D686: Operating Systems for Computer Scientists (chapter 12)

hard disk drive (HDD)

A secondary storage device based on mechanical components, including spinning magnetic media platters and moving read-write heads.

nonvolatile memory (NVM)

Persistent storage based on circuits and electric charges.

platter

An HDD component that has a magnetic media layer for holding charges.

disk arm

An HDD component that holds the read-write head and moves over cylinders of platters.

track

On an HDD platter, the medium that is under the read-write head during a rotation of the platter.

sectors

On an HDD platter, a fixed-size section of a track.

cylinder

On an HDD, the set of tracks under the read-write heads on all platters in the device.

transfer rate

The rate at which data flows.

positioning time

On an HDD, the time it takes the read-write head to position over the desired track.

seek time

On an HDD, the time it takes the read-write head to position over the desired cylinder.

rotational latency

On an HDD, the time it takes the read-write head, once over the desired cylinder, to access the desired track.

head crash

On an HDD, a mechanical problem involving the read-write head touching a platter.

effective transfer rate

The actual, measured transfer rate of data between two devices (such as a computer and a disk drive).

solid-state disk

A disk-drive-like storage device that uses flash-memory-based nonvolatile memory.

USB drive

Nonvolatile memory in the form of a device that plugs into a USB port.

flash translation layer (FTL)

For nonvolatile memory, a table that tracks currently valid blocks.

garbage collection

In general, recovery of space containing no-longer-valid data.

over-provisioning

In non-volatile memory, space set aside for data writes that is not counted in the device free space.

wear leveling

In nonvolatile memory, the effort to select all NAND cells over time as write targets to avoid premature media failure due to wearing out a subset of cells.

RAM drives

Sections of a system's DRAM presented to the rest of the system as if they were secondary storage devices.

magnetic tape

A magnetic media storage device consisting of magnetic tape spooled on reels and passing over a read-write head. Used mostly for backups.

I/O bus

A physical connection of an I/O device to a computer system.

advanced technology attachment (ATA)

An older-generation I/O bus.

eSATA

A type of I/O bus.

serial-attached SCSI (SAS)

A common type of I/O bus.

universal serial bus (USB)

A type of I/O bus.

fibre channel (FC)

A type of storage I/O bus used in data centers to connect computers to storage arrays. A storage-attachment network.

NVM express (NVMe)

A high-speed I/O bus for NVM storage.

controller

A special processor that manages I/O devices.

host bus adapter (HBA)

A device controller installed in a host bus port to allow connection of one or more devices to the host.

host controller

The I/O-managing processors within a computer (e.g., inside a host bus adapter).

device controller

The I/O managing processor within a device.

logical blocks

Logical addresses used to access blocks on storage devices.

constant linear velocity (CLV)

A device-recording method that keeps a constant density of bits per track by varying the rotational speed of the medium.

constant angular velocity (CAV)

A device-recording method in which the medium spins at a constant velocity and the bit density decreases from inner to outer tracks

solid-state disk (SSD)

a disk-drive-like storage device that uses flash-memory-based nonvolatile memory

USB drive

nonvolatile memory in the form of a device that plugs into a USB port

flash translation layer (FTL)

for nonvolatile memory, a table that tracks currently valid blocks

garbage collection

recovery of space containing no-longer-valid data

over-provisioning

in nonvolatile memory, this is space set aside for data writes that is not counted in the device’s free space

wear-leveling

in nonvolatile memory, the effort to select all NAND cells over time as write targets to avoid premature media failure due to wearing out a subset of cel

RAM drives

sections of a system's dram presented to the rest of the system as if they were secondary storage devices

magnetic tape

a magnetic media storage device consisting of magnetic tape spooled on reels and passing over a read-write head; used mostly for backups

NVM express (NVMe)

a high-speed I/O bus for NVM storage

controller

a special processor that manages I/O devices

logical blocks

 logical addresses used to access blocks on storage devices

constant linear velocity (CLV)

a device-recording method that keeps a constant density of bits per track by varying the rotational speed of the medium

constant angular velocity (CAV)

a device-recording method in which the medium spins at a constant speed and the bit density decreases from inner to outer tracks

bandwidth

The total amount of data transferred divided by the total time between the first request for service and the completion of the last transfer.

SCAN algorithm

An HDD I/O scheduling algorithm in which the disk head moves from one end of the disk to the other performing I/O as the head passes the desired cylinders; the head then reverses direction and repeats.

Circular SCAN (CSCAN) scheduling

An HDD I/O scheduling algorithm in which the disk head moves from one end of the disk to the other performing I/O as the head passes the desired cylinders; the head then reverses direction and continues.

Completely Fair Queuing (CFQ)

In Linux, the default I/O scheduler in kernel 2.6 and later versions.

disk scheduling

the process of managing the order in which I/O requests are served by the hard disk

access time

the total time required to fetch data from the disk, including both seek time and rotational latency

first come, first served (FCFS) scheduling

a disk scheduling algorithm where requests are processed in the order they arrive

shortest seek time first (SSTF) scheduling

a disk scheduling algorithm that selects I/O requests requiring the minimum seek time first

LOOK Scheduling

a variation of SCAN scheduling where the disk head only moves as far as the final request in each direction, then reverses

C-LOOK scheduling

a variation of C-SCAN scheduling where the disk head only serves requests in one direction, similar to LOOK but in a circular manner

starvation

a situation in SSTF scheduling where long-distance requests may suffer delays if nearer requests frequently arrive

parity bit

a bit added to data to help detect errors by ensuring the number of bits set to 1 is even or odd

cyclic redundancy check (CRC)

a method using a hash function to detect multiple-bit errors in data

low-level formatting

The initialization of a storage medium in preparation for its use as a computer storage device.

physical formatting

The initialization of a storage medium in preparation for its use as a computer storage device.

partition

Logical segregation of storage space into multiple area; e.g., on HDDs, creating several groups of contiguous cylinders from the devices' full set of cylinders.

mounting

Making a file system available for use by logically attaching it to the root file system.

volume

A container of storage; frequently, a device containing a mountable file system (including a file containing an image of the contents of a device).

logical formatting

The creation of a file system in a volume to ready it for use.

cluster

In Windows storage, a power-of-2 number of disk sectors collected for I/O optimization.

raw disk

Direct access to a secondary storage device as an array of blocks with no file system.

bootstrap

The set of steps taken at computer power-on to bring the system to full operation.

boot disk

A disk that has a boot partition and a kernel to load to boot the system. A device that has a boot partition and can store an operating system for booting the computer.

system disk

A storage device that has a boot partition and can store an operating system and other information for booting the computer.

boot partition

A storage device partition containing an executable operating system

master boot record (MBR)

Windows boot code, stored in the first sector of a boot partition

boot sector

The first sector of a Windows boot device, containing the bootstrap code.

bad block

An unusable sector on an HDD.

sector sparing

The replacement of an unusable HDD sector with another sector at some other location on the device.

sector slipping

The renaming of sectors to avoid using a bad sector.

host-attached storage

Storage accessed through local I/O ports (directly attached to a computer, rather than across a network or SAN).

fibre channel (FC)

A type of storage I/O bus used in data centers to connect computers to storage arrays. A storage-attachment network.

network-attached storage (NAS)

Storage accessed from a computer over a network.

iSCSI

The protocol used to communicate with SCSI devices; used across a network for more distant access

cloud storage

Storage accessed from a computer over a network to a distant, shared resource data center.

storage-area network (SAN)

A local-area storage network allowing multiple computers to connect to one or more storage devices.

InfiniBand (IB)

A high-speed network communications link