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QNX4FS
QNX4FS is an extent-based file system used by the QNX4 and QNX6 operating systems. As the file system uses soft updates, it remains consistent even after a power failure, without using journaling. Instead, the writes are carefully ordered and flushed to disk at appropriate intervals so that the on-disk structure always remains consistent, no matter if the operation is interrupted. However, unflushed changes to the file system are nevertheless lost, as the disk cache is typically stored in volatile memory. This design has a considerable performance gain over journaling, by just bypassing that step. Another notable property of this file system is that its actual metadata, like inode information and disk bitmaps, are accessible in the same way as any other file on the file system (as /.inodes and /.bitmap, respectively). This is consistent with QNX's (in fact, Plan 9 from Bell Labs's, or historically Unix's) philosophy that "everything is a file Everything is a file is an i ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
File System
In computing, file system or filesystem (often abbreviated to fs) is a method and data structure that the operating system uses to control how data is stored and retrieved. Without a file system, data placed in a storage medium would be one large body of data with no way to tell where one piece of data stopped and the next began, or where any piece of data was located when it was time to retrieve it. By separating the data into pieces and giving each piece a name, the data are easily isolated and identified. Taking its name from the way a paper-based data management system is named, each group of data is called a " file". The structure and logic rules used to manage the groups of data and their names is called a "file system." There are many kinds of file systems, each with unique structure and logic, properties of speed, flexibility, security, size and more. Some file systems have been designed to be used for specific applications. For example, the ISO 9660 file system is desig ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Soft Updates
Soft updates is an approach to maintaining file system meta-data integrity in the event of a crash or power outage. Soft updates work by tracking and enforcing dependencies among updates to file system meta-data. Soft updates are an alternative to the more commonly used approach of journaling file systems. Method of operation Soft updates allow only asynchronous metadata writes that do not render the on-disk file system inconsistent, or that the only inconsistency that ever happens is a storage space leak (space marked allocated when not used by any file). It avoids having to do ordered synchronous metadata writes by temporarily "rolling back" any part of a metadata block that depends on another potentially non-flushed or partially rolled-back block when writing it. In effect, blocks may be flushed at any time and the soft-update code will always provide the disk a consistent version of it (as long as it knows which blocks have physically been flushed). Recovery then simply b ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Power Failure
A power outage (also called a powercut, a power out, a power failure, a power blackout, a power loss, or a blackout) is the loss of the electrical power network supply to an end user. There are many causes of power failures in an electricity network. Examples of these causes include faults at power stations, damage to electric transmission lines, substations or other parts of the distribution system, a short circuit, cascading failure, fuse or circuit breaker operation. Power failures are particularly critical at sites where the environment and public safety are at risk. Institutions such as hospitals, sewage treatment plants, and mines will usually have backup power sources such as standby generators, which will automatically start up when electrical power is lost. Other critical systems, such as telecommunication, are also required to have emergency power. The battery room of a telephone exchange usually has arrays of lead–acid batteries for backup and also a socket ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Journaling File System
A journaling file system is a file system that keeps track of changes not yet committed to the file system's main part by recording the goal of such changes in a data structure known as a " journal", which is usually a circular log. In the event of a system crash or power failure, such file systems can be brought back online more quickly with a lower likelihood of becoming corrupted. Depending on the actual implementation, a journaling file system may only keep track of stored metadata, resulting in improved performance at the expense of increased possibility for data corruption. Alternatively, a journaling file system may track both stored data and related metadata, while some implementations allow selectable behavior in this regard. History In 1990 IBM introduced JFS in AIX 3.1 as one of the first UNIX commercial filesystems that implemented journaling. The next year the idea was popularized in a widely cited paper on log-structured file systems. This was subsequently i ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Hard Disk
A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magnetic material. The platters are paired with magnetic heads, usually arranged on a moving actuator arm, which read and write data to the platter surfaces. Data is accessed in a random-access manner, meaning that individual blocks of data can be stored and retrieved in any order. HDDs are a type of non-volatile storage, retaining stored data when powered off. Modern HDDs are typically in the form of a small rectangular box. Introduced by IBM in 1956, HDDs were the dominant secondary storage device for general-purpose computers beginning in the early 1960s. HDDs maintained this position into the modern era of servers and personal computers, though personal computing devices produced in large volume, like cell phones and tablets, rely ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Page Cache
In computing, a page cache, sometimes also called disk cache, is a transparent cache for the pages originating from a secondary storage device such as a hard disk drive (HDD) or a solid-state drive (SSD). The operating system keeps a page cache in otherwise unused portions of the main memory (RAM), resulting in quicker access to the contents of cached pages and overall performance improvements. A page cache is implemented in kernels with the paging memory management, and is mostly transparent to applications. Usually, all physical memory not directly allocated to applications is used by the operating system for the page cache. Since the memory would otherwise be idle and is easily reclaimed when applications request it, there is generally no associated performance penalty and the operating system might even report such memory as "free" or "available". When compared to main memory, hard disk drive read/writes are slow and random accesses require expensive disk seeks; as a resul ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Volatile Memory
