All about Solid State Drives

Detailed information about Solid State Drives, how those are different from Hard Disk Drives and types of SSD NAND Flashes.

What is Solid-state Drive (SSD)?

Solid-state drives are Secondary Storage devices to store persistent data in computer systems. It makes use of NAND Flash memory to store the data in semiconductor cells. SSDs are also called Solid State Devices or Solid State Disks.

How SSD different from Hard Disk Drive?

Hard Disk Drives(HDD) utilize mechanical moving parts such as rotating discs (Platters) and read/write Heads to store and retrieve the data whereas SSD uses instantly accessible memory chips. Due to this lack of moving components, SSDs are much more energy-efficient, faster, and reliable than HDDs.

Brief information about Hard Disk Drives

Hard Disk Drive is invented by IBM engineers in 1953 at San Jose California laboratory and started using commercially on its computer IBM 305 RAMAC in 1957.

Hard Disk Drive Component Diagram

HDD uses rotating platters coated with magnetic material paired with magnetic heads to write and read the data. Once written the magnetic platters hold the data even the HDD powered off. Data on the HDD can be randomly accessed in contrast with the sequential access devices such as Magnetic Tapes.

It is ideal to consider the below parameters while buying the Hard disk drives.

  • Amount of data can be stored.(eg: 500GiB, 1TiB,..)
  • Speed in which the spindles/platters rotate. (5000rpm, 7200rpm, 10,000rpm,..)
  • Type of interface for connecting HDD to the computer. (IDE, SCSI, SATA,..)

Comparison Table - HDD vs SSD

AttributeHard Disk Drive (HDD)Solid-state Drive (SSD)
Technology Contains Mechanical moving parts like rotating platters/discs and read/write heads. Made by using Integrated Chips - Flash Memory
Weight Heavy Lightweight
Power Consumption 6 to 15 watts 2 to 5 watts
Noise 27db (due to the moving parts) 0db (no moving parts)
Heat Generate heat due to the moving parts. Produce less heat compare to HDD.
Access Time 5.5 to 8 ms 0.1 ms
Random IO Performance 400 io/s up to 2.5 Million io/s
Failure Rate 2 to 5 % less than 0.5 %
Average service time for an IO below 20 ms 400 to 500 ms
Endurance MTBF > 20,00,000 Hrs MTBF < 7,00,000 Hrs
Operating Temperature 0°C to 70°C 0°C to 60°C

How Solid-state Drives Works?

Data in SSD stored using NAND Flash represented by an electrical charge on each NAND cell. NAND Flash is a type of non-volatile storage technology that does not require power to retain data.

SSD Components

Solid State Drive Component Diagram

The main components in the Solid State Drives are the controller and the memory used to store the data. On the initial days of SSD, DRAM volatile memory has been used as the primary memory component. But from 2009 onwards NAND Flash non-volatile become more common and replaced the DRAMs.

The controller is the brain of the SSD that bridges the NAND Flash memory to the SSD Input/Output interfaces connected with the computer. Following are the main function of the Controller.

  • Encryption
  • Read and Write Caching
  • Garbage Collection
  • Bad Block Mapping
  • Error correction (ECC)
  • Wear leveling

Types of SSD Drives

Hard Disk Drive Component Diagram

SSDs are available commonly in the following form factors.

  • Traditional Drives ( 1.8inch, 2.5 inch, 3.5 inch)
  • PCIe Drives
  • M.2 – Keys and slots

The common interface types to connect SSDs with the computer system are below.

  • Serial-ATA (SATA)
  • PCIe (PCI Express)
  • NVMe (Non-Volatile Memory express)

SSDs can be grouped into different categories depending on the NAND Flash used in them, form factor(physical size) attributes, and the interface that is used to connect them to the computer system.

The performance, cost, and durability of an SSD primarily depend on the type of NAND Flash used in them. There are different types of NAND based on the number of bits that can be stored in each NAND cell at a one-time point of time.

