DRBD Kernel Module

The DRBD kernel module is the core component of DRBD (Distributed Replicated Block Device) that runs in the Linux kernel. It implements the low-level functionality required to mirror block devices between servers in real time. By handling data replication at the kernel level, the module ensures efficient, synchronous data transfer between nodes, making it a key element in building high-availability storage clusters.

Purpose

• Block-Level Replication: The DRBD kernel module mirrors data written to one block device to a corresponding device on a remote node. This real-time replication provides data redundancy and high availability.

• Synchronous Data Transfer: Operating within the kernel, the module ensures minimal latency in data replication, which is critical for maintaining data consistency between the primary and secondary nodes.

• Integration with User-Space Tools: The kernel module works in conjunction with user-space utilities (e.g., drbdadm, drbdsetup, drbdmeta) to configure, monitor, and manage DRBD resources. It exposes status and configuration details via virtual files (e.g., /proc/drbd).

Key Characteristics

• Kernel-Space Operation: As a kernel module (often named drbd.ko), it is loaded into the Linux kernel at boot or on demand via tools like modprobe.

• Performance Optimization: By handling replication in kernel space, DRBD minimizes context switches and data copies between user and kernel space, resulting in high throughput and low latency.

• Dynamic Configuration: The module supports dynamic configuration changes through user-space tools, allowing administrators to adjust replication settings and monitor device health in real time.

Typical Usage

1. Loading the Module: The DRBD kernel module is typically loaded at boot via the system's module loader or manually using:

   modprobe drbd

   This command loads drbd.ko into the kernel, enabling DRBD functionality.

2. Verifying Module Status: To check if the DRBD kernel module is loaded, use:

   lsmod | grep drbd

   This lists the loaded modules and confirms that DRBD is active.

3. Monitoring via /proc/drbd: Once loaded, the module exposes its status and configuration through the /proc/drbd file. Viewing this file:

   cat /proc/drbd

   provides real-time details about DRBD devices, including their connection states and synchronization progress.

4. Integration with User-Space Utilities: Tools such as drbdsetup, drbdadm, and drbdmeta interact with the DRBD kernel module to manage replication, initialize metadata, and perform device configuration. These utilities rely on the kernel module to implement the actual replication logic.

Conclusion

The DRBD kernel module is the backbone of the DRBD system, enabling real-time, synchronous block-level replication between Linux systems. Its integration into the kernel allows it to deliver high performance and low latency, making it an essential component for high-availability storage clusters. When used alongside user-space management tools, the module provides robust data replication, ensuring that critical data remains consistent and available even in the event of node failures.