A hard disk drive (HDD) uses magnetic recording platters coated with magnetic material for data storage. Data is written to and read from quickly rotating platters using read/write heads that float just above the platters on an air bearing. The platters and read/write heads are enclosed in an airtight enclosure.
Quick Answers
Here are quick answers to some common questions about what hard disk drives use for data storage:
– Hard disk drives use magnetic recording platters coated with a thin magnetic film for data storage.
– Data is stored and retrieved using read/write heads that hover over the spinning platters.
– The platters, heads, and other components are sealed in an airtight enclosure to prevent contamination.
– Changes in magnetic polarity on the platter surfaces represent binary 1s and 0s for storing data.
– The platters are made from non-magnetic materials like aluminum or glass and are coated on both sides.
– Common magnetic coatings include iron oxide, cobalt-alloy, and thin film metals like cobalt-chrome and cobalt-platinum.
– Platters spin at speeds typically between 5,400 rpm and 15,000 rpm during drive operation.
– Higher platter rotational speeds allow for faster data access but use more power.
How Hard Drives Store Data
Hard disk drives rely on the ability to change and detect the polarity of magnetic fields on the platter surfaces. By controlling the magnetic field at microscopic areas on the platters, binary 1s and 0s can be recorded. The read/write heads contain electromagnetic coils that produce changing magnetic fields to polarize small regions of the disk surface.
To write data, the disk controller sends electrical pulses to the read/write head to induce a pattern of magnetic polarity changes in the magnetic platter coating. The magnetic material retains the applied charge even after the field is removed. These residual magnetic fields representing 1s and 0s can be detected during reads back from the platters.
Typical HDD platters are made of glass or aluminum and are coated on both sides with a thin magnetic layer only a few nanometers thick. Common magnetic coatings include iron oxide, cobalt-alloy, and thin film metals like cobalt-chrome and cobalt-platinum. More advanced materials allow for smaller magnetic regions to support increased data density.
The platters are precisely mounted and balanced on a spindle motor that rotates the disks at high speeds during operation. Common spindle motor speeds range between 5,400 rpm for consumer HDDs up to 15,000 rpm for high-performance enterprise drives. Faster platter rotation reduces latency when accessing data from different locations on the platters.
Read/Write Heads
While the magnetic platters provide long-term data storage, the read/write heads allow data to be accessed by changing or sensing the magnetic fields on the platters. The heads are affixed on sliders attached to actuator arms driven by a voice coil motor to position the heads over the desired track radius.
Modern read/write heads use separate elements for reading and writing data. The write element induces magnetic polarity changes by generating a local magnetic field from an electrical coil. The read element uses a magnetoresistive material to detect changes in magnetic orientation from the platter surface. Materials like giant magnetoresistance (GMR) and tunneling magnetoresistance (TMR) are commonly used.
The read/write heads float extremely close above the rotating platters on an air bearing surface just nanometers above the magnetic coating. Closer spacing allows for increased data density. Special head sliders are designed to maintain the precise tiny distance across variable conditions and platter speeds.
Servos and Actuators
To accurately position the read/write heads over the desired track and sector locations, HDDs use servo control systems. Embedded servo patterns are written onto disk surfaces that provide feedback for accurately hitting narrow tracks. Feedback from the servo patterns allow for precisely controlling the voice coil actuator arm.
The actuator arm holds the read/write head sliders and pivots using a rotary voice coil actuator to swing the arms across the platters. Strong permanent magnets interact with the fixed voice coils to quickly and accurately move the arms. This provides fast access times by being able to swiftly move the heads across the platters.
Platter Enclosure
The platters, spindle motor, read/write heads, actuator arms, and other components are enclosed in a precisely manufactured HDD case that seals the interior environment. The enclosure provides an airtight seal filled with filtered air or helium gas to prevent contamination or damage to the platters and heads from dust particles.
The cover, baseplate, spindle motor, and internal frame provide rigid support to prevent platters from vibrating or flexing during operation. Vibration can interfere with proper head positioning and data access reliability. Some drives use breather filters to equalize interior pressure changes due to temperature fluctuations.
