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A memory-mapped file is a segment of virtual memory [1] that has been assigned a direct byte-for-byte correlation with some portion of a file or file-like resource. This resource is typically a file that is physically present on disk, but can also be a device, shared memory object, or other resource that an operating system can reference through a file descriptor.
The main difference between System V shared memory (shmem) and memory mapped I/O (mmap) is that System V shared memory is persistent: unless explicitly removed by a process, it is kept in memory and remains available until the system is shut down. mmap'd memory is not persistent between application executions (unless it is backed by a file).
This makes it possible for multiple users on multiple machines to share files and storage resources. Distributed file systems differ in their performance, mutability of content, handling of concurrent writes, handling of permanent or temporary loss of nodes or storage, and their policy of storing content.
Drive mapping is how MS-DOS and Microsoft Windows associate a local drive letter (A-Z) with a shared storage area to another computer (often referred as a File Server) over a network. After a drive has been mapped , a software application on a client 's computer can read and write files from the shared storage area by accessing that drive, just ...
A distributed data store is a computer network where information is stored on more than one node, often in a replicated fashion. [1] It is usually specifically used to refer to either a distributed database where users store information on a number of nodes, or a computer network in which users store information on a number of peer network nodes.
A full file reference (pathname in today's parlance) consists of a filename, a filetype, and a disk letter called a filemode (e.g. A or B). Minidisks can correspond to physical disk drives, but more typically refer to logical drives, which are mapped automatically onto shared devices by the operating system as sets of virtual cylinders.
POSIX also provides the mmap API for mapping files into memory; a mapping can be shared, allowing the file's contents to be used as shared memory. Linux distributions based on the 2.6 kernel and later offer /dev/shm as shared memory in the form of a RAM disk , more specifically as a world-writable directory (a directory in which every user of ...
A block, a contiguous number of bytes, is the minimum unit of storage that is read from and written to a disk by a disk driver.The earliest disk drives had fixed block sizes (e.g. the IBM 350 disk storage unit (of the late 1950s) block size was 100 six-bit characters) but starting with the 1301 [8] IBM marketed subsystems that featured variable block sizes: a particular track could have blocks ...