Solid state drives (SSDs) are a type of computer storage device that uses flash memory to store data. Unlike traditional hard disk drives (HDDs) that use spinning platters, SSDs have no moving parts and instead store data in memory chips. This makes SSDs much faster, more durable, and energy efficient than HDDs. But what exactly is flash memory, and how does it allow SSDs to work so effectively? Here we’ll take a closer look at how flash memory works and its role in powering modern SSDs.
What is Flash Memory?
Flash memory is a type of electronically erasable programmable read-only memory (EEPROM) chip that can be electronically erased and reprogrammed. It is non-volatile, meaning it retains stored data even when power is removed. The name “flash” comes from its ability to be erased in blocks instead of one byte at a time.
There are two main types of flash memory:
- NAND flash – Named after the NAND logic gate, this is the most common type used in SSDs and USB flash drives. It is lower cost but slower than NOR flash.
- NOR flash – Named after the NOR logic gate. It is faster and more expensive than NAND flash. Used primarily for storing firmware or boot instructions that need fast random access.
NAND flash is well-suited for data storage applications like SSDs because it prioritizes higher density and lower cost per bit over performance. It can store more data in the same physical space.
How NAND Flash Stores Data
NAND flash memory chips contain a grid-like arrangement of memory cells composed of floating gate transistors. These cells use quantum tunneling to trap electrons on a floating gate electrode, storing one bit of data – a 0 or 1 – based on the number of electrons:
- Lower number of electrons = Logic 1
- Higher number of electrons = Logic 0
Reading the cell determines if it contains a 1 or 0 based on its electron charge. To write or erase a cell, controlled electrical charges are applied to add or remove electrons from the floating gate.
NAND flash writes and erases data in “pages” or blocks, typically 16 to 64kB in size. It is well-suited for sequential data writes. However, erase commands can only be done on an entire block of cells, not individual pages. This necessitates some behind-the-scenes data management known as the Flash Translation Layer.
Flash Translation Layer
Because entire blocks of NAND flash cells must be erased together, SSD controllers use a Flash Translation Layer (FTL) to manage where data is stored. When a file is updated or rewritten, rather than erasing the original block it is written to a new empty block. The old block is marked for erasure later.
This out-of-place update style requires a logical-to-physical mapping system to track where current valid data is located. The FTL remaps logical block addresses that the operating system uses to physical block addresses on the SSD where data is actually stored.
This helps distribute writes evenly to improve performance and device longevity. The FTL also performs wear leveling to ensure all flash blocks experience a similar number of erasures over the SSD’s lifetime.
SSD Architecture and Components
Now that we’ve looked at how the NAND flash memory itself works, let’s examine how flash memory is utilized in solid state drive architecture and design. SSDs contain several key components:
NAND Flash Memory Chips
The NAND flash chips make up the bulk of an SSD’s hardware and provide the core data storage capacity. Common flash chip form factors used include:
- SATA – For standard 2.5″ SSDs
- BGA – Ball grid array chips soldered directly onto circuit boards
- M.2 – For M.2 SSDs using the PCIe bus
Higher capacity SSDs may contain multiple flash memory chips in parallel. Performance also scales with more packages as data can be striped across them.
SSD Controller
The controller is the processor that manages all data I/O operations on the SSD and runs the flash translation layer firmware. It provides:
- Interface support – SATA, PCIe, etc
- Data pathing and management
- Error correction and detection
- Wear leveling algorithms
- Encryption support
Many SSD controllers also incorporate a DRAM chip as memory to cache mapping tables and help accelerate performance.
Host Interface
The host interface enables the SSD controller to connect with the host computer system and may include:
- SATA – Up to 6Gbps speeds
- PCIe – NVMe SSDs, higher performance
- U.2 – Enterprise SSDs
DRAM Cache
In addition to the main NAND flash storage, higher performance SSDs add a smaller amount of DRAM memory. This DRAM cache buffers incoming writes to help speed up the process. The FTL mapping data may also be cached in DRAM for faster lookups.
Power Circuitry
The SSD requires power circuitry to regulate and distribute power from the host system to components like the controller and NAND flash memory. Proper power management is important for stable operation.
SSD vs. HDD Architecture Comparison
To better understand what sets SSDs apart, it helps to contrast their internal architecture against traditional hard disk drives:
Storage Media
SSDs use interconnected NAND flash memory chips able to electronically store data without moving parts. HDDs rely on mechanical spinning disks coated in magnetic material for data storage.