Managing Data Storage
In addition to the physical components that store and access the magnetic data, HDDs contain onboard controllers and firmware that manage how data is organized, stored, cached, and retrieved from the platters. These include:
Disk Controller
The onboard disk controller is a circuit board with the electronics needed to manage the HDD. It provides the processors, memory, interfaces, and firmware that perform critical drive operations. This includes spindle motor control, head position control, data reads/writes, caching, error detection/correction, defect management, diagnostics, data interfaces, and more.
Logical Block Addressing
Data is stored in fixed-size blocks of 512 bytes or 4,096 bytes on the physical sectors of each platter. But the operating system addresses data using logical block addressing (LBA) to simplify linear addressing and data management. The drive maps LBAs to physical sectors and handles this conversion internally.
Caching and Buffering
For faster data access, HDD controllers use RAM caches and read/write buffers to temporarily store frequently accessed data. Cache hit rates help minimize the latency of physical platter accesses. Larger caches provide performance benefits for repetitive reads and writes.
Error Detection and Correction
Data integrity is vital for HDD reliability. Drives use cyclic redundancy checks (CRC) and error correcting codes (ECC) to detect and repair corrupt data bits. CRC detects common errors while more advanced ECC can fix a certain number of faulty bits without needing a re-write.
Defect Management
HDD firmware maps out any bad sectors or tracks that develop physical defects over time. Reserves of spare sectors are set aside to replace damaged areas transparently so operating systems are not impacted by rewritten sectors.
Host Interfaces
HDDs use standard interfaces to connect to computers and other devices. Common interfaces include SATA, SAS, USB, Fibre Channel, and Ethernet. Faster interface speeds reduce latency for accessing and transferring data to and from the platters.
Evolution of Hard Drive Data Density
Engineers have steadily improved the areal data density of HDDs over decades, resulting in massive increases in cost-effective storage capacity. Some key innovations that boosted density include:
Magnetic Coatings
Advances in magnetic particle materials like barium ferrite in the 1950s increased linear bit density. Thin film coatings later enabled further gains in density.
MR Head
Magnetoresistive (MR) read heads based on variable resistivity significantly improved linear density versus earlier inductive heads. Later GMR and TMR heads provided additional density gains through greater magnetic sensitivity.
PRML and ECC
Incorporating partial-response maximum-likelihood (PRML) decoding and strong error correction codes (ECC) enabled more dense data encoding along with error detection and correction.
Multi-Platter Designs
Increasing the number of platters and heads per drive effectively multiplied total data capacity and density.
Areal Density Jumping
New technologies steadily improved both linear density of tracks as well as track density of concentric data tracks on platters, resulting in density doubling every few years.
Perpendicular Recording
Shifting from longitudinal to perpendicular magnetic recording in 2005 enabled further large leaps in data density.
Shingled Magnetic Recording
Overlapping shingled write tracks like roof shingles boosted track density at the cost of rewrites.
Helium-Filled Drives
Using lighter helium instead of air in HDD enclosures reduced turbulence to allow closer head positioning and higher platter densities.
HAMR and SMR
New technologies like heat-assisted magnetic recording (HAMR) and shingled magnetic recording (SMR) aim to continue pushing conventional platter densities higher.
Together these innovations have enabled HDD storage capacity to scale enormously over the decades. Data center drives now exceed capacities of 10 TB while consumer HDDs provide cost-effective multi-terabyte storage.
Manufacturing Hard Disk Drives
While HDD technology has evolved considerably, hard drive manufacturing retains an intense focus on quality control and precision processes. Main steps in HDD production include:
Sub-Component Manufacturing
Highly specialized suppliers provide the wafers, substrates, magnetoresistive heads, and other advanced sub-components to HDD companies. Tight tolerances and stringent standards for defect rates are enforced.
Media Patterning
Servo patterns are precisely written onto the media during manufacturing using specialized servo writers. These provide embedded position information for accurate head positioning.
Head-Disk Integration
The read/write heads are integrated with the actuator arms and positioned over the platters with microscopic tolerances. Modern HDI enables spacings under 10 nanometers.
Drive Assembly
The HDI, platters, spindle motor, and onboard electronics are integrated into a sealed enclosure with meticulous clean room assembly procedures.
Component Burn-In
Individual drives undergo extended burn-in testing and calibration to ensure reliable operation over a long lifetime.
