Raw device

Last updated

In computing, specifically in Unix and Unix-like operating systems, a raw device is a special kind of logical device associated with a character device file that allows a storage device such as a hard disk drive to be accessed directly, bypassing the operating system's caches and buffers (although the hardware caches might still be used). Applications like a database management system can use raw devices directly, enabling them to manage how data is cached, rather than deferring this task to the operating system.

In FreeBSD, all device files are in fact raw devices. Support for non-raw devices was removed in FreeBSD 4.0 in order to simplify buffer management and increase scalability and performance. [1]

In Linux kernel, raw devices were deprecated and scheduled for removal at one point, because the O_DIRECT flag can be used instead. [2] However, later the decision was made to keep raw devices support since some software cannot use the O_DIRECT flag. [3] Raw devices simply open block devices as if the O_DIRECT flag would have been specified. Raw devices are character devices (major number 162). The first minor number (i.e. 0) is reserved as a control interface and is usually found at /dev/rawctl. A command-line utility called raw [4] can be used to bind a raw device to an existing block device. These "existing block devices" may be disks or CD-ROMs/DVDs whose underlying interface can be anything supported by the Linux kernel (for example, IDE/ATA or SCSI). [5]

Related Research Articles

In computing, a device driver is a computer program that operates or controls a particular type of device that is attached to a computer or automaton. A driver provides a software interface to hardware devices, enabling operating systems and other computer programs to access hardware functions without needing to know precise details about the hardware being used.

XFS is a high-performance 64-bit journaling file system created by Silicon Graphics, Inc (SGI) in 1993. It was the default file system in SGI's IRIX operating system starting with its version 5.3. XFS was ported to the Linux kernel in 2001; as of June 2014, XFS is supported by most Linux distributions, Red Hat Enterprise Linux uses it as default filesystem.

A disk array controller is a device that manages the physical disk drives and presents them to the computer as logical units. It almost always implements hardware RAID, thus it is sometimes referred to as RAID controller. It also often provides additional disk cache.

XNU

XNU is the computer operating system (OS) kernel developed at Apple Inc. since December 1996 for use in the Mac OS X operating system and released as free and open-source software as part of the Darwin OS, which is the basis for the Apple TV Software, iOS, iPadOS, watchOS, and tvOS OSes. XNU is an abbreviation of X is Not Unix.

Native Command Queuing

In computing, Native Command Queuing (NCQ) is an extension of the Serial ATA protocol allowing hard disk drives to internally optimize the order in which received read and write commands are executed. This can reduce the amount of unnecessary drive head movement, resulting in increased performance for workloads where multiple simultaneous read/write requests are outstanding, most often occurring in server-type applications.

USB mass storage device class

The USB mass storage device class is a set of computing communications protocols, specifically a USB Device Class, defined by the USB Implementers Forum that makes a USB device accessible to a host computing device and enables file transfers between the host and the USB device. To a host, the USB device acts as an external hard drive; the protocol set interfaces with a number of storage devices.

The proc filesystem (procfs) is a special filesystem in Unix-like operating systems that presents information about processes and other system information in a hierarchical file-like structure, providing a more convenient and standardized method for dynamically accessing process data held in the kernel than traditional tracing methods or direct access to kernel memory. Typically, it is mapped to a mount point named /proc at boot time. The proc file system acts as an interface to internal data structures about running processes in the kernel. In Linux, it can also be used to obtain information about the kernel and to change certain kernel parameters at runtime (sysctl).

The device mapper is a framework provided by the Linux kernel for mapping physical block devices onto higher-level virtual block devices. It forms the foundation of the logical volume manager (LVM), software RAIDs and dm-crypt disk encryption, and offers additional features such as file system snapshots.