Memory Cells
NAND flash comprises thousands of electrically addressable blocks containing pages of memory cells. HDD platters contain billions of magnetically addressable sectors.
Read/Write Mechanism
SSDs read and write data by applying controlled electrical charges to trap or release electrons on floating gate transistors. HDDs use a movable head to detect and modify the magnetism of microscopic dots on the disk platter.
File System
The Flash Translation Layer provides a logical-to-physical mapping system for SSD block addresses. HDDs directly address sectors via the disk’s geometry and firmware.
Performance
SSDs have no moving parts, allowing for very fast random I/O performance up to 550,000 IOPS. HDDs rely on physical movement, limiting IOPS typically below 200.
This comparison shows why SSDs with their streamlined electrical storage offer big performance and reliability gains over traditional magnetic hard disk drives.
Types of SSDs
Not all solid state drives are built the same. There are several SSD form factors and interfaces available:
2.5″ SATA SSD
The most common type, designed for use in laptops and desktops. Connects via the SATA interface and is packaged in a standard 2.5″ drive enclosure. Typical capacities 120GB to 4TB.
M.2 SSD
Designed for smaller devices and motherboards with the M.2 slot. Very compact, usually uses PCIe bus for faster speeds. Popular for ultrabooks and advanced PCs.
PCIe Add-In Card
SSD on a PCI Express add-in card interface for very high performance, low latency. Offer 500K+ IOPS. Ideal for servers and high end workstations.
U.2 / U.3 SSD
Enterprise SSD form factor, connects via PCIe, SAS or SATA. Designed for data centers and servers. Offer advanced performance, endurance, and reliability.
SSD vs HDD Comparison
Although HDDs still offer a better value per gigabyte, SSD prices have fallen steadily. And the performance benefits of flash memory storage give SSDs big advantages in many use cases. Let’s compare some of the differences:
Price per GB
SSDs are pricier per gigabyte than HDDs. However, steady advances in NAND flash technology continue to reduce SSD prices over time. As of 2022, 1TB SATA SSDs cost around $100 while 1TB HDDs are $40-50.
Maximum Storage Capacity
Current HDD technology allows capacities up to 20TB for consumer models. High capacity SSDs aimed at consumers are typically limited to 8TB, but enterprise SSDs can reach substantially higher.
Interface Speeds
SSDs using the SATA interface top out around 550 MB/s. However, PCIe 4.0 x4 NVMe SSDs can reach 7,000 MB/s – over 12x faster. Even SATA SSDs are faster than HDDs.
Reliability and Durability
With no moving parts, SSDs are less susceptible to damage or component failures over time. They offer far greater shock, vibration, and temperature resistance.
| Specification | SSD | HDD |
| Shock Resistance | 1,500 Gs | 300 Gs |
| Vibration Tolerance | 20 Gs RMS | 1.5 Gs RMS |
Power Efficiency
SSDs consume a fraction of the power that HDDs require – up to 8-10x less when active, and 100x less when idle. This provides longer battery life.
Noise Levels
With no spinning platters or physical movement, SSDs operate silently with no audible noise. HDDs generate low but noticeable humming and whirring sounds from the drive motor.
Access Latency
NAND flash memory has microseconds of read latency, 1,000x faster than HDDs. This allows very fast random access.
File Transfer Speeds
SSDs excel at sequential read/write performance due to lack of seek time. This delivers faster load times for large files, programs, games, and operating systems.
Conclusion
Solid state drives rely on NAND flash memory chips as the core storage component in place of hard disk platter media. The NAND flash memory cells store data based on the amount of electrons trapped on floating gate transistors in each cell. Reads and writes are done by applying controlled electrical charges without any moving parts.
SSDs incorporate NAND flash, a controller, host interface, DRAM cache, and power circuitry to make up the complete storage device. The controller runs the Flash Translation Layer firmware that maps logical block addresses from the host to physical locations on the flash memory. This allows SSDs to optimize write performance and lifespan by evenly distributing writes across the drive.
When compared to traditional spinning hard disk drives, SSDs excel in critical areas like speed, latency, longevity, power efficiency, shock resistance, and noise levels due to their streamlined electrical storage architecture. Their lack of moving parts allows for very high input/output speeds not hampered by seek time or mechanical delays. SSDs deliver substantial performance and reliability gains over HDDs, although currently at a higher initial cost per gigabyte. However, steady technological advancements continue to drive down SSD pricing year after year, making flash memory the preferred storage medium for most computing applications.
