What is hard drive flash?

Hard drive flash refers to the flash memory chips that are used in solid state drives (SSDs) to store data. SSDs use flash memory instead of the spinning magnetic disks used in traditional hard disk drives. The use of flash memory provides SSDs with advantages over traditional hard drives such as faster access times, improved reliability, and lower power consumption. However, flash memory also has limitations such as lower storage densities and higher costs per gigabyte compared to hard disk drives. Understanding how flash memory works in SSDs can help explain the strengths and weaknesses of this increasingly popular storage technology.

What is Flash Memory?

Flash memory is a type of electronically erasable programmable read-only memory (EEPROM) that can be electrically erased and reprogrammed. It is a non-volatile memory, meaning it retains stored data even when power is removed. The name “flash” comes from the fact that data can be erased and rewritten in blocks instead of one byte at a time.

There are two main types of flash memory used in SSDs:

  • NAND flash – Cheaper and denser but slower than NOR flash.
  • NOR flash – Faster read times but more expensive and lower densities than NAND.

How NAND Flash Works

NAND is the primary flash memory used in SSDs due to its higher density and lower cost compared to NOR flash. NAND flash stores data in an array of memory cells made up of floating-gate transistors. These transistors have two gates instead of just one like in normal transistors:

  • Control gate – Activates the transistor, allowing current to flow.
  • Floating gate – Stores charge to represent data stored in the cell.

To write data, a high voltage is applied to the control gate and drain of the transistors. This allows electrons to tunnel through the thin oxide layer onto the floating gate, changing its threshold voltage. Erasing is done by applying a high negative voltage to the control gate, allowing the electrons to tunnel back off the floating gate.

Reading the data simply requires applying a voltage to the control gate and sensing the transistor’s threshold voltage to determine if the floating gate is charged or discharged, representing a 1 or 0 bit.

NAND flash writes and erases data in pages and blocks:

  • Page – Smallest unit that can be written, typically 4-16KB.
  • Block – Smallest unit that can be erased, typically 16-256 pages.

This architecture means that while reads are fast, erases and writes are slow since whole blocks have to be rewritten even if just updating a small amount of data. Wear-leveling techniques help distribute writes across all blocks to extend the lifespan.


NAND flash comes in several varieties characterized by the number of bits stored per memory cell:

  • SLC – 1 bit per cell
  • MLC – 2 bits per cell
  • TLC – 3 bits per cell
  • QLC – 4 bits per cell

SLC is the fastest and most durable, but also the most expensive per gigabyte. TLC and QLC are slower and wear out faster, but provide much higher densities at lower cost. Most consumer SSDs now use TLC NAND while high-performance models may use MLC NAND. The diagram below compares the different NAND types visually:

NAND Type Bits per Cell Durability Speed Density
SLC 1 High Fast Low
MLC 2 Good Medium Medium
TLC 3 Moderate Slow High
QLC 4 Low Slowest Highest

SSD Hardware Components

While the flash memory provides the actual storage in an SSD, there are other important components that enable the SSD interface and manage the flash:


The controller is the brain of the SSD, managing all communications between the SSD and host system. It has the following responsibilities:

  • Interface with host – Supports SATA, NVMe, etc interface protocols.
  • Error correction – ECC detects and fixes errors from NAND.
  • Encryption – Full disk encryption secures data.
  • Wear leveling – Ensures all NAND blocks wear evenly.
  • Garbage collection – Recovers unused pages.
  • Caching – Improves write speeds using SLC cache.

The controller greatly impacts overall SSD performance and endurance. More channels allow parallel access to the NAND chips, improving speed. An effective wear-leveling algorithm improves endurance and lifespan. A DRAM cache also accelerates writes and reduces wear on the NAND.

NAND Flash Memory

The NAND flash provides the non-volatile storage capacity in the SSD. Consumer SSDs typically use lower cost TLC NAND while high-performance models use MLC NAND. The number of NAND packages directly affects performance by enabling parallel access. Early SSDs used a single NAND chip but modern drives have up to 16 or more packages.

DRAM Cache

High performance SSDs augment the NAND flash with a small amount of fast DRAM, usually 1-2GB. This serves as a cache to improve write speeds since data can be written to the faster DRAM first before being committed to the slower NAND. It also helps reduce wear on the NAND since fewer write operations reach the flash. This temporary storage powered by standby power or supercapacitors when the SSD is powered off.

SSD Form Factors

SSDs come in several physical designs known as form factors. The form factor impacts available interfaces, performance, and compatibility. Common SSD form factors include:

2.5″ SATA

The most common SSD format, used as a direct replacement for 2.5″ hard drives in laptops and desktops. Connects via the SATA interface and comes in 7mm or 9.5mm Z-heights. Typical capacities of 128GB to 4TB. Performance around 500-550MB/s sequential reads and writes.


Compact, flat form factor that plugs directly into an M.2 slot on the motherboard. Available in different lengths from 30mm up to 110mm and in SATA or PCIe/NVMe interfaces. High performance NVMe models can reach over 3,000MB/s. Difficult to upgrade or replace in most laptops.

Add-In Card (AIC)

Older format for early SSDs using PCIe slots. Look like graphics cards or other add-on cards. Very high performance but larger size and power draw. Enterprise use only now. Replaced by M.2 and U.2.

U.2 / U.3

Enterprise SSD format that connects via PCIe/NVMe like M.2 but uses a larger, rectangular form factor. Designed for data centers to enable easy installation and replacement. Performance over 3,000MB/s. Not typically used in consumer PCs.

