In recent years, concerns have arisen because of long latencies associated with some kernel run-times, sometimes on the order of 100ms or more in systems with monolithic kernels. These latencies often produce noticeable slowness in desktop systems, and can prevent operating systems from performing time-sensitive operations such as audio recording and some communications.Modern operating systems extend the concepts of application preemption to device drivers and kernel code, so that the operating system has preemptive control over internal run-times as well. Under Windows Vista, the introduction of the Windows Display Driver Model (WDDM) accomplishes this for display drivers, and in Linux, the preemptable kernel model introduced in version 2.6 allows all device driversand some other parts of kernel code to take advantage of preemptive multi-tasking.
Under Windows prior to Windows Vista and Linux prior to version 2.6 all driver execution was co-operative, meaning that if a driver entered an infinite loop it would freeze the system.
Disk access and file systems
Access to data stored on disks is a central feature of all operating systems. Computers store dataon disks using files, which are structured in specific ways in order to allow for faster access, higher reliability, and to make better use out of the drive's available space. The specific way in which files are stored on a disk is called a file system, and enables files to have names and attributes. It also allows them to be stored in a hierarchy of directories or folders arranged in a directorytree.
Early operating systems generally supported a single type of disk drive and only one kind of file system. Early file systems were limited in their capacity, speed, and in the kinds of file names and directory structures they could use. These limitations often reflected limitations in the operating systems they were designed for, making it very difficult for an operating system to supportmore than one file system.
While many simpler operating systems support a limited range of options for accessing storage systems, operating systems like UNIX and Linux support a technology known as a virtual file system or VFS. An operating system like UNIX supports a wide array of storage devices, regardless of their design or file systems to be accessed through a common application programminginterface (API). This makes it unnecessary for programs to have any knowledge about the device they are accessing. A VFS allows the operating system to provide programs with access to an unlimited number of devices with an infinite variety of file systems installed on them through the use of specific device drivers and file system drivers.
A connected storage device such as a hard drive isaccessed through a device driver. The device driver understands the specific language of the drive and is able to translate that language into a standard language used by the operating system to access all disk drives. On UNIX this is the language of block devices.
When the kernel has an appropriate device driver in place, it can then access the contents of the disk drive in raw format, which maycontain one or more file systems. A file system driver is used to translate the commands used to access each specific file system into a standard set of commands that the operating system can use to talk to all file systems. Programs can then deal with these file systems on the basis of filenames, and directories/folders, contained within a hierarchical structure. They can create, delete, open, andclose files, as well as gather various information about them, including access permissions, size, free space, and creation and modification dates.
Various differences between file systems make supporting all file systems difficult. Allowed characters in file names, case sensitivity, and the presence of various kinds of file attributes makes the implementation of a single interface for every file...