Solid state storage devices are increasingly being used to store data in place of traditional mechanical hard disk drives (HDDs). Solid state drives (SSDs) offer faster read/write speeds, better reliability, and lower latency compared to HDDs. There are several types of solid state storage technologies that are used for data storage in personal computers, servers, and consumer devices.
Flash Storage
Flash storage is the most common type of solid state storage used today. Flash storage uses flash memory chips to store data persistently. The flash memory chips contain floating gate transistors that can store electric charge to represent binary data. Some key advantages of flash storage include:
- Faster access speeds – Flash storage has very fast random read speeds, allowing quick boot times and fast access to applications and files.
- Higher reliability – Flash contains no moving parts and is less prone to mechanical failure.
- Lower power usage – Flash chips consume much less power per gigabyte compared to spinning hard drives.
- Lighter weight – Flash has a higher storage density, allowing lighter and more compact devices.
There are two main types of flash storage:
NAND Flash
NAND flash is the most common type used in SSDs, USB drives, and memory cards. NAND flash reads and writes data in blocks. It is non-volatile, retaining data even when power is removed. NAND flash has higher storage density but slower writes than NOR flash.
NOR Flash
NOR flash reads and writes data in bytes and is volatile. It offers faster read and write performance compared to NAND but lower storage density. NOR flash is primarily used when random access performance and fast read times are critical.
3D XPoint
3D XPoint is an emerging solid state storage technology developed by Intel and Micron. It uses a different architecture from flash memory to store data by changing the electrical resistance of cells arranged in a 3D lattice structure. Key advantages of 3D XPoint include:
- Faster performance – 3D XPoint has lower latency and higher endurance than NAND flash.
- Higher density – 3D XPoint cells can be stacked in three dimensions for greater density.
- Non-volatile – Data is retained when power is removed.
Intel’s Optane brand 3D XPoint SSDs are now commercially available and provide an intermediate option between fast but expensive DRAM and slower NAND flash in servers and high-end PCs.
Resistive RAM (ReRAM)
ReRAM is an emerging non-volatile storage technology that contains resistors that can switch between low and high resistance states to store data. ReRAM cells are simple in construction compared to flash but provide similar non-volatile capabilities. Advantages of ReRAM include:
- Faster writes – ReRAM has lower write latency than flash.
- Increased endurance – ReRAM cells can endure significantly more write cycles before failure.
- Scalability – The simple structure of ReRAM cells makes them easier to scale down.
ReRAM is still in early stages of development and not yet widely used, but has potential for high speed and high density storage applications.
Magnetoresistive RAM (MRAM)
MRAM uses magnetic storage elements instead of electric charge or resistance to store data bits. Each MRAM cell contains two ferromagnetic plates separated by a thin insulating layer. Data is stored by magnetizing one of the plates, and read operations are performed by measuring the electrical resistance. Advantages of MRAM include:
- Non-volatility – Data remains when power is removed.
- Fast access – MRAM offers low latency close to DRAM levels.
- Unlimited endurance – MRAM cells can be rewritten virtually indefinitely.
- High density – MRAM has the potential for greater storage density than DRAM.
MRAM is starting to see usage in embedded and niche applications where fast persistent memory and unlimited endurance are critical.
Applications of Solid State Storage
Some of the major applications where solid state storage is replacing conventional hard disk drives include:
Laptop and Desktop Computers
SSDs are commonly used as the primary storage in laptops, ultrabooks, and high-performance desktop PCs. The combination of flash storage and SATA/PCIe interfaces provide tremendous improvements to boot times and application launch speeds over hard disk drives. SSD reliability and shock resistance advantages are also desirable for mobile computing.
Servers
Enterprise and data center servers extensively use SSDs for storing databases, virtual machine images, caches and indexes. The random access performance of SSDs provides greater throughput for transactional workloads and I/O intensive applications. SSDs also reduce power and cooling costs and datacenter footprint compared to large spinning HDD arrays.
Consumer Electronics
Portable consumer electronics like smartphones and tablets use NAND flash as primary storage. The small form factor, low power usage, and absence of moving parts provided by solid state storage allows for slim and compact device designs with long battery life. Large flash storage capacities are achievable with very small physical packaging.
Industrial Applications
Rugged industrial computing devices benefit from the resilience of flash storage to shock, vibration and extreme temperatures. Avoiding mechanical failures improves uptime and reduces maintenance costs. The steady performance of SSDs across a wide temperature range is also preferred for industrial control systems and embedded devices.
Solid State vs Hard Disk Drives
While HDD technology has gradually evolved over decades, solid state storage represents a completely different approach to storing data. Some key differences between traditional hard drives and modern solid state storage include:
| Characteristic | Hard Disk Drive (HDD) | Solid State Drive (SSD) |
|---|---|---|
| Construction | Mechanical platters with magnetic coating, read/write heads, moving parts. | Integrated circuits with no moving parts. |
| Speed | Sequential data transfer performance limited by mechanical parts. HDDs have ~100 MB/s transfer rates. | No moving limitations on access patterns. SATA SSDs over 500 MB/s, NVMe SSDs over 3 GB/s transfer rates. |
| Latency | Seek time is typically 2-5 ms for consumer HDDs. 15,000 rpm enterprise HDDs can have ~1 ms seeks. | Consistent microseconds of latency for SSDs. No seek time. |
| Reliability | Vulnerable to physical shock. MTBF is typically 1-2 million hours. | No moving parts, better tolerance to vibration/shock. MTBF over 1.5 million hours. |
| Power | HDD start-up current spikes can be over 2A. Typical idle power is 2-6W. | Low startup current below 150 mA. Idle power less than 100 mW. |
| Density | Areally density approaching 1 Tb/in2 with perpendicular recording and shingled magnetic recording (SMR). | MLM NAND flash is over 10 Tb/in2 currently. 3D flash technology roadmap exceeding 100 Tb/in2. |
| Cost | Typically under $0.03 per GB for HDD capacity. | Roughly $0.10 to $0.30 per GB for SATA SSD capacity. |
While HDDs retain advantages in price per gigabyte and maximum capacity, SSDs are superior across most other metrics that impact client and enterprise application performance. The combination of better speed, latency, reliability, power efficiency, form factor, and steadily decreasing cost per GB are driving the adoption of solid state storage for a growing range of usage scenarios.
Conclusion
Solid state storage devices are poised to become the new standard for storing data across personal computing, enterprise, and consumer electronics segments. The limitations of mechanical hard drives are bypassed by integrated circuit storage media like NAND flash or emerging technologies such as 3D XPoint. The advantages in performance, reliability, power efficiency and density offered by solid state storage are substantial enough to justify their higher cost over hard drives for many applications. As solid state storage technology continuously evolves and improves, its superiority over the venerable hard disk drive will become even more prominent.