Virtual File System and the layering of a file system call from API to driver

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Virtual File System and Layering of File System Calls

Virtual File System and Layering of File System Calls

A Virtual File System (VFS) is an abstraction layer that separates the file system operations from the physical file system implementation. It is a software layer that sits between the file system driver and the application programming interface (API). The VFS provides a standard set of system calls for file system access, allowing different file systems to be plugged in and used interchangeably without the need for modifications to the application or operating system.

The layering of a file system call from the API to the driver involves several steps, each of which is handled by a different component of the VFS. When an application issues a file system operation, such as opening a file or creating a directory, the VFS intercepts the call and passes it down through the layers to the appropriate file system driver.

System Call Layer

The first layer in the VFS is the system call layer, which is responsible for handling file system operations requested by the application. The system call layer provides a standard set of interfaces for file system access, such as open(), read(), write(), and close(). These interfaces are defined in the operating system's header files and are called by the application to access the file system.

Virtual File System Layer

The second layer is the virtual file system layer, which provides an abstraction of the file system operations. It translates the system calls into generic file system operations that can be handled by any file system driver. This layer also provides a namespace for the file system hierarchy, allowing different file systems to be mounted at different points in the namespace.

File System Switch Layer

The third layer is the file system switch layer, which is responsible for choosing the appropriate file system driver to handle the file system operation. The file system switch layer maintains a list of all the file systems that are currently mounted and selects the appropriate driver based on the file system type and mount point.

File System Driver Layer

The fourth and final layer is the file system driver layer, which provides the physical implementation of the file system operations. Each file system driver is responsible for translating the generic file system operations received from the VFS into operations that are specific to the file system. For example, a driver for a FAT file system would translate the generic read() and write() operations into FAT-specific operations to read and write data from the file system.

Conclusion

In summary, the VFS is a software layer that separates the file system operations from the physical file system implementation. The layering of a file system call from the API to the driver involves several steps, each of which is handled by a different component of the VFS. The system call layer intercepts the file system operation requested by the application and passes it down through the virtual file system layer, the file system switch layer, and finally to the appropriate file system driver. The VFS allows different file systems to be plugged in and used interchangeably without the need for modifications to the application or operating system.

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