Chapter 2. Persistent naming attributes

The way you identify and manage storage devices ensures the stability and predictability of the system. Red Hat Enterprise Linux 9 uses two primary naming schemes for this purpose: traditional device names and persistent naming attributes.

Traditional device names

Traditional device names are determined by the Linux kernel based on the physical location of the device in the system. For example, the first SATA drive is usually labeled as /dev/sda, the second as /dev/sdb, and so on. While these names are straightforward, they are subject to change when devices are added or removed or when the hardware configuration is modified. This can pose challenges for scripting and configuration files. Furthermore, traditional names lack descriptive information about the purpose or characteristics of the device.

Persistent naming attributes

Persistent naming attributes are based on unique characteristics of the storage devices, making them more stable and predictable across system reboots. Implementing PNAs involves a more detailed initial configuration compared to traditional naming. One of the key benefits of PNAs is their resilience to changes in hardware configurations, making them ideal for maintaining consistent naming conventions. When using PNAs, you can reference storage devices within scripts, configuration files, and management tools without concerns about unexpected name changes. Additionally, PNAs often include valuable metadata, such as device type or manufacturer information, enhancing their descriptiveness for effective device identification and management.

2.1. Persistent attributes for identifying file systems and block devices

In Red Hat Enterprise Linux 9 storage, persistent naming attributes (PNAs) are mechanisms that provide consistent and reliable naming for storage devices across system reboots, hardware changes, or other events. These attributes are used to identify storage devices consistently, even if the storage devices are added, removed, or reconfigured.

PNAs are used to identify both file systems and block devices, but they serve different purposes:

Persistent attributes for identifying file systems

  • Universally unique identifier (UUID)

    UUIDs are primarily used to uniquely identify file systems on storage devices. Each file system has its own UUID, and this identifier remains constant even if the file system is unmounted, remounted, or the device is detached and reattached.

  • Label

    Labels are user-assigned names for file systems. While they can be used to identify and reference file systems, they are not as standardized as UUIDs. Labels are often used as alternatives to UUIDs to specify file systems in configuration files.

    When you assign a label to a file system, it becomes part of the file system metadata. This label persists with the file system even if you mount the file system on different mount points or different systems.

Persistent attributes for identifying block devices

  • Universally unique identifier (UUID)

    UUIDs can be used to identify storage block devices. When a storage device is formatted or when a file system is created on it, a UUID is often assigned to the device itself. This UUID is embedded within the file system metadata or partition table and is used as a reference for persistent device naming. It allows you to uniquely identify the block device, even if you change the file system or reformat it.

  • World Wide Identifier (WWID)

    WWIDs are globally unique identifiers associated with storage block devices. They are commonly used in Fibre Channel Storage Area Networks (SANs) to identify Host Bus Adapters (HBAs) or network interfaces that connect servers to SAN storage devices. WWIDs ensure consistent communication between servers and SAN storage devices and help manage redundant paths to storage devices.

  • Serial number

    The serial number is a unique identifier assigned to each storage block device by the manufacturer. It can be used to differentiate between storage devices and may be used in combination with other attributes like UUIDs or WWIDs for device management.

2.2. udev device naming rules

Userspace device manager (udev) subsystem allows you to define rules for assigning persistent names to devices. These rules are stored in a file with a .rules extension in the /etc/udev/rules.d/ directory. The purpose of these rules is to ensure that storage devices are consistently and predictably identified, even across system reboots and configuration changes.

udev rules are written in a human-readable format using key-value pairs. When a device is detected or initialized, udev evaluates these rules sequentially, based on the order they are defined. The first matching rule is applied to the device, determining its name and how it will be identified within the system.

In the case of storage devices, udev rules create symbolic links in the /dev/disk/ directory. These symbolic links provide user-friendly aliases for storage devices, making it more convenient to refer to and manage these devices.

You can create custom udev rules to specify how devices should be named based on various attributes such as serial numbers, WWN (World Wide Name) identifiers, or other device-specific characteristics. By defining specific naming rules, you have precise control over how devices are identified within the system.

There are two primary locations for udev rules:

  • /lib/udev/rules.d/ directory contains default rules that come with the udev package.
  • /etc/udev/rules.d directory is intended for custom udev rules.

While udev rules are very flexible, it is important to be aware of udev limitations:

  • Accessibility Timing: Some storage devices might not be accessible at the time of a udev query.
  • Event-Based Processing: The kernel can send udev events at any time, potentially triggering rule processing and link removal if a device is inaccessible.
  • Processing Delay: There might be a delay between event generation and processing, especially with numerous devices, causing a lag between kernel detection and link availability.
  • Device Accessibility: External programs invoked by udev rules, like blkid, might briefly open the device, making it temporarily inaccessible for other tasks.
  • Link Updates: Device names managed by udev in /dev/disk/ can change between major releases, requiring link updates.

Additional resources

  • The udev man page