Form Factor Interface Size Performance Use Cases
2.5″ SATA SATA 7-9.5mm thick Up to 550MB/s Laptops, desktops
M.2 SATA or PCIe NVMe 22×30/42/60/80/110mm Up to 3,500MB/s (NVMe) Laptops, desktops, servers
AIC PCIe Full size card Up to 3,500MB/s Older desktops, servers
U.2 PCIe NVMe 15×35/70/110mm Up to 3,500MB/s Servers, data centers

SSD Interfaces

The interface connects the SSD to the host computer and determines maximum performance. Common SSD interface protocols include:


Serial ATA is the traditional hard drive interface, also used by early SSDs. SATA is limited to 600MB/s throughput due to protocol overhead. SATA SSD speeds reached a plateau around 550MB/s. Still used widely in budget and legacy systems.


NVMe (Non-Volatile Memory Express) is a new protocol designed specifically for SSDs on PCI Express bus. Reduced overhead and parallel access allows for speeds over 3,000MB/s. Requires NVMe drivers and BIOS/UEFI support. Primarily used in high performance SSDs and servers.


Serial Attached SCSI is an enterprise interface used in servers and data centers. Provides performance over 550MB/s and advanced features like dual-port access. Rarely used in consumer SSDs. Being replaced by NVMe in enterprise.

Interface Max Throughput Drivers Needed? Use Cases
SATA 600MB/s No Consumer HDD replacement
PCIe NVMe 32GB/s (PCIe 3.0 x4) Yes High performance, servers
SAS 12Gb/s Yes Servers and data centers

SSD Performance Factors

Many technical factors impact SSD speeds. The following parameters help compare SSD performance:

Sequential Read/Write Speed

The sequential transfer rate when accessing large continuous files. Important for moving large files but most PC workloads are random I/O.

Random Read/Write Speed

The random access speed when reading or writing small files in random locations. Critical for snappy OS and application response.

IOPS (Input/Output Operations per Second)

The number of 4KB random read/write operations the SSD can perform per second. Important metric for transactional workloads.


The response time for I/O operations. Lower latency provides better real-world performance. NVMe SSDs greatly reduce latency compared to SATA.


Total data that can be written over the SSD’s lifetime, measured in terabytes written (TBW). Higher endurance ratings indicate the drive can handle more writes before wearing out.


Length of warranty coverage in years. Higher endurance drives typically come with 5 year or longer warranties while budget models may have only 2 or 3 years.

Metric Typical Range Faster is…
Sequential Read/Write 300 – 3,500+ MB/s Higher
Random Read/Write 20K – 600K+ IOPS Higher
Latency 20 – 100 microseconds Lower
Endurance 100 – 5,000 TBW Higher
Warranty 2 – 5+ years Longer

SSD Reliability Factors

SSDs improve reliability versus hard drives with no moving parts. However, flash memory still has limitations for data integrity:

Bit Errors

As flash memory cells wear out, they are more prone to errors when reading data. SSD controllers use ECC (error correcting code) to detect and repair bit errors, but can be overwhelmed if too many occur.

Read Disturb

Reading data from a flash cell can cause nearby cells in the same block to change and lose data. The voltage applied during reads can disturb neighboring floating gates.

Write Endurance

Flash memory cells have a limited number of P/E cycles before they can no longer reliably store data. Careful write endurance management is needed to distribute writes and avoid “burning out” cells.

Retention Loss

Stored charge in flash cells can degrade over time leading to data loss. This may occur over months or years at high temperatures. Refreshing data can help avoid retention issues.

Internal Data Path Errors

Faulty wiring and connections between flash dies, controller, and host interface can lead to data corruption. High quality design, manufacturing, and testing helps minimize errors.

To maximize reliability, SSDs utilize RAID-like technologies such as RAIN (Redundant Array of Independent NAND). Critical data can be written across multiple flash dies to provide redundancy against any die failures. However, this increases costs due to higher over-provisioning requirements.

Ideal SSD Workloads

SSD performance advantages over hard drives are most pronounced in certain workloads:

Boot Drives

SSDs provide much faster boot times than hard drives due to very low latency and random IOPS. This provides near-instantaneous loading of operating systems and programs on startup.

Transactional Databases

Databases with frequent small updates benefit greatly from SSD performance. The random write capability reduces transaction commit times.

Caching and Tiered Storage

High performance accompanied by lower storage densities make SSDs ideal for caching and tiered storage. Hot data can be stored on SSD while cooler data goes on slower HDDs.

Single-threaded Applications

Applications that rarely multitask or run parallel operations see larger benefit from SSDs versus HDDs due to the drastic reduction in latency.

Virtual Desktop Infrastructure (VDI)

Booting multiple virtual machine images simultaneously strains hard drives but is easily handled by SSDs with high random IOPS capability.

In contrast, video editing and other large file sequential operations see diminishing returns from SSDs versus hard drives. Understanding application access patterns helps determine when SSDs provide the biggest return on investment.

Application Access Pattern
Operating System / Programs Many small random reads
Transactional databases Small random reads and writes
Virtual desktop infrastructure Large number of random I/O
Caching tier Mixed large and small reads and writes
Single-threaded apps Very low latency requirements


Hard drive flash provides the non-volatile storage in SSDs using NAND flash memory chips. The special characteristics of NAND technology provide SSDs with performance and reliability advantages compared to traditional hard disk drives. Factors such as bus interfaces, wear leveling algorithms, and caching technologies all impact SSD capabilities and suitability for different applications. Understanding the inner workings of hard drive flash enables matching the strengths of solid state storage appropriately to unlock significant speed benefits over magnetic hard drives. With storage technology rapidly evolving, hard drive flash will continue playing a major role powering faster computers and data centers into the future.