Solid State Drives (SSDs) and Hard Disk Drives (HDDs) are two different types of computer storage devices. Both serve the same basic function of storing data, but they have very different physical designs and performance characteristics.
What is an SSD?
An SSD, or solid state drive, is a type of storage device that uses flash memory chips to store data. The name “solid state” refers to the fact that SSDs contain no moving mechanical parts, unlike traditional hard drives which use physical platters and a read/write head to access data.
Some key characteristics of SSDs:
- Faster access speeds – SSDs can access data much faster than HDDs, often 10-100x faster for random reads and writes.
- Silent operation – No moving parts so SSDs make no noise when accessing data.
- Lower power consumption – SSDs use less energy than HDDs.
- Lighter weight – SSDs weigh less than HDDs for the same capacity.
- Shock and vibration resistant – The lack of moving parts makes SSDs more durable if dropped or in high vibration environments.
However, SSDs also have some downsides compared to HDDs:
- Higher cost per gigabyte – SSDs have a higher cost per GB compared to HDDs.
- Limited number of write cycles – NAND flash memory cells wear out after a certain number of writes. HDDs do not have this limitation.
What is an HDD?
A hard disk drive, or HDD, uses spinning magnetic platters to store data. A read/write head moves over the platters to read or write data. HDDs have been the traditional form of high capacity storage in computers for decades.
Some key advantages of HDDs:
- Lower cost per gigabyte – HDDs are cheaper than SSDs in terms of cost per GB.
- Virtually unlimited writes – HDDs can be written to an unlimited number of times.
- Higher capacities available – HDDs are available in larger capacities than SSDs, up to 10TB+ in consumer models.
However, HDDs also have some disadvantages compared to SSDs:
- Slower access speeds – HDDs are slower than SSDs, especially for random access.
- Noise – The spinning platters and read/write heads make noise when accessing data.
- Moving parts – The mechanical parts of HDDs mean they are less shock/vibration resistant.
- Higher power consumption – HDDs require more power to spin the platters.
- Heavier – The mechanical internals make HDDs much heavier than SSDs.
Internal Components and Operation
SSD Internal Components
Inside an SSD are printed circuit boards, an SSD controller, and NAND flash memory chips. The controller manages all read/write operations on the NAND chips. Interfaces like SATA, PCIe, or NVMe are used to connect the SSD to the computer.
NAND flash memory stores data in an array of memory cells made from floating gate transistors. Unlike volatile RAM, NAND flash retains data even when power is removed. However, the cells have a limited lifespan and will eventually wear out after repeated writes.
HDD Internal Components
Inside a hard disk are one or more circular platters coated with a magnetic recording material, stacked on a spindle and rotated at high speed. A read/write head moves back and forth over the platters on an actuator arm, reading and writing data.
The platters are divided into billions of tiny regions called sectors. Data is stored by magnetizing a region with one orientation to represent a “1” or the opposite orientation to represent a “0”. The sectors are accessed in concentric tracks across the platters.
One of the most significant differences between SSDs and HDDs is performance.
SSDs are significantly faster than HDDs in almost every category of performance measurement:
|Sequential Read||Up to 3500 MB/s||Up to 200 MB/s|
|Sequential Write||Up to 3000 MB/s||Up to 150 MB/s|
|Random Read||Up to 680K IOPS||Up to 1500 IOPS|
|Random Write||Up to 600K IOPS||Up to 600 IOPS|
This massive performance advantage comes from the fact that SSDs have no moving parts. When data is requested from an SSD, it can retrieve it nearly instantly from the NAND chips. HDDs must physically spin the platter and move the read head to find the requested data, which takes significantly longer.
Closely related to speed is latency, which is the delay between requesting data and receiving it from the drive. Again, SSDs have a huge advantage due to their lack of moving parts:
|Operation||SSD Latency||HDD Latency|
|Read||0.1 ms||10-20 ms|
|Write||0.2 ms||10-20 ms|
For perspective, the blink of a human eye takes about 300-400 milliseconds. This is why SSDs feel so much faster in normal use – programs launch instantly, files open right away, etc. due to the extremely low latency.
Reliability and Endurance
When it comes to reliability, SSDs and HDDs both have advantages and disadvantages:
- SSDs have no moving parts, making them more resistant to shocks and vibration.
- However, SSDs have a limit on how many writes can be done to their memory cells before they wear out.
- HDDs can be written hundreds of thousands of times or more before failure.
- But HDDs are susceptible to damage or data loss due to impacts during operation.
Most modern SSDs are rated for hundreds to thousands of terabytes written over the lifespan of the drive. For typical consumer workloads, this often equates to many years of use before wear becomes a concern.
When SSDs first hit the market, their capacities were very limited compared to HDDs. But over time, SSD capacities have increased while prices have declined. Here’s a look at typical capacities available today:
|SSD||250GB to 8TB|
|HDD||500GB to 14TB|
While HDDs still offer larger maximum capacities, SSD sizes have gotten large enough for most general consumer and business needs. And SSD capacities continue to grow – 16TB and larger SSDs are on the horizon.
In the past, HDDs held a decisive price advantage over SSDs for the same capacities. But with SSD prices plummeting in recent years, the cost gap has narrowed substantially. Here’s a rough comparison between SSD and HDD costs for popular consumer models:
|Drive Type||Price Range (1TB)|
|SSD (SATA)||$80 – $150|
|HDD (SATA)||$40 – $60|
While HDDs are still cheaper per gigabyte, SSD prices have fallen enough that the speed and performance benefits often outweigh the higher initial cost for many consumers. Heavy workloads like gaming, CAD/CAM design, HD video editing, and programming all benefit tremendously from the speed of SSDs.
