Hard disk

A hard disk drive in computing is a type of data storage device made up of hard disk platters, a spindle, read and write heads, read and write arms, electrical motors, and integrated electronics contained inside an airtight enclosure.

Table of contents

1 Hard vs. floppy
2 Hard disk hardware
3 Access and interfaces
4 Performance
5 Operating system use of hard disks
6 Manufacturers
7 Hard Disk Usage
8 Further reading

Hard vs. floppy

Using rigid platters and sealing the unit allows much tighter tolerances than in a floppy disk. Consequently, hard disks can store much more data than floppy disk, and access and transmit it faster. In 2003, a typical workstation hard disk might store between 60 GB and 120 GB of data, rotate at 5400 to 10,000 rpm (revolutions per minute), and have an average transfer rate of about 30 MByte/sec. The fastest workstation hard drives spin at 15,000 rpm. Notebook hard drives are generally smaller and slower than their desktop counterparts, they spin usually at 4200 rpm.

Hard disk hardware

Typical hard drives of the mid-1990s.
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The disk drive is a type of disk storage that stores and retrieves digital data from a planar magnetic surface. Information is written to the disk by transmitting an electromagnetic flux through an antenna or write head that is very close to a magnetically polarizable material that changes its polarization due to the flux. The information can be read back in a reverse manner, as the magnetic fields cause electrical change in the coil or read head that passes over it.

A typical hard disk drive design consists of a central axis or spindle upon which the platters spin at a constant speed. Moving along and between the platters on a common armature are the read-write heads, with one head for each platter face. The armature moves the heads radially across the platters as they spin, allowing each head access to the entirety of its platter.

The integrated electronics control the movement of the read-write armature and the rotation of the disk, and perform reads and writes upon demand from the disk controller. Some modern drive electronics are capable of scheduling reads and writes efficiently across the disk, and of remapping sectors of the disk which have failed.

The sealed enclosure protects the drive internals from dust, condensation, and other sources of contamination. Any contamination of the read-write heads or disk platters can lead to a head crash—a failure of the disk in which the head scrapes across the platter surface, grinding away the thin magnetic film. Head crashes can also be caused by electronic failure, wear and tear, or poorly manufactured disks.

Access and interfaces

A hard disk is generally accessed over one of a number of bus types, including ATA (IDE, EIDE), SCSI, FireWire/IEEE 1394, and Fibre Channel. From late 2002 Serial ATA was introduced to improve upon the performance of ATA.

Performance

There are three primary factors that determine hard drive performance: seek time, latency and data transfer rate, plus several subsiduary factors:

Seek time is a measure of the speed with which the drive can position its read/write heads over any particular data track. Because neither the starting position of the head nor the distance from there to the desired track is fixed, seek time varies greatly, and it is almost always measured as an average seek time, though full-track (the longest possible) and track-to-track (the shortest possible) seeks are also quoted sometimes. The standard way to measure seek time is to time a large number of disk accesses to random locations, subtract the latency (see below) and take the mean. Note, however, that two different drives with identical average seek times can display quite different performance characteristics. Seek time is always measured in milliseconds (ms), and often regarded as the single most important determinant of drive performance, though this claim is hotly debated. (More on seek time.)
All drives have rotational latency: the time that elapses between the moment when the read/write head settles over the desired data track and the moment when the first byte of the required data appeares under the head. For any individual read or write operation, latency is random between zero (if the first data sector happpens to be directly under the head at the exact moment that the head is ready to begin reading or writing) or anything up to the full rotational period of the drive (for a typical 7200 RPM drive, just under 8.4ms). However, on average, latency is always equal to one half of the rotational period. Thus, all 5400 RPM drives of any make or model have 5.56ms latency; all 7200 RPM drives, 4.17ms; and all 15,000 RPM drives have 2.0ms latency. Like seek time, latency is considered a critical performance factor and is always measured in milliseconds. (More on latency.)
Access time is simply the sum of the seek time and the latency. It is important not to mistake seek time figures for access time figures!
The internal data rate is the speed with which the drive's internal read channel can transfer data from the magnetic media. (Or, less commonly, in the reverse direction.) Previously a very important factor in drive performance, it remains significant but less so than in prior years, as all modern drives have very high internal data rates. Internal data rates are normally measured in Megabits per second (Mb/s—note the lower-case "b".)
The external data rate is the speed with which the drive can transfer data from its buffer to the host computer system. Although in theory this is vital, in practice it is usually a non-issue. It is a relatively trivial matter to design an electronic interface capable of outpacing any possible mechanical read/write mechanism, and it is routine for computer makers to include a hard drive controller interface that is significantly faster than the drive it will be attached to. As a general rule, modern ATA and SCSI interfaces are capable of dealing with at least twice as much data as any single drive can deliver; they are, after all, designed to handle two or more drives per bus even though a consumer desktop computer almost always mounts only one. For a single-drive computer, the difference between ATA-100 and ATA-133, for example, is primarily one of marketing rather than performance. No drive yet manufactured can utilise the full bandwidth of an ATA-100 interface, and few are able to send more data than an ATA-66 interface can accept. The external data rate is usually measured in Megabytes per second. (MB/s—note the upper-case "B".)
Command overhead is the time it takes the drive electronics to interpret instructions from the host computer and issue commands to the read/write mechanism. In modern drives it is negligible.

