Architecture and Design of the Linux Storage Stack by Muhammad Umer

Author:Muhammad Umer
Language: eng
Format: epub, pdf
Publisher: Packt
Published: 2023-11-15T00:00:00+00:00

Looking at the device mapper framework

By default, managing physical block devices is rigid in that there are only a handful of ways in which an application can make use of them. When dealing with block devices, informed decisions have to be made regarding disk partitioning and space management to ensure optimal usage of available resources. In the past, features such as thin provisioning, snapshots, volume management, and encryption were exclusive to enterprise storage arrays. However, over time, these features have become crucial components of any local storage infrastructure. When operating with physical drives, it is expected that the upper layers of the operating system will possess the necessary capabilities to implement and sustain these functionalities. The Linux kernel provides the device mapper framework for implementing these concepts. The device mapper is used by the kernel to map physical block devices to higher-level virtual block devices. The primary goal of the device mapper framework is to create a high-level layer of abstraction on top of physical devices. The device mapper provides a mechanism to modify bio structures in transit and map them to block devices. The use of the device mapper framework lays the foundation for implementing features such as logical volume management.

The device mapper provides a generic way to create virtual layers of block devices on top of physical devices and implement features such as striping, mirroring, snapshots, and multipathing. Like most things in Linux, the functionality of the device mapper framework is divided into kernel space and user space. The policy-related work, such as defining physical-to-logical mappings, is contained in the user space, while the functions that implement the policies to establish these mappings lie in the kernel space.

The device mapper’s application interface is the ioctl system call. This system call adjusts the special file’s underlying device parameters. The logical devices that employ the device mapper framework are managed via the dmsetup command and the libdevmapper library, which implement the respective user interface, as depicted in the following figure:

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