Explaining the need for a Virtual Filesystem
A standard filesystem is a set of data structures that determine how user data is organized on a disk. End users are able to interact with this standard filesystem through regular file access methods and perform common tasks. Every operating system (Linux or non-Linux) provides at least one such filesystem, and naturally, each of them claims to be better, faster, and more secure than the other. A great majority of modern Linux distributions use XFS or Ext4 as the default filesystem. These filesystems have several features and are considered stable and reliable for daily usage.
However, the support for filesystems in Linux is not limited to only these two. One of the great benefits of using Linux is that it offers support for multiple filesystems, all of which can be considered perfectly acceptable alternatives to Ext4 and XFS. Because of this, Linux can peacefully coexist with other operating systems. Some of the more commonly used filesystems include older versions of Ext4, such as Ext2 and Ext3, Btrfs, ReiserFS, OpenZFS, FAT, and NTFS. When using multiple partitions, users can choose from a long list of available filesystems and create a different one on every disk partition as per their needs.
The smallest addressable unit of a physical hard drive is a sector. For filesystems, the smallest writable unit is called a block. A block can be considered a group of consecutive sectors. All operations by a filesystem are performed in terms of blocks. There is no singular way in which these blocks are addressed and organized by different filesystems. Each filesystem may use a different set of data structures to allocate and store data on these blocks. The presence of a different filesystem on each storage partition can be difficult to manage. Given the wide range of supported filesystems in Linux, imagine if applications needed to understand the distinct details of every filesystem. In order to be compatible with a filesystem, the application would need to implement a unique access method for each filesystem it uses. This would make the design of an application almost impractical.
Abstraction interfaces play a critical role in the Linux kernel. In Linux, regardless of the filesystem being used, the end users or applications can interact with the filesystem using uniform access methods. All this is achieved through the Virtual Filesystem layer, which hides the filesystem implementations under an all-inclusive interface.