SSDs and HDDs come in essentially the same physical form factors, making it easy to switch between them or use both in the same computer.
Common form factors include:
- 2.5″ SATA – Used in laptops and desktops
- M.2 – Compact, designed for laptops and mini PCs
- U.2 – Enterprise SSD standard
- PCIe Add-In Card – For high performance needs
- HHHL/FHHL – Server-sized SSDs
M.2, U.2, and PCIe SSDs utilize faster interfaces than SATA, allowing even greater speeds. But the physical connectors are interchangeable, permitting easy swap between SSDs and HDDs if desired.
Both SSDs and HDDs have a limited lifespan and will eventually fail with use over time. However, SSDs and HDDs tend to fail in different ways:
- SSDs – Gradual performance degradation as cells wear out. Still functions but speed reduces over time.
- HDDs – Mechanical failure tends to be sudden. May completely seize up with no warning.
With proper care, both types of drives can still last 5 years or longer in typical consumer use. Following manufacturer’s guidelines for use and avoiding excessive writes when possible will extend the usable life of both SSDs and HDDs.
Since they lack moving parts that need to be spun at high speed, SSDs require less power overall than HDDs – typically 30-50% less at peak load. This helps extend battery life in mobile devices. Here is an example comparison of max power draw:
|Drive Type||Max Power Draw|
|SSD (SATA)||3-4 Watts|
|HDD (SATA)||6-7 Watts|
High performance SSDs using PCIe or M.2 may require slightly more peak power, but still less than an equivalently performing HDD.
With no moving parts generating sound, SSDs run completely silent. HDDs have whirring platters and clicking read/write heads that make audible noise during operation. While not extremely loud, the noise level is noticeable, especially in quiet environments.
Both HDDs and SSDs offer full disk encryption options to protect data from unauthorized access. SSDs often have very high performance levels even with encryption enabled – sometimes even faster than unencrypted HDDs. Enabling encryption carries little to no speed penalty on most SSDs.
SSDs and HDDs work with the same file systems – like NTFS, exFAT, ext4, etc. Operating systems don’t differentiate between storage types.
However, SSDs may offer specific optimizations or benefits for some file systems over others. For example, the exFAT system is optimized for flash memory and offers high performance on SSDs.
Shock and Vibration Resistance
The lack of moving parts gives SSDs far greater tolerance to shocks and vibration. HDDs must be carefully protected to avoid damage that could ruin the drive and corrupt data. SSDs are more rugged if regularly moved or in high vibration environments like cars.
No special drivers are needed for SSDs or HDDs – both work with the standard storage drivers included in all modern operating systems. The SATA or PCIe interfaces used to connect the drives is handled automatically by the OS and applications require no adjustments or optimizations.
SSDs dramatically improve boot times for operating systems and programs. This is one of the most noticeable benefits of switching to an SSD. As an example, here is a typical Windows 10 boot time comparison:
|Drive Type||Boot Time|
The lower latency and faster data access of SSDs provides huge improvements to start up times. Upgrading an older HDD computer to an SSD will make it feel much quicker.
File fragmentation on traditional HDDs results in gradual performance degradation over time. More fragments means longer read times while the drive seeks between fragments. SSDs are not impacted by fragmentation nearly as much due to their fast random access speeds.
Lifespan and Long-Term Reliability
Both SSDs and HDDs slowly lose capacity through normal wear over time. However, SSDs will show more consistent gradual failure through cell degradation, while mechanical HDDs are more prone to sudden unexpected failure from impacts or other stress.
Under normal use and with proper maintenance, modern drives of both types can remain reliable for many years. Following manufacturer guidelines regarding temperature, shock protection, and write volumes will maximize lifespan.
Susceptibility to Magnetism
HDDs can be corrupted or damaged by strong magnetic fields, as their operation depends on magnetizing bits on the platter surfaces. SSDs are entirely electronic and immune to magnetic fields. This makes SSDs better suited for use around magnets or in high magnetic environments.
“Hybrid” drives combining both flash memory and HDD technology in a single unit are also available. These drives use a small SSD as a cache in front of a larger HDD, offering some benefits of both worlds. However, true SSDs are much faster than hybrids in most use cases.
Both SSDs and HDDs can be used in external enclosures to create large RAID arrays. There are some advantages to using SSDs in RAID configurations:
- Faster rebuilds – If a drive fails or is replaced, the RAID will rebuild much quicker with SSDs.
- Better parallelism – The fast speed of SSDs allows more drives to be added to RAID without slowdowns.
- Higher reliability – More drives can be used for redundancy while still getting great performance.
“Trim” is a command SSDs support that allows the drive to efficiently handle garbage collection and impacts write performance. Operating systems send TRIM commands to SSDs during idle time so future writes will be optimized. HDDs have no need for TRIM.
SSDs and HDDs share the same connectivity and form factors, making switching between them or mixing them easy. But SSDs offer huge performance benefits thanks to their lack of moving parts – they are faster, more reliable, and use less power. HDDs remain a cost-effective choice for high capacity storage, but for the best all-around performance, SSDs are now the preferred choice for most PC and server workloads.