Hard Drives 101 Hugh Stroupe December 12, 2013

What is a Hard Drive? A hard disk drive (HDD)is a data storage device used for storing and retrieving digital information using rapidly rotating disks (platters) coated with magnetic material An HDD retains its data even when powered off. Data is read in a random-access manner, meaning individual blocks of data can be stored or retrieved in any order rather than sequentially. An HDD consists of one or more rigid ("hard") rapidly rotating disks (platters) with magnetic heads arranged on a moving actuator arm to read and write data to the surfaces.

History The first IBM drive, the 350 RAMAC developed in 1956, was approximately the size of two refrigerators and stored 5 million 6-bit characters (the equivalent of 3.75 million 8-bit bytes or 3.75 MB or megabytes) on a stack of Fifty 24 inch disks. In 1961 IBM introduced the model 1311 disk drive, which was about the size of a washing machine and stored two million characters on a removable disk pack.

History Some high performance HDDs were manufactured with one head per track, e.g., IBM 2305 so that no time was lost physically moving the heads to a track. Known as Fixed-Head or Head-Per- Track disk drives they were very expensive and are no longer in production. In 1973, IBM introduced a new type of HDD codenamed "Winchester". Its primary distinguishing feature was that the disk heads were not withdrawn completely from the stack of disk platters when the drive was powered down. Instead, the heads were allowed to "land" on a special area of the disk surface upon spin-down, "taking off" again when the disk was later powered on

Let’s Look at the Internals

Make up of the Platter

Technology A HDD records data by magnetizing a thin film of ferromagnetic material on a disk. Sequential changes in the direction of magnetization represent binary data bits. The data is read from the disk by detecting the transitions in magnetization. User data is encoded using an encoding scheme, such as run-length limited encoding, which determines how the data is represented by the magnetic transitions.

Technology Due to the extremely close spacing between the heads and the disk surface, HDDs are vulnerable to being damaged by a head crash. The HDD's spindle system relies on air density inside the disk enclosure to support the heads at their proper flying height while the disk rotates. HDDs require a certain range of air densities in order to operate properly. If the air density is too low, then there is not enough lift for the flying head, and there is a risk of head crashes and data loss.

Size Comparisons

Technology Perpendicular recording can deliver more than three times the storage density of traditional longitudinal recording. Hard disk technology with longitudinal recording has an estimated limit of 100 to 200 gigabit per square inch due to the superparamagnetic effect. Perpendicular recording is predicted to allow information densities of up to around 1 Tbit/sq. inch (1000 Gbit/sq. inch). Perpendicular recording was first used by Toshiba in 3.5" floppy disks to permit 2.88 MB of capacity (ED or extended density), but they failed to succeed in the market place. Since about 2005 the technology has come into use for hard disk drives.

New Technology According to Seagate, its latest 1TB platter 3.5" drives have shrunk read/write heads as small as they can physically go. Similarly, tracks on those platters are placed as close together as physically possible. Seagate's solution is something it calls Shingled Magneting Recording (SMR). The process is pretty simple. Track size is traditionally defined by the size of the write heads, as they are larger than the read heads. The track width is larger than necessary from the perspective of reading data back in order to decrease the chances of reading data from adjacent tracks. Seagate's SMR exploits this reality.

New Technology SMR shrinks the guard space between tracks and allows tracks to overlap one another, like roofing shingles. Although data is written to the entire width of the track, a smaller/trimmed portion of the track (the width of the read head) is all that the drive cares about. By allowing tracks to overlap, areal density can continue to scale without further shrinking the size of the heads. 

Hard Drive Interfaces Modified Frequency Modulation, commonly MFM, is a run-length limited (RLL) coding scheme used to encode the actual data-bits on most floppy disks. It was first introduced in disk drives with the IBM 3330 hard disk drive in 1970. Floppy disk drive hardware examples include Amiga, most CP/M machines as well as IBM PC compatibles. In the ST-506 interface, the drive was connected to a controller card with two cables; a third cable provided power. The drives were "dumb", so-called because the control card translated requests for a particular track and sector from the host system into a sequence of head positioning commands, then read the signal from the drive head and recovered the data from it.

Enhanced Small Disk Interface Enhanced Small Disk Interface (ESDI) was a disk interface designed by Maxtor Corporation in the early 1980s to be a follow-on to the ST-506 interface. ESDI improved on ST-506 by moving certain parts that were traditionally kept on the controller into the drives themselves, and also generalizing the control bus such that more kinds of devices (such as removable disks and tape drives) could be connected. ESDI used the same cabling as ST-506, and thus could easily be retrofitted to ST-506 applications.

SCSI Since its standardization in 1986, SCSI has been commonly used in the Amiga, Apple Macintosh and Sun Microsystems computer lines and PC server systems. SCSI is available in a variety of interfaces. The first, still very common, was parallel SCSI (now also called SPI), which uses a parallel bus design. Serial Storage Architecture (SSA), SCSI-over-Fibre Channel Protocol (FCP), Serial Attached SCSI (SAS), Automation/Drive Interface − Transport Protocol (ADT), and USB Attached SCSI (UAS) – break from the traditional parallel SCSI standards and perform data transfer via serial communications. iSCSI preserves the basic SCSI paradigm, especially the command set, almost unchanged, through embedding of SCSI-3 over TCP/IP.

SCSI Connectors

Parallel ATA Parallel ATA (PATA), originally AT Attachment, is an interface standard for the connection of storage devices such as hard disks, floppy drives, and optical disc drives in computers. It uses the underlying AT Attachment (ATA) and AT Attachment Packet Interface (ATAPI) standards.

Fibre Channel Fibre Channel (FC) is a successor to parallel SCSI interface on enterprise market. It is a serial protocol. In disk drives usually the Fibre Channel Arbitrated Loop (FC-AL) connection topology is used. FC has much broader usage than mere disk interfaces, and it is the cornerstone of storage area networks (SANs). Recently other protocols for this field, like iSCSI and ATA over Ethernet have been developed as well. Confusingly, drives usually use copper twisted-pair cables for Fibre Channel, not fibre optics. The latter are traditionally reserved for larger devices, such as servers or disk array controllers.

SATA Serial ATA (SATA). The SATA data cable has one data pair for differential transmission of data to the device, and one pair for differential receiving from the device, just like EIA-422. That requires that data be transmitted serially. A similar differential signaling system is used in RS485, LocalTalk, USB, Firewire, and differential SCSI. SATA-1 Revision interface provides a 150 Mbps connection. SATA-2Revision interface provides a 300 Mbps connection. SATA-3 Revision interface provides a 600 Mbps connection. Also can be externally connected as an eSATA.

SAS Serial Attached SCSI (SAS). The SAS is a new generation serial communication protocol for devices designed to allow for much higher speed data transfers and is compatible with SATA. SAS uses a mechanically identical data and power connector to standard 3.5-inch SATA1/SATA2 HDDs, and many server-oriented SAS RAID controllers are also capable of addressing SATA hard drives. SAS uses serial communication instead of the parallel method found in traditional SCSI devices but still uses SCSI commands.

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