In computing, ioctl is a system call for device-specific input/output operations and other operations which cannot be expressed by regular system calls. It takes a parameter specifying a request code; the effect of a call depends completely on the request code. Request codes are often device-specific. For instance, a CD-ROM device driver which can instruct a physical device to eject a disc would provide an ioctl request code to do so. Device-independent request codes are sometimes used to give userspace access to kernel functions which are only used by core system software or still under development.

sync is a standard system call in the Unix operating system, which commits all data in the kernel filesystem to non-volatile storage buffers, i.e., data which has been scheduled for writing via low-level I/O system calls. Higher-level I/O layers such as stdio may maintain separate buffers of their own.

The following tables compare general and technical information for a number of file systems.

TrueNAS is the branding for a range of free and open-source network-attached storage (NAS) operating systems produced by iXsystems, and based on FreeBSD and Linux, using the OpenZFS file system. It is licensed under the terms of the BSD License and runs on commodity x86-64 hardware.

HAL is a software subsystem for UNIX-like operating systems providing hardware abstraction.

Kernel-based Virtual Machine Virtualization module in the Linux kernel

Kernel-based Virtual Machine (KVM) is a virtualization module in the Linux kernel that allows the kernel to function as a hypervisor. It was merged into the mainline Linux kernel in version 2.6.20, which was released on February 5, 2007. KVM requires a processor with hardware virtualization extensions, such as Intel VT or AMD-V. KVM has also been ported to other operating systems such as FreeBSD and illumos in the form of loadable kernel modules.

Disk buffer

In computer storage, disk buffer is the embedded memory in a hard disk drive (HDD) acting as a buffer between the rest of the computer and the physical hard disk platter that is used for storage. Modern hard disk drives come with 8 to 256 MiB of such memory, and solid-state drives come with up to 4 GB of cache memory.

The Berkeley Packet Filter (BPF) is a technology used in certain computer operating systems for programs that need to, among other things, analyze network traffic. It provides a raw interface to data link layers, permitting raw link-layer packets to be sent and received. BPF is available on most Unix-like operating systems and eBPF for Linux and for Microsoft Windows. In addition, if the driver for the network interface supports promiscuous mode, it allows the interface to be put into that mode so that all packets on the network can be received, even those destined to other hosts.

Network block device

On Linux, network block device (NBD) is a network protocol that can be used to forward a block device from one machine to a second machine. As an example, a local machine can access a hard disk drive that is attached to another computer.

In Unix-like operating systems, a device file or special file is an interface to a device driver that appears in a file system as if it were an ordinary file. There are also special files in DOS, OS/2, and Windows. These special files allow an application program to interact with a device by using its device driver via standard input/output system calls. Using standard system calls simplifies many programming tasks, and leads to consistent user-space I/O mechanisms regardless of device features and functions.

The bio(4) pseudo-device driver and the bioctl(8) utility implement a generic RAID volume management interface in OpenBSD and NetBSD. The idea behind this software is similar to ifconfig, where a single utility from the operating system can be used to control any RAID controller using a generic interface, instead of having to rely on many proprietary and custom RAID management utilities specific for each given hardware RAID manufacturer. Features include monitoring of the health status of the arrays, controlling identification through blinking the LEDs and managing of sound alarms, and specifying hot spare disks. Additionally, the softraid configuration in OpenBSD is delegated to bioctl as well; whereas the initial creation of volumes and configuration of hardware RAID is left to card BIOS as non-essential after the operating system has already been booted. Interfacing between the kernel and userland is performed through the ioctl system call through the /dev/bio pseudo-device.

References

  1. ↑ "FreeBSD Architecture Handbook: 9.4. Block Devices (Are Gone)" . Retrieved 2017-06-29.
  2. ↑ Day, Robert P. J. (2007-02-16). "[PATCH] Remove obsolete raw device support" . Retrieved 2017-06-29.
  3. ↑ Jones, Dave (2007-05-13). "undeprecate raw driver" . Retrieved 2017-06-29.
  4. ↑ "util-linux: raw(8)". August 1999. Retrieved 2017-06-29.
  5. ↑ "The Linux 2.4 SCSI subsystem HOWTO: Chapter 11. Raw devices". Linux Documentation Project . 2004-09-13. Retrieved 2017-06-29.


This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.