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= WinFsp - Windows File System Proxy = WinFsp - Windows File System Proxy
This documents presents the design of WinFsp, the Windows File System Proxy. WinFsp is a set of software components for Windows computers that allows the creation of user mode file systems. In this sense it is similar to FUSE (Filesystem in Userspace), which provides the same functionality on UNIX-like computers. This document presents the design of WinFsp, the Windows File System Proxy. WinFsp is a set of software components for Windows computers that allows the creation of user mode file systems. In this sense it is similar to FUSE (Filesystem in Userspace), which provides the same functionality on UNIX-like computers.
== Overview == Overview
Developing file systems is in general a challenging proposition. Not only it requires the correct and efficient design and implementation of a software component that manages files, but to also present an interface to it that adheres to historical standards and expectations that are times difficult to get right or perhaps even ill-conceived (atime in POSIX). Developing file systems is a challenging proposition. It requires the correct and efficient design and implementation of a software component that manages files and it also requires the creation of an interface that adheres to historical standards and expectations that are times difficult to get right or perhaps even ill-conceived (e.g. atime in POSIX).
Developing file systems in Windows is an order of magnitude more difficult. Not only the operating system imposes some very hard (and often-times conflicting) limitations on what a file system driver can do, it is not well-documented and requires a lot of trial and error and long debugging sessions to get right. On top of this kernel mode API's and Driver entry points are heavily overloaded and can be invoked in "unexpected" ways and the system can crash or deadlock for a variety of reasons (including ones that are clearly the result of bad design and you can only avoid them if you follow the "magic incantation" that is not documented anywhere (hello FastIo AcquireForModWrite)). Developing file systems for Windows is an order of magnitude more difficult. Not only the operating system imposes some very hard and at times conflicting limitations on what a file system driver can do, it is not well-documented and requires a lot of trial and error and long debugging sessions to get right. On top of this kernel mode API's and Driver entry points are heavily overloaded and can be invoked in "unexpected" ways and the system can crash or deadlock for a variety of reasons.
To compound all this there is a single book that describes how to write file systems for Windows and it was publised in 1997! While the book is invaluable, it is out of date and contains many errors and omissions. Still if you are thinking about writing file systems get a copy of "Windows NT File System Internals: A Developer's Guide" by Rajeev Nagar (and be prepared to not understand anything). Also study the Microsoft FastFat source! Compounding all this there is a single book that describes how to write file systems for Windows and it was publised in 1997! While the book is invaluable, it is out of date and contains many errors and omissions. Still if you are thinking about writing file systems get a copy of "Windows NT File System Internals: A Developer's Guide" by Rajeev Nagar. Also study the Microsoft FastFat source!
WinFsp attempts to ease the task of writing a new file system for Windows in the same way that FUSE has done so for UNIX. File system writers only need to understand the general semantics of a file in Windows and the simple WinFsp interface. They do not need to understand the intricacies of kernel-mode file system programming or the myriad of details surrounding access control, file sharing, conflicts between open and delete/rename, etc. In this sense WinFsp may save 6-12 months of development cost (and pain) for a file system writer. It also allows developers who have no inclication to understand kernel mode programming to write their own file systems. WinFsp attempts to ease the task of writing a new file system for Windows in the same way that FUSE has done so for UNIX. File system writers only need to understand the general semantics of a file in Windows and the simple WinFsp interface. They do not need to understand the intricacies of kernel-mode file system programming or the myriad of details surrounding access control, file sharing, conflicts between open and delete/rename, etc. In this sense WinFsp may save 6-12 months of development cost (and pain) for a file system writer. It also allows developers who have no inclication to understand kernel mode programming to write their own file systems.