Life Testing
Statistical life testing under a range of temperatures, vibrations, and workloads ensures drive reliability targets are achievable.
QA Inspections
At various manufacturing stages, components and assembled drives undergo inspections and testing to meet stringent quality standards. Defect limits are tightly controlled.
Volume Manufacturing
Highly refined and automated manufacturing with rigorous quality standards allow economies of scale for mass production of drives with small form factors.
HDD Interface Standards
Hard drives use standard connector interfaces that enable them to be integrated into computers and storage systems. Here are common industry interface standards used with modern HDDs:
SATA
Serial ATA is the pervasive interface used in desktop PCs, notebooks, gaming systems, DAS, and NAS. SATA has gone through several revisions with transfer speeds increasing from 1.5 Gbit/s to 16 Gbit/s for SATA 3.2.
SAS
Serial attached SCSI is a high-speed enterprise and data center HDD interface designed for performance and reliability. SAS speeds have increased from 3 Gbit/s to 22.5 Gbit/s for SAS-4 supporting up to 65,535 devices.
PCIe
NVMe SSDs are displacing HDDs in high-performance applications, but NVMe over PCI Express is also an emerging HDD interface with very low latency and high speeds. NVM Express enables PCIe HDDs for all-flash data centers.
USB
External portable and removable HDDs utilize USB interfaces. USB 3.2 Gen 2×2 provides 20 Gbit/s transfer rates. Thunderbolt 3 also integrates with USB Type-C connectors.
Fibre Channel
Fibre Channel is a key HDD interface for enterprise storage area networks. 16 and 32 Gbit/s FC speeds enable SAN architects to design high-capacity storage solutions.
Ethernet
Some data center HDDs support Ethernet interfaces using iSCSI, FCoE, or FC-NVMe to provide shared network access to drives. Fast Ethernet and 100GbE speeds help minimize latency.
Comparing HDDs vs. SSDs
Hard disk drives share the data storage market with SSDs based on flash memory chips and RAM-based drives. HDDs excel for bulk data due to low cost per GB, but SSDs are superior for performance-sensitive uses. Key differences:
Price per Capacity
HDDs are far less expensive in terms of dollars per gigabyte, with a large gap versus SSDs. HDDs are optimal for high capacity bulk storage needs.
Performance
SSDs have much faster data access due to no moving parts, allowing higher IOPS and low latency. NVMe SSDs are up to 16x faster than HDDs for I/O intensive workloads.
Shock and Vibration
With no moving parts, SSDs are more tolerant to shocks and vibration versus HDDs with sensitive spinning platters.
Power Efficiency
SSDs consume less active power than HDDs. But HDDs can use very little power when idle or spun-down. Hybrid drives combine benefits.
Reliability and Lifetime
SSD wear-out depends on writes. HDD reliability declines over time. SSD lifespans tend to be shorter than HDDs currently.
Noise
With no moving parts, SSDs are silent while HDD platters and motors create audible noise during activity.
| Specification | Hard Disk Drive (HDD) | Solid State Drive (SSD) |
|---|---|---|
| Price per capacity | Very low, bulk storage | Higher than HDDs |
| Max performance (IOPS, Throughput) | Higher latency, 100s of IOPS | Very high IOPS, 1000s to 100,000s, low latency |
| Shock and vibration tolerance | Sensitive to impact and vibration | No moving parts, high tolerance |
| Power efficiency | Higher active power, low idle power | Lower active power usage |
| Reliability and lifetime | Gradual decline in reliability | Limited write cycles, component failures |
| Noise | Audible platter and seek noise | Silent operation |
Conclusion
In summary, hard disk drives provide a unique combination of very low cost per gigabyte and high capacity storage capabilities that make them well-suited for bulk data storage needs. The spinning magnetic platters, precisely positioned read/write heads, and sealed enclosures have been refined over decades to deliver continually increasing areal densities at affordable prices.
While SSDs outshine HDDs for performance-sensitive workloads requiring fast random I/O, the strength of hard drives remains serving massive data at the lowest possible cost. HDD technology continues evolving with new recording methods and materials to stay ahead as the world’s insatiable data storage needs keep growing.