Solid State Drives (SSDs) and Hard Disk Drives (HDDs) are two different types of computer data storage devices. Both serve the same basic function of storing and retrieving digital information, but they have some key differences in how they work, their performance characteristics, and their cost.
What is a SSD?
A Solid State Drive (SSD) is a data storage device that uses integrated circuit assemblies to store data persistently. SSDs use flash memory, meaning they store data in memory chips rather than on spinning platters like hard disk drives (HDDs).
The lack of moving parts allows SSDs to operate silently and gives them advantages in physical shock resistance. SSDs also provide much faster access to data stored on them compared to HDDs. They have higher data transfer rates and lower access latency. However, SSDs are typically more expensive per gigabyte than HDDs.
What is a HDD?
A Hard Disk Drive (HDD) is a data storage device that uses magnetic storage to store and retrieve digital data. HDDs write data on spinning metal platters inside the drive. A read/write head floats above the platters on an actuator arm, allowing data to be accessed rapidly.
HDDs have been the predominant form of computer data storage since the 1960s. They feature moving parts like the spinning platters and actuator arm. This makes them more susceptible to physical damage from shocks or vibration. However, HDDs offer larger storage capacities than SSDs for a lower price point.
Main Differences Between SSDs and HDDs
1. Construction and operation
SSDs have no moving parts and use integrated circuits to persistently store data. HDDs store data on spinning metal platters that are read by a moving read/write head.
2. Shock and vibration resistance
SSDs’ lack of moving parts gives them greater resilience to damage from physical shocks and vibration. HDDs’ spinning platters and head make them more prone to damage.
3. Speed and performance
SSDs provide faster access to stored data and much higher data transfer speeds. Their speed is not affected by where data is physically stored on the drive. HDDs can access data on the outer edges of platters fastest.
4. Latency
SSDs have much lower access latency than HDDs. Latency is the delay between a request for data and the start of its transfer. SSD latency is in the microsecond range, while HDDs are in the millisecond range.
5. Noise
SSDs have no moving parts so make no noise when operating. HDDs’ spinning platters and moving heads create audible noise.
6. Weight and size
Due to having fewer components, SSDs are lighter and have a smaller physical profile than HDDs. HDDs require more space for spinning platters.
7. Power consumption
SSDs consume less power and produce less heat than spinning HDDs. This gives SSDs advantages in mobile devices. HDDs require more electricity to operate.
8. Storage capacity
HDDs are available in larger storage capacities than SSDs. High capacity HDDs provide up to tens of terabytes of storage, while SSD capacities top out below 10 terabytes.
9. Price and cost per gigabyte
HDDs are significantly less expensive in terms of cost per gigabyte of storage. High capacity HDDs provide the lowest storage costs.
10. Lifespan and durability
SSDs can withstand more write/erase cycles before drive failure. HDDs’ moving platters have finite life expectancy of a few years. SSDs can last much longer under normal use.
Internal Components and Operation
SSDs and HDDs have very different internal designs and methods of operation. These underlying differences lead to the performance distinctions between the two technologies.
SSD Internal Components
– Flash memory chips – Stores data persistently using transistors in an integrated circuit. Different types of flash memory used include MLC, TLC, and SLC.
– Controller – The SSD controller manages communications between the flash memory and host computer. It also manages memory operations.
– DRAM cache – Provides faster access to frequently used data. Helps address slower write speeds in flash memory.
– Host interface – Connects the SSD to the computer it is installed in. Common interfaces are SATA, PCIe, and NVMe.
How SSDs Write Data
To write new data to an SSD:
1. The host computer sends a write command to the SSD controller.
2. The controller identifies empty blocks in the flash memory chips to accept the new data.
3. It writes the new data to the flash memory.
4. The controller updates the address mapping table that locates data on the SSD.
How SSDs Read Data
To read data from an SSD:
1. The host computer sends a read request for a piece of data.
2. The SSD controller consults the address mapping table to find the data’s physical location.
3. It reads the data from the flash memory chips.
4. The data is sent to the host computer.
HDD Internal Components
– Platters – Circular disks that store data magnetically. Stacked platters spin on a central spindle.
– Read/write head – Hovers over the platters on an actuator arm to read and write data.
– Spindle motor – Spins the platters at high speeds during drive operation.
– Actuator mechanism – Moves the read/write head across the platters.
– Firmware – Provides the HDD’s basic control software and functionality.
How HDDs Write Data
To write new data to a HDD:
1. The actuator arm positions the read/write head over the correct track on a platter.
2. As the platter spins, the head magnetically records data along the track.
3. The new data is verified through a read process.
4. The drive confirms the data is now stored on the track.
How HDDs Read Data
To read data from a HDD:
1. The actuator arm positions the read/write head over the track holding the data.
2. As the platter spins, the head detects magnetic polarity changes, producing electrical signals.
3. The drive converts the signals into binary data that the computer can understand.
4. The binary data is sent to the host computer.
Speed and Performance Comparisons
SSDs significantly outperform HDDs in data access speeds and drive performance. This section explores quantifiable speed differences between the two technologies.
