What is HDD and SSD?

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What is HDD and SSD

HDD and SSD are two common types of computer data storage devices. HDD stands for Hard Disk Drive and SSD stands for Solid State Drive. Both serve the same basic function of storing data, but they have some key differences that make each better suited for certain use cases.

What is a Hard Disk Drive (HDD)?

A hard disk drive (HDD) is a traditional storage device that uses magnetic recording to store and retrieve digital data. It consists of one or more rigid platters coated with magnetic material, with an arm that has read/write heads that move across the platters to read or write data.

Data is stored on the platters in concentric tracks divided into sectors. As the platters spin at high speeds, the read/write head can access data on any part of the drive. HDDs use electromagnetism to store data on the platters. The write head changes the magnetic orientation of bits on the platter to either a 1 or 0 to encode data. The read head detects the magnetic orientation of the bits to read the data.

Some key characteristics of HDDs are:

  • High capacity – Modern HDDs can store anywhere from 500GB to 10TB of data or more.
  • Non-volatile storage – Data persists on HDD platters even when the power is off.
  • Lower cost per GB compared to SSD.
  • Moving parts – The use of physical platters and read/write heads makes HDDs more mechanically complex.
  • Slower access times – HDDs have higher latency when accessing random data due to the moving parts.

What is a Solid State Drive (SSD)?

A solid state drive (SSD) is a storage device that uses flash memory chips to store persistent data. Unlike HDDs, SSDs have no moving mechanical components. Instead, SSDs use microchips arranged in a grid that retain data in non-volatile flash memory cells.

The grid architecture provides excellent throughput and access times for random I/O performance. SSDs are typically much faster than HDDs at reading and writing data because they can access any data instantly without needing to physically move a read/write head.

Some key characteristics of SSDs are:

  • Faster access times – SSDs have near instantaneous access times for random I/O performance.
  • Higher cost per GB compared to HDD currently.
  • Limited number of write cycles – Each NAND flash cell can only be overwritten a finite number of times before it fails.
  • Shock and vibration resistant – Having no moving parts makes SSDs more durable.
  • Lower capacity – Maximum capacities on the order of 2-4TB currently.

HDD vs SSD Comparison

Here is a comparison of some key attributes between HDD and SSD drives:

Attribute HDD SSD
Storage medium Magnetic platters NAND flash memory
Capacity 500GB – 10TB+ 128GB – 4TB
Cost per GB Around $0.03 per GB Around $0.20 per GB
File transfer speed Around 100 MB/s Around 550 MB/s
Random access time 2-5 ms average 0.1 ms average
Durability Susceptible to shock damage No moving parts, more durable
Power usage 5-8 watts typical 2-4 watts typical
Operating noise Audible magnetic platters spinning Silent, no moving parts

As the table illustrates, HDDs and SSDs each have pros and cons across different attributes. HDDs offer more storage capacity for less cost, while SSDs provide substantial performance advantages.

How does an HDD work?

Here is a deeper look at the key components and functioning of a hard disk drive:

  • Platters – The disks that data is stored on. Platters are made of non-magnetic material like aluminum or glass and are coated on both sides with a thin magnetic material for storing data. Most HDDs have multiple platters stacked on top of each other in the drive.
  • Spindle – The central axis that platters are mounted on. The spindle rotates the platters at very high speeds up to 15,000 RPM in some drives.
  • Read/Write Heads – Located at the end of the actuator arm, these are electromagnetic devices that read and write data on the platter surfaces.
  • Actuator Arm – Moves the read/write heads across the platters as needed to access data.
  • Voice-coil Motor – Controls the motion of the actuator arm allowing precise positioning of the heads.
  • Firmware – Low-level software that handles I/O requests and controls the hardware.
  • Buffer – Small amount of fast DRAM memory that caches data to smooth out differences between media speeds and interface speeds.
  • Interface – Connects to the computer’s motherboard, usually through SATA but also SAS or USB connections.

In operation, platters spin continuously while the read/write heads move in unison over the surfaces of all the disk surfaces. The actuator arm rapidly repositions the heads to access different tracks and sectors. Data is stored in tracks as magnetized spots on the platters. As platters spin, a head can read these changing magnetic fields to reconstruct the binary 1s and 0s. For writing data, the head generates a strong changing magnetic field that aligns magnetic spots on the platter to record 1s and 0s.

HDD Form Factors

Some common form factors that HDDs come in are:

  • 3.5″ desktop drives – The most common size, used for desktop PCs and DAS (direct attached storage).
  • 2.5″ laptop drives – Smaller drives used in laptops and some external portable HDDs. Can use less power.
  • 1.8″ microdrives – Very small HDDs for mobile devices. Lower capacities.
  • Enterprise drives – Optimized for performance, RAID scalability, and 24/7 operation. Larger capacities.

HDD Interface Types

Some common interfaces HDDs use to connect to computers are:

  • SATA – Serial ATA, the most common HDD interface for both desktop and laptop PCs. SATA-III runs at 6Gbps speeds.
  • SAS – Serial Attached SCSI, used in enterprise servers and storage. Runs at 12Gbps.
  • USB – Universal serial bus, often used for external portable HDDs.

How does an SSD work?

Here is an overview of the internal components and operation of a solid state drive:

  • NAND flash memory – The primary storage media composed of flash memory cells organized in a grid.
  • Controller – Microprocessor that manages all data flow and I/O operations. Also handles error correction and wear levelling.
  • DRAM cache – Provides fast access buffer to frequently used data.
  • Firmware – Software that handles data I/O requests and security functions.
  • Host interface – Connects the SSD to the computer, typically with SATA, PCIe, or NVMe interfaces.

At the core, flash memory cells retain charge in a cell made up of a floating gate transistor. Data is stored by changing the cell charge to enable different voltage states signifying 1s and 0s. Reads and writes target blocks of cells. The controller provides a mapping layer called the Flash Translation Layer (FTL) that handles logical to physical address mapping and read/write operations.

The controller also handles bad block mapping, wear levelling, and garbage collection to maximize performance and lifespan. An SSD has no moving mechanical parts, allowing very fast random I/O performance. DRAM cache buffers frequently accessed data for faster access. Data persistence is maintained even when power is removed via the non-volatile flash memory cells.

SSD Form Factors

Common physical form factors that SSDs come in are:

  • 2.5″ SATA – The most common SSD format, uses a 2.5″ chassis like a laptop HDD and SATA host interface.
  • M.2 – Compact, blade-like card edge connector SSDs used in laptops and high performance systems.
  • PCIe Add-in Card – SSDs in PCI Express card form factors for very high performance.
  • mSATA – Mini-SATA drives in compact sizes for mobile devices and embedded systems.

SSD Interface Types

Interface protocols commonly used with SSDs include:

  • SATA – Serial ATA, with capacities around 4TB max and 6Gbps speed.
  • NVMe – Optimized PCIe interface for SSDs. Very low latency, throughputs over 3.5GB/s.
  • PCIe – PCI Express, attach SSD directly to PCIe lanes for highest possible performance.
  • USB – Universal Serial Bus, typically used with external portable SSDs.

HDD vs SSD: Strengths and Weaknesses

Let’s compare some of the relative strengths and weaknesses between HDD and SSD storage:

HDD Strengths

  • Cheap cost per gigabyte – HDDs are much more affordable per unit of storage compared to SSDs.
  • Good for sequential data access – HDDs can efficiently stream large files sequentially.
  • Massive capacity – HDD capacities range from hundreds of GB to tens of TB.
  • Long-term data archiving – Magnetic storage can retain data for years.

HDD Weaknesses

  • Slower random I/O performance – HDDs are poor at accessing small random data chunks.
  • Fragile mechanical parts – HDDs can fail due to mechanical stresses and shocks.
  • High power draw – Spinning platters use more power, produce heat.
  • Noise – Audible clicks and spin noise.

SSD Strengths

  • Blazing fast random I/O – Excellent for random data access, low latency.
  • Silent operation – No noise from moving parts.
  • Low power draw – Use less electricity and run cooler.
  • Shock and vibration resistant – Can withstand more physical stress.

SSD Weaknesses

  • More expensive per GB than HDDs currently.
  • Lower capacities than HDDs.
  • Wear out over time with limited write cycles.
  • File deletion less secure – Data remnants may remain.

Typical Uses for HDD vs SSD

Due to their different strengths, HDDs and SSDs each tend to be used for certain use cases more often. Some typical uses are:

HDD Usage Cases

  • External storage and backups – Large and affordable storage.
  • Desktop PCs – Mass storage for photos, media, files.
  • Gaming PCs – Can store large game installs.
  • Media centers – DVRs, media streaming.
  • Data archiving – Long term mass storage.

SSD Usage Cases

  • Boot drives – Fast boot and app launch times.
  • High performance PCs – Excellent for frequent random access.
  • External portable storage – Compact and durable.
  • Servers – Faster access to cached and random data.
  • Industrial uses – Rugged, operates in wide temp range.

Hybrid Drives

A hybrid drive, sometimes called SSHD (solid state hybrid drive), combines features of both HDDs and SSDs. Hybrid drives contain a large HDD platter for mass storage combined with a smaller NAND flash module that acts as a cache. Frequently accessed data is cached on the flash module for much faster access. This makes hybrid drives a compromise aiming to provide some of the speed of SSDs along with the larger storage capacity of HDDs. However, performance is still slower than a pure SSD.

Conclusion

In summary, HDDs and SSDs take different approaches to providing non-volatile digital storage. HDDs utilize mechanically spinning magnetic platters to store data, while SSDs use microchips storing data in flash memory cells. Both have advantages and disadvantages. HDDs are more affordable per gigabyte and offer much higher capacities. SSDs are substantially faster, use less power, generate no noise, and are more physically robust.

For consumers, HDDs are suitable for bulk data storage needs where access speed is less critical. SSDs provide a major performance boost for more performance-intensive uses like operating systems and applications. For businesses, HDDs allow affordable mass archiving while SSDs help accelerate databases and high transaction volume servers. Moving forward, SSD prices will continue to fall, allowing them to displace HDDs in more and more applications.