Operating system use of hard disks

hard disk drive partitioning, master boot record, drive letter assignment

addressing modes

There are two modes of addressing the data blocks on hard disks. The older one is the CHS addressing (Cylinder-Head-Sector), used on old ST-506 and ATA drives and internally by the PC BIOS, and the more recent one the LBA (Logical Block Addressing), used by SCSI drives and newer ATA drives (ATA drives power up in CHS mode for historical reasons).

CHS describes the disk space in terms of its physical dimensions, data-wise; this is the traditional way of accessing a disk on IBM PC compatible hardware, and while it works well for floppies (for which it was originally designed) and small hard disks, it caused problems when disks started to exceed certain limits of the PC's CHS implementation. The original CHS limit was 1024 cylinders, 16 heads and 63 sectors; on a drive with 512-byte sectors, this comes to 504 megabytes. This comes from a combination of the limitations of IBM's BIOS interface (which allowed 1024 cylinders, 256 heads and 64 sectors; sectors were counted from 1, reducing that number to 63, giving an addressing limit of 8064 megabytes or just under 8GB), and a hardware limitation of the AT's hard disk controller (which allowed up to 65536 cylinders and 256 sectors, but only 16 heads, putting its addressing limit at 2^24 bits or 128GB).

When drives larger than 504MB began to appear in the mid-1990s, many system BIOSes had problems communicating with them, requiring BIOS upgrades or special driver software to work correctly; this limitation appeared again at the 8 GB mark as drives increased in size in the late 1990s. This varied in seriousness from PC to PC; some PCs would just ignore the extra capacity, whereas some would refuse to boot, complaining of a hard disk error.

SCSI drives, however, have always used LBA addressing, which describes the disk as a linear, sequentially-numbered set of blocks. SCSI mode page commands can be used to get the physical specifications of the disk, but this is not used to read or write data; this is an artifact of the early days of SCSI, circa 1986, when a disk attached to a SCSI bus could just as well be an ST-506 or ESDI drive attached through a bridge (and therefore having a CHS configuration that was subject to change) as it could a native SCSI device. Because PCs use CHS addressing internally, the BIOS code on PC SCSI host adapters does CHS-to-LBA translation, and provides a set of CHS drive parameters that tries to match the total number of LBA blocks as closely as possible.

ATA drives can either use their native CHS parameters (mainly on early drives; most hard drives made since the early 1990s use multiple-zone recording, and thus don't have a set number of sectors per track), use a "translated" CHS profile (similar to what SCSI host adapters provide), or run in ATA LBA mode, as specified by ATA-2. To maintain some degree of compatibility with older computers, LBA mode generally has to be requested explicitly by the host computer. ATA drives larger than 8 GB are always accessed by LBA, due to the 8GB limit described above.

Manufacturers

Most of the world's hard disks are manufactured by a handful of firms: Seagate, Maxtor, Western Digital, Samsung, and the former drive manufacturing division of IBM, now sold to Hitachi. Fujitsu continue to make specialist SCSI drives but exited the mass market in 2001. Toshiba is a major maker of 2.5 inch notebook drives.

Dozens of former hard drive manufacturers have gone out of business, merged, or closed their hard drive divisions, notably Conner (merged with Seagate in 1996), Quantum (now a tape drive specialist with the hard drive division sold to Maxtor), Micropolis (sold to Singapore Technologies, who eventually wound it down), JTS (went bankrupt in early 1999), and Miniscribe (who went bankrupt in 1990 after cooking their books; they were eventually purchased by Maxtor).

Hard Disk Usage

From the original use of a hard drive in a single computer, techniques for guarding against hard disk failure were developed such as the redundant array of independent disks (RAID). Hard disks are also found in network attached storage devices, but for large volumes of data are most efficiently used in a Storage Area Network.