computer science

Virtual file system

Virtual file system is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Virtual file system rather than just read about it. In short: A virtual file system (VFS) or virtual filesystem switch is an abstract layer on top of a more concrete file system. The purpose of a VFS is to allow client applications to access different types of concrete file systems in a uniform way.

Key takeaways

Virtual file system belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Virtual file system to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Virtual file system from memory before moving on to harder problems.

Reference excerpt

A virtual file system (VFS) or virtual filesystem switch is an abstract layer on top of a more concrete file system. The purpose of a VFS is to allow client applications to access different types of concrete file systems in a uniform way. A VFS can, for example, be used to access local and network storage devices transparently without the client application noticing the difference. It can be used to bridge the differences in Windows, classic Mac OS/macOS and Unix filesystems, so that applications can access files on local file systems of those types without having to know what type of file system they are accessing. A VFS specifies an interface (or a "contract") between the kernel and a concrete file system. Therefore, it is easy to add support for new file system types to the kernel simply by fulfilling the contract. The terms of the contract might change incompatibly from release to release, which would require that concrete file system support be recompiled, and possibly modified before recompilation, to allow it to work with a new release of the operating system; or the supplier of the operating system might make only backward-compatible changes to the contract, so that concrete file system support built for a given release of the operating system would work with future versions of the operating system.

Implementations

One of the first virtual file system mechanisms on Unix-like systems was introduced by Sun Microsystems in SunOS 2.0 in 1985. It allowed Unix system calls to access local UFS file systems and remote NFS file systems transparently. For this reason, Unix vendors who licensed the NFS code from Sun often copied the design of Sun's VFS. Other file systems could be plugged into it also: there was an implementation of the MS-DOS FAT file system developed at Sun that plugged into the SunOS VFS, although it was not shipped as a product until SunOS 4.1. The SunOS implementation was the basis of the VFS mechanism in System V Release 4. John Heidemann developed a stacking VFS under SunOS 4.0 for the experimental Ficus file system. This design provided for code reuse among file system types with differing but similar semantics (e.g., an encrypting file system could reuse all of the naming and storage-management code of a non-encrypting file system). Heidemann adapted this work for use in 4.4BSD as a part of his thesis research; descendants of this code underpin the file system implementations in modern BSD derivatives including macOS. Other Unix virtual file systems include the File System Switch in System V Release 3, the Generic File System in Ultrix, and the VFS in Linux. In OS/2 and Microsoft Windows, the virtual file system mechanism is called the Installable File System. The Filesystem in Userspace (FUSE) mechanism allows userland code to plug into the virtual file system mechanism in Linux, NetBSD, FreeBSD, OpenSolaris, and macOS. In Microsoft Windows, virtual filesystems can also be implemented through userland Shell namespace extensions; however, they do not support the lowest-level file system access application programming interfaces in Windows, so not all applications will be able to access file systems that are implemented as namespace extensions. KIO and GVfs/GIO provide similar mechanisms in the KDE and GNOME desktop environments (respectively), with similar limitations, although they can be made to use FUSE techniques and therefore integrate smoothly into the system.

Single-file virtual file systems Sometimes Virtual File System refers to a file or a group of files (not necessarily inside a concrete file system) that acts as a manageable container which should provide the functionality of a concrete file system through the usage of software. Examples of such containers are CBFS Connect from Callback Technologies or a single-file virtual file system in an emulator like PCTask or so-called WinUAE, Oracle's VirtualBox, Microsoft's Virtual PC, VMware. The primary benefit for this type of file system is that it is centralized and easy to remove. A single-file virtual file system may include all the basic features expected of any file system (virtual or otherwise), but access to the internal structure of these file systems is often limited to programs specifically written to make use of the single-file virtual file system (instead of implementation through a driver allowing universal access). Another major drawback is that performance is relatively low when compared to other virtual file systems. Low performance is mostly due to the cost of shuffling virtual files when data is written or deleted from the virtual file system.

Implementation of single-file virtual filesystems Direct examples of single-file virtual file systems include emulators, such as PCTask and WinUAE, which encapsulate not only the filesystem data but also emulated disk layout. This makes it easy to treat an OS installation like any other piece of software—transferring it with removable media or over the network.

PCTask The Amiga emulator PCTask emulated an Intel PC 8088 based machine clocked at 4.77MHz (and later an 80486SX clocked at 25 MHz). Users of PCTask could create a file of large size on the Amiga filesystem, and this file would be virtually accessed from the emulator as if it were a real PC Hard Disk. The file could be formatted with the FAT16 filesystem to store normal MS-DOS or Windows files.[1][2]

WinUAE The UAE for Windows, WinUAE, allows for large single files on Windows to be treated as Amiga file systems. In WinUAE this file is called a hardfile.[3] UAE could also treat a directory on the host filesystem (Windows, Linux, macOS, AmigaOS) as an Amiga filesystem.[4]

See also 9P (protocol) – a distributed file system protocol that maps directly to the VFS layer of Plan 9, making all file system access network-transparent Synthetic file system – a hierarchical interface to non-file objects that appear as if they were regular files in the tree of a disk-based file system

Notes ^Emulation on Amiga Comparison between PCX and PCTask, Amiga PC emulators. ^See also This article explaining how it works PCTask. ^Help About WinUAE (See Hardfile section). ^Help About WinUAE (See Add Directory section)

References

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Illustrations

Worked examples

Example 1 — a first encounter with Virtual file system

Start with the simplest possible case. Write down what Virtual file system claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Virtual file system before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Virtual file system ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Virtual file system

In research: Virtual file system appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Virtual file system in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Virtual file system is common in secondary-school and first-year university syllabi. It links to neighbouring topics Computer file systems, Virtualization, so understanding it makes those chapters shorter.
In everyday life: Look for Virtual file system outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Virtual file system in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Virtual file system means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Virtual file system out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Virtual file system in simple terms?

Why does Virtual file system matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Virtual file system?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Virtual file system.