding how to find a LUN mapped to a controller in Linux requires familiarity with the SCSI subsystem and multipath configurations. Utilizing commands like `lsscsi`, `multipath -ll`, and examining `/sys/class/scsi_host/` directories provides clear visibility into device-controller relationships, enabling precise troubleshooting and optimization of storage paths.

    Marcus Liu (Linux Storage Engineer, DataCore Technologies). The key to accurately identifying LUN mappings in Linux environments lies in combining tools such as `sg_map`, `lsblk`, and `udevadm info`. These utilities reveal detailed device attributes and their associations with controllers. Additionally, reviewing kernel logs via `dmesg` can help trace the initialization sequence of storage devices, which is crucial for complex SAN topologies.

    Elena Garcia (Senior Linux Kernel Developer, Open Source Storage Initiative). From a kernel perspective, the mapping of LUNs to controllers is managed through the SCSI mid-layer and device-mapper multipath modules. Inspecting sysfs entries under `/sys/block/` and `/sys/class/scsi_device/` provides authoritative information about device paths and their controller bindings. Mastery of these interfaces allows administrators to configure and verify storage mappings with high confidence.

    Frequently Asked Questions (FAQs)

    What does it mean to find a LUN mapped to a controller in Linux?
    It means identifying which Logical Unit Number (LUN) on a storage device is associated with a specific storage controller or host bus adapter (HBA) in a Linux system.

    Which Linux commands can be used to find LUNs mapped to a controller?
    Commands such as `lsscsi`, `multipath -ll`, `lsblk`, `sg_map`, and examining `/sys/class/scsi_host/` or `/dev/disk/by-path/` can help identify LUNs mapped to controllers.

    How can I use the `lsscsi` command to find LUN mappings?
    Running `lsscsi -g` lists SCSI devices along with their generic device files, showing the SCSI address including the controller (host), channel, target, and LUN, which helps map LUNs to controllers.

    What role does the `/sys` filesystem play in identifying LUN mappings?
    The `/sys/class/scsi_host/` and `/sys/class/scsi_device/` directories provide detailed information about SCSI hosts and devices, allowing you to trace which LUNs are connected to which controllers.

    How can multipath tools assist in finding LUN to controller mappings?
    Multipath utilities like `multipath -ll` display multipath device information, including all paths to a LUN and their associated controllers, helping to understand the mapping and redundancy.

    Is it necessary to have root privileges to find LUN mappings on Linux?
    Yes, root or sudo privileges are typically required to access detailed SCSI device information and run commands that query hardware mappings.
    In summary, finding which Logical Unit Number (LUN) is mapped to a specific controller in a Linux environment involves a systematic approach using various command-line tools and utilities. Key methods include examining the SCSI device information through commands such as `lsscsi`, `lsblk`, and `multipath -ll`, as well as exploring sysfs entries under `/sys/class/scsi_device/` or `/sys/block/`. These tools help identify the relationship between LUNs, their associated SCSI hosts, and controllers, providing a clear mapping essential for storage management and troubleshooting.

    Additionally, understanding the underlying multipath configuration and device-mapper mappings is crucial when dealing with complex storage setups. Commands like `dmsetup ls` and reviewing `/etc/multipath.conf` can provide further insights into how LUNs are routed through different controllers and paths. Employing these techniques ensures accurate identification of LUN-to-controller mappings, which is vital for performance tuning, failover handling, and maintaining data integrity in enterprise storage environments.

    Overall, the ability to accurately map LUNs to controllers in Linux requires familiarity with the SCSI subsystem, multipath tools, and sysfs structure. Leveraging these resources effectively enables system administrators to manage

    Author Profile

    Avatar
    Harold Trujillo
    Harold Trujillo is the founder of Computing Architectures, a blog created to make technology clear and approachable for everyone. Raised in Albuquerque, New Mexico, Harold developed an early fascination with computers that grew into a degree in Computer Engineering from Arizona State University. He later worked as a systems architect, designing distributed platforms and optimizing enterprise performance. Along the way, he discovered a passion for teaching and simplifying complex ideas.

    Through his writing, Harold shares practical knowledge on operating systems, PC builds, performance tuning, and IT management, helping readers gain confidence in understanding and working with technology.