SSD vs HDD which is better in 2021? Comparison between Solid State Drive vs Hard Disk Drive

A solid-state drive (SSD) is a solid-state storage device that uses integrated circuit assemblies to store data persistently, typically using flash memory, and functioning as secondary storage in the hierarchy of computer storage. It is also sometimes called a solid-state device or a solid-state disk, even though SSDs lack the physical spinning disks and movable read–write heads used in hard disk drives (HDDs) and floppy disks.
Compared with electromechanical drives, SSDs are typically more resistant to physical shock, run silently, and have quicker access time and lower latency. SSDs store data in semiconductor cells. As of 2019, cells can contain between 1 and 4 bits of data. SSD storage devices vary in their properties according to the number of bits stored in each cell, with single-bit cells ("SLC") being generally the most reliable, durable, fast, and expensive type, compared with 2- and 3-bit cells ("MLC" and "TLC"), and finally quad-bit cells ("QLC") being used for consumer devices that do not require such extreme properties and are the cheapest of the four. In addition, 3D XPoint memory (sold by Intel under the Optane brand), stores data by changing the electrical resistance of cells instead of storing electrical charges in cells, and SSDs made from RAM can be used for high speed, when data persistence after power loss is not required, or may use battery power to retain data when its usual power source is unavailable. Hybrid drives or solid-state hybrid drives (SSHDs), such as Apple's Fusion Drive, combine features of SSDs and HDDs in the same unit using both flash memory and a HDD in order to improve the performance of frequently-accessed data.
SSDs based on NAND Flash will slowly leak charge over time if left for long periods without power. This causes worn-out drives (that have exceeded their endurance rating) to start losing data typically after one year (if stored at 30 °C) to two years (at 25 °C) in storage; for new drives it takes longer. Therefore, SSDs are not suitable for archival storage. 3D XPoint is a possible exception to this rule; it is a relatively new technology with unknown long-term data-retention characteristics.

SSDs can use traditional HDD interfaces and form factors, or newer interfaces and form factors that exploit specific advantages of the flash memory in SSDs. Traditional interfaces (e.g. SATA and SAS) and standard HDD form factors allow