Volatile memory, in contrast to non-volatile memory, is computer memory that requires power to maintain the stored information; it retains its contents while powered on but when the power is interrupted, the stored data is quickly lost. Volatile memory has several uses including as primary storage. In addition to usually being faster than forms of mass storage such as a hard disk drive, volatility can protect sensitive information, as it becomes unavailable on power-down. Most general-purpose random-access memory (RAM) is volatile. Types There are two kinds of volatile RAM: dynamic and static. Even though both types need continuous electrical current to retain data, there are some important differences between them. Dynamic RAM (DRAM) is very popular due to its cost-effectiveness. DRAM stores each bit of information in a different capacitor within the integrated circuit. DRAM chips need just one single capacitor and one transistor to store each bit of information. This makes it s ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Inode
The inode (index node) is a data structure in a Unix-style file system that describes a file-system object such as a file or a directory. Each inode stores the attributes and disk block locations of the object's data. File-system object attributes may include metadata (times of last change, access, modification), as well as owner and permission data. A directory is a list of inodes with their assigned names. The list includes an entry for itself, its parent, and each of its children. Etymology There has been uncertainty on the Linux kernel mailing list about the reason for the "i" in "inode". In 2002, the question was brought to Unix pioneer Dennis Ritchie, who replied: A 1978 paper by Ritchie and Ken Thompson bolsters the notion of "index" being the etymological origin of inodes. They wrote: Additionally, Maurice J. Bach wrote that an inode "is a contraction of the term index node and is commonly used in literature on the UNIX system". Details A file system relies on d ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Plan 9 From Bell Labs
Plan 9 from Bell Labs is a distributed operating system which originated from the Computing Science Research Center (CSRC) at Bell Labs in the mid-1980s and built on UNIX concepts first developed there in the late 1960s. Since 2000, Plan 9 has been free and open-source. The final official release was in early 2015. Under Plan 9, UNIX's ''everything is a file'' metaphor is extended via a pervasive network-centric filesystem, and the cursor-addressed, terminal-based I/O at the heart of UNIX-like operating systems is replaced by a windowing system and graphical user interface without cursor addressing, although rc, the Plan 9 shell, is text-based. The name ''Plan 9 from Bell Labs'' is a reference to the Ed Wood 1957 cult science fiction Z-movie ''Plan 9 from Outer Space''. The system continues to be used and developed by operating system researchers and hobbyists. History Plan 9 from Bell Labs was originally developed, starting in the late 1980s, by members of the Computin ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Unix Philosophy
The Unix philosophy, originated by Ken Thompson, is a set of cultural norms and philosophical approaches to minimalist, modular software development. It is based on the experience of leading developers of the Unix operating system. Early Unix developers were important in bringing the concepts of modularity and reusability into software engineering practice, spawning a " software tools" movement. Over time, the leading developers of Unix (and programs that ran on it) established a set of cultural norms for developing software; these norms became as important and influential as the technology of Unix itself, and have been termed the "Unix philosophy." The Unix philosophy emphasizes building simple, compact, clear, modular, and extensible code that can be easily maintained and repurposed by developers other than its creators. The Unix philosophy favors composability as opposed to monolithic design. Origin The Unix philosophy is documented by Doug McIlroy in the Bell System Tec ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Everything Is A File
Everything is a file is an idea that Unix, and its derivatives handle input/output to and from resources such as documents, hard-drives, modems, keyboards, printers and even some inter-process and network communications as simple streams of bytes exposed through the filesystem name space. Exceptions include shared memory, semaphores, datagram sockets, symbolic links, directories (which are read-only are not accessed as a byte-stream), processes and threads. The advantage of this approach is that the same set of tools, utilities and APIs can be used on a wide range of resources. There are a number of file types. When a file is opened, a file descriptor is created. The file path becomes the addressing system and the file descriptor becomes the byte stream I/O interface. But file descriptors are also created for things like anonymous pipes and network sockets. Therefore, a more accurate description of this feature is ''Everything is a file descriptor''. Additionally, a range of ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
BlackBerry
The blackberry is an edible fruit produced by many species in the genus ''Rubus'' in the family Rosaceae, hybrids among these species within the subgenus ''Rubus'', and hybrids between the subgenera ''Rubus'' and ''Idaeobatus''. The taxonomy of blackberries has historically been confused because of hybridization and apomixis, so that species have often been grouped together and called species aggregates. For example, the entire subgenus ''Rubus'' has been called the '' Rubus fruticosus'' aggregate, although the species ''R. fruticosus'' is considered a synonym of '' R. plicatus''. '' Rubus armeniacus'' ("Himalayan" blackberry) is considered a noxious weed and invasive species in many regions of the Pacific Northwest of Canada and the United States, where it grows out of control in urban and suburban parks and woodlands. Description What distinguishes the blackberry from its raspberry relatives is whether or not the torus ( receptacle or stem) "picks with" (i.e., stays w ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