Access Time and Latency
Access time measures the delay between initiating a command to read or write data, and when the operation begins. It encompasses the full process of the system locating data, positioning the head, and beginning the transfer.
Latency refers more narrowly to just the time it takes to locate the data, without including physically moving components to access it.
Below are typical latency and access time values for HDDs and SSDs:
| Drive Type | Average Latency | Average Access Time |
| HDD | 5,000 microseconds | 10,000 microseconds (10 ms) |
| SSD | 20-100 microseconds | 0.2 ms |
This shows SSDs provide at least 50X faster data access latency than hard drives. Access times are faster for SSDs by a factor of at least 20X.
Interface Transfer Speeds
SSDs support much faster interface standards than HDDs. Below are typical sustained transfer speeds for drives using different interfaces:
| Interface | HDD Speed | SSD Speed |
| SATA 3 | 180 MB/s | 550 MB/s |
| SAS 12 Gb/s | 900 MB/s | 900-1200 MB/s |
| PCIe 3.0 x4 NVMe | N/A | 3,500 MB/s |
| PCIe 4.0 x4 NVMe | N/A | 7,000 MB/s |
SSDs achieve at least 3X faster speeds than HDDs on the older SATA interface. Newer PCIe NVMe SSDs are 6-10X faster than SATA SSDs.
IOPS Ratings
IOPS (input/output operations per second) measures the number of data transactions a drive can perform per second. SSDs achieve dramatically higher IOPS ratings than HDDs:
| Drive Type | Typical IOPS Rating |
| HDD | 100-200 IOPS |
| SATA SSD | 20,000 IOPS |
| PCIe NVMe SSD | 400,000+ IOPS |
Top-performing NVMe SSDs can handle over 4,000X more IOPS than HDDs. This demonstrates the massive random I/O performance benefits of SSDs.
Reliability and Durability
While HDDs can provide years of reliable service, SSDs are more durable for typical use cases. Their lack of moving parts increases lifespan.
Drive Writes Per Day (DWPD)
Drive writes per day (DWPD) indicates a drive’s endurance – how much data can be written to it daily over a 5-year period before failure. Most consumer HDDs support 1-2 DWPD. Mainstream SATA SSDs support 5-10 DWPD, while high-end models boast 10+ DWPD.
Mean Time Between Failures (MTBF)
MTBF predicts average drive lifespan before failure. HDD MTBF rates range from 500,000 to 1.5 million hours. SSDs show MTBF between 1 to 2 million hours. Higher quality SSDs last longer.
Bad Block Rate
Bad block rate measures the percentage of memory cells on a drive that become unusable over its lifetime. HDDs have relatively low bad block rates of less than 0.005%. Budget SSDs can have higher bad block rates around 0.05-0.1%. High-end SSDs use better flash memory with rates as low as 0.0001%.
Cost Comparisons
Hard drives continue to provide the most affordable storage in terms of dollars per gigabyte. However, SSD pricing has been decreasing steadily.
Here are approximate current price ranges for HDDs and SSDs with different storage capacities:
| Capacity | HDD Price Range | SSD Price Range |
| 128GB | N/A | $25-$50 |
| 256GB | N/A | $40-$70 |
| 512GB | N/A | $50-$100 |
| 1TB | $35-$60 | $80-$150 |
| 2TB | $50-$80 | $140-$250 |
| 4TB | $80-$150 | $400-$750 |
While HDDs are cheaper per gigabyte, SSD prices have dropped to be competitive with HDDs for smaller storage capacities under 1TB. At higher capacities, HDDs retain a significant price advantage.
Power users requiring massive storage still get better value from HDDs. But SSDs provide a compelling choice for mainstream consumers focused on performance over maximum capacity.
Use Cases and Applications
The unique strengths and weaknesses of HDDs and SSDs make each technology better suited for certain applications.
SSD Use Cases
SSDs provide the best performance for the following uses:
– Primary or secondary storage in desktop PCs, laptops, tablets, phones, and IoT devices
– Boot drives holding operating systems
– Programs where fast load times are critical
– Servers hosting latency-sensitive applications like databases
– High-performance computing and data analytics tasks
– Video editing, 3D modelling, and other content creation
HDD Use Cases
HDDs are a better choice for these scenarios:
– Archival storage and backups
– Bulk storage of rarely accessed data
– Media libraries holding videos, photos, and music
– Big data analytics servers focused on capacity over speed
– Cloud storage and data centers on a tight budget
– NAS devices and DAS in home users’ desktops
– Gaming PCs needing massive storage for games
Conclusion
SSDs and HDDs will continue to coexist as key complementary technologies in computing and data storage. HDDs still rule supreme for high capacity bulk storage needs, while SSDs provide unmatched speed and smooth operation.
For typical mainstream personal computing, SSDs have become the default choice. Their continually lower costs and higher performance deliver a better overall user experience. HDDs retain purpose for specialized scenarios benefiting from massive slow storage.
Moving forward, expect SSDs to continue displacing HDDs in more and more applications. Advances like QLC NAND flash, PCIe 5.0, and new form factors will cement SSDs as the future of storage. But the venerable HDD still has a role to play when massive terabyte capacity is required.