16.4: Disk Drives in the OS

Hard Disks

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 and 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 even when powered off.

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 on flash memory storage devices. More than 224 companies have produced HDDs historically, though after extensive industry consolidation most units are manufactured by Seagate, Toshiba, and Western Digital. HDDs dominate the volume of storage produced (exabytes per year) for servers. Though production is growing slowly (by exabytes shipped), sales revenues and unit shipments are declining because solid-state drives (SSDs) have higher data-transfer rates, higher areal storage density, better reliability, and much lower latency and access times.

Performance

The are several factors which impact the performance of a disk drive. A drive's performance has an impact on the over all performance of the operating system, since much of the computer's processing does interface with the disk drive. The factors that limit the time to access the data on an HDD are mostly related to the mechanical nature of the rotating disks and moving heads. 

Access Time

The access time or response time of a rotating drive is a measure of the time it takes before the drive can actually transfer data. The factors that control this time on a rotating drive are mostly related to the mechanical nature of the rotating disks and moving heads. It is composed of a few independently measurable elements that are added together to get a single value when evaluating the performance of a storage device. The access time can vary significantly, so it is typically provided by manufacturers or measured in benchmarks as an average.

The key components that are typically added together to obtain the access time are:

• Seek time
• Rotational latency
• Command processing time
• Settle time

Seek time

With rotating drives, the seek time measures the time it takes the head assembly on the actuator arm to travel to the track of the disk where the data will be read or written. The data on the media is stored in sectors which are arranged in parallel circular tracks (concentric or spiral depending upon the device type) and there is an actuator with an arm that suspends a head that can transfer data with that media. When the drive needs to read or write a certain sector it determines in which track the sector is located. It then uses the actuator to move the head to that particular track. If the initial location of the head was the desired track then the seek time would be zero. If the initial track was the outermost edge of the media and the desired track was at the innermost edge then the seek time would be the maximum for that drive. Seek times are not linear compared with the seek distance traveled because of factors of acceleration and deceleration of the actuator arm.

Rotational Latency

Rotational latency (sometimes called rotational delay or just latency) is the delay waiting for the rotation of the disk to bring the required disk sector under the read-write head. It depends on the rotational speed of a disk (or spindle motor), measured in revolutions per minute (RPM). For most magnetic media-based drives, the average rotational latency is typically based on the empirical relation that the average latency in milliseconds for such a drive is one-half the rotational period. Maximum rotational latency is the time it takes to do a full rotation excluding any spin-up time (as the relevant part of the disk may have just passed the head when the request arrived)

Other

The command processing time or command overhead is the time it takes for the drive electronics to set up the necessary communication between the various components in the device so it can read or write the data. This is of the order of 3 μs, very much less than other overhead times, so it is usually ignored when benchmarking hardware.

The settle time is the time it takes the heads to settle on the target track and stop vibrating so they do not read or write off track. This time is usually very small, typically less than 100 μs, and modern HDD manufacturers account for it in their seek time specifications

Data Transfer Rate

The data transfer rate of a drive (also called throughput) covers both the internal rate (moving data between the disk surface and the controller on the drive) and the external rate (moving data between the controller on the drive and the host system). The measurable data transfer rate will be the lower (slower) of the two rates. The sustained data transfer rate or sustained throughput of a drive will be the lower of the sustained internal and sustained external rates. The sustained rate is less than or equal to the maximum or burst rate because it does not have the benefit of any cache or buffer memory in the drive. The internal rate is further determined by the media rate, sector overhead time, head switch time, and cylinder switch time.

Solid State Drives

Over time, the performance gap between the central processing units (CPUs) and electromechanical storage (hard disk drives and their RAID setups) widened, requiring advancements in the secondary storage technology. A solution was found in flash memory, which is an electronic non-volatile computer storage media that can be electrically erased and reprogrammed. Solid-state storage typically uses the NAND type of flash memory, which may be written and read in chunks much smaller than the entire size of the storage device. The size of a minimal chunk (page) for read operations is much smaller than the minimal chunk size (block) for write/erase operations, resulting in an undesirable phenomenon called write amplification that limits the random write performance and write endurance of flash-based solid-state storage devices. Another type of solid-state storage devices uses volatile random-access memory (RAM) combined with a battery that allows the contents of RAM to be preserved for a limited amount of time after the device's power supply is interrupted. As an advantage, RAM-based solid-state storage is much faster compared to flash, and does not experience write amplification.

As a result of having no moving mechanical parts, solid-state storage virtually eliminates the data access latencies present in electromechanical storage devices, and allows significantly higher rates of I/O operations per second (IOPS). Additionally, solid-state storage allows much faster sequential access to stored data, consumes less power, has better physical shock resistance, and produces less heat and no vibrations during operation. As a downside, solid-state storage devices have much higher per-megabyte prices than electromechanical storage devices, and generally come in significantly smaller per-device capacities. Moreover, flash-based devices experience the memory wear that reduces their service life by imposing a limited amount of data that may be written to them, resulting from the limitations of flash memory that impose a finite number of program–erase cycles used to write data. As a result, solid-state storage is frequently used for the creation of hybrid drives, in which solid-state storage serves as a cache for frequently accessed data instead of being a complete substitute for the traditional secondary storage.

Adapted from:
"Hard disk drive" by Multiple Contributors, Wikipedia is licensed under CC BY-SA 3.0
"Hard disk drive performance characteristics" by Multiple Contributors, Wikipedia is licensed under CC BY-SA 3.0

"Solid-state storage" by Multiple Contributors, Wikipedia is licensed under CC BY-SA 3.0