What happens if computer hangs while persisting a memory-mapped file?

Question

I'm very interested in using managed memory-mapped files available since .NET 4.0 .

Check the following statement extracted from MSDN:

Persisted files are memory-mapped files that are associated with a source file on a disk. When the last process has finished working with the file, the data is saved to the source file on the disk. These memory-mapped files are suitable for working with extremely large source files.

My question is: what happens if computer hangs while persisting a memory-mapped file?

I mean, since memory-mapped files are stored in virtual memory (I understand that this is in the page file), maybe a file can be restored from virtual memory and try to store it again to the source file after restarting Windows.

Answer 1

The data pages that underlie a memory mapped file reside in the OS cache (file cache). Whenever you shutdown Windows it writes all modified cache pages to the file system.

The pages in the cache are either ordinary file data (from processes doing reads/writes from/to files) or memory mapped pages which are read/written by the paging system.

If Windows is unable (eg crashes or freezes) to flush cache contents to disk then that data is lost.

Answer 2

If enable persistence , memory map file not remove after reboot .

you can use atomic action process with a flag that show data is valid or not if valid you can restore else data lost

If your os support (kernel or filesystem lifetime) like unix you can use share memory with synchronisation that is more faster than map file

Modern Operating Systems 3e (2007) book memory map file: Shared libraries are really a special case of a more general facility called memory-mapped files. The idea here is that a process can issue a system call to map a file onto a portion of its virtual address space. In most implementations, no pages are brought in at the time of the mapping, but as pages are touched, they are demand paged in one at a time, using the disk file as the backing store. When the process exits, or explicitly unmaps the file, all the modified pages are written back to the file. Mapped files provide an alternative model for I/O. Instead of doing reads and writes, the file can be accessed as a big character array in memory. In some situations, programmers find this model more convenient. If two or more processes map onto the same file at the same time, they can communicate over shared memory. Writes done by one process to the shared memory are immediately visible when the other one reads from the part of its virtual address spaced mapped onto the file. This mechanism thus provides a high bandwidth channel between processes and is often used as such (even to the extent of mapping a scratch file). Now it should be clear that if memory-mapped files are available, shared libraries can use this mechanism

In http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n2044.html

Shared Memory

POSIX defines a shared memory object as "An object that represents memory that can be mapped concurrently into the address space of more than one process." Shared memory is similar to file mapping, and the user can map several regions of a shared memory object, just like with memory mapped files. In some operating systems, like Windows, shared memory is an special case of file mapping, where the file mapping object accesses memory backed by the system paging file. However, in Windows, the lifetime of this memory ends when the last process connected to the shared memory object closes connection or the application exits, so there is no data persistence. If an application creates shared memory, fills it with data and exits, the data is lost. This lifetime is known as process lifetime In POSIX operating systems the shared memory lifetime is different since for semaphores, shared memory, and message queues it's mandatory that the object and its state (including data, if any) is preserved after the object is no longer referenced by any process. Persistence of an object does not imply that the state of the object is preserved after a system crash or reboot, but this can be achieved since shared memory objects can actually be implemented as mapped files of a permanent file system. The shared memory destruction happens with an explicit call to unlink(), which is similar to the file destruction mechanism. POSIX shared memory is required to have kernel lifetime (the object is explicitly destroyed or it's destroyed when the operating system reboots) or filesystem persistence (the shared memory object has the same lifetime as a file). This lifetime difference is important to achieve portability. Many portable runtimes have tried to achieve perfect portability between Windows and POSIX shared memory but the author of this paper has not seen any satisfactory effort. Adding a reference count to POSIX shared memory is effective only as long as a process does not crash, something that it's very usual. Emulating POSIX behaviour in Windows using native shared memory is not possible since we could try to dump shared memory to a file to obtain persistence, but a process crash would avoid persistence. The only viable alternative is to use memory mapped files in Windows simulating shared memory, but avoiding file-memory synchronization as much as possible. Many other named synchronization primitives (like named mutexes or semaphores) suffer the same lifetime portability problem. Automatic shared memory cleanup is useful in many contexts, like shared libraries or DLL-s communicating with other DLL-s or processes. Even when there is a crash, resources are automatically cleaned up by the operating systems. POSIX persistence is also useful when a launcher program can create and fill shared memory that another process can read or modify. Persistence also allows data recovery if a server process crashes. All the data is still in the shared memory, and the server can recover its state. This paper proposes POSIX lifetime (kernel or filesystem lifetime) as a more portable solution, but has no strong opinion about this. The C++ committee should take into account the use cases of both approaches to decide which behaviour is better or if both options should be available, forcing the modification of both POSIX and Windows systems.