NAND Flash - SLC, MLC, TLC, QLC Comparison

The fewer bits per cell, the smaller the capacity, but data is written and retrieved faster. In this way, the NAND chip has a higher endurance level so will last much longer. SLC is the fastest and has the highest endurance but lower capacities - typically up to 100GB.

QLC is much slower, with low endurance, but has a much higher capacity threshold - 1TB to 8TB.

Following are the various types of NAND Flashes used in SSD.

Single-Level Cell (SLC) - 1 bit of data per NAND cell

Single Level Flash holds a single bit in its cell while charged. It has long-lasting data read/write cycles and great accuracy while reading and writing the data. This type of flash is mostly used in the enterprise market because of its accuracy, life span, and overall performance - very rare to see in home computers due to its high cost and limited storage capacities.

Representation of one Byte(8 bits) stored in SLC

Pros :

  • Higher lifespan and reliability than any other flash type.
  • Has the highest number of charge cycles( 90,000 and 100,000).
  • Able to operate in a broader temperature range.

Cons :

  • Most expensive NAND Flash in the market.
  • Commercially available in smaller capacities.

Multi-Level Cell (MLC) - 2 bits of data per NAND cell

This type of NAND stores multi bits of data in a single cell. This makes it a lower cost of manufacturing compared to SLC. Since this lower cost of production is passed over to the consumer, it is popular among several brands.

Representation of one Byte(8 bits) stored in MLC

Pros :

  • Lower production cost compared to SLC.
  • More reliable than TLC and QLC ( around 10,000 cycles per cell).

Cons :

  • Less reliable than SLC.

Enterprise Multi-Level Cell (eMLC) - 2 bits of data per NAND cell

eMLC stores the data in the same manner as SLC, but optimized for the enterprise sector provides better performance and durability. The life span of eMLC is less compared to SLC - between 20,000 and 30,000 Read/Write cycles per cell.

Pros :

  • A cheaper alternative to SLC for an enterprise SSD.
  • More reliable and deliver better performance than TLC.

Cons :

  • Inferior compared to SLC.

Triple-Level Cell (TLC) - 3 bits of data per NAND cell

This type of NAND Flash stores 3 bits of data per cell with varying voltage levels. It is cheaper than SLC and TLC but comes with the cost of reduced performance.

Representation of one Byte(8 bits) stored in TLC

Pros :

  • Lower production cost compared to SLC and TLC

Cons :

  • Less reliable and limited lifespan ( 3,000 to 5,000 cycles per cell).

Quadruple-Level Cell (QLC) - 4 bits of data per NAND cell

This is the cheapest among all the NAND Flash types. It can hold 4 times data in a single cell compared to the SLC. This enables to produces of these types of drives in a larger capacity.

Representation of one Byte(8 bits) stored in QLC

Pros :

  • Inexpensive compared to all other Flash types.
  • Comes in large sizes( More than a Terabyte).

Cons :

  • Least reliable among all with shorter lifespan(1,000 P/E cycles).

3D V-NAND - The latest in the Flash Memory world

In traditional NAND Flash memory, thousands of cells arranged in a 2D plane to read and write the data. Over a period of time, the demand for performance and capacity requirements increased. This pushed the drive manufacturers to reduce the size of individual cells thus accommodating more cells in the same plane area. But that introduced an additional problem - increased instances of errors and power usage. V-NAND or 3D NAND is introduced as a solution to this problem.

In this type of SSDs, the cells are stacked vertically( Thus the name V[vertical]- NAND). Due to this vertical alignment of cells, these SSDs have higher capacities at lower production costs. Also, this arrangement greatly reduces the power requirements, resulting in higher speed and increased durability compared to normal planar(2D) NAND.

This new 3D V-NAND technology helped SAMSUNG to introduce the world's first 2TB SSD 850 Pro in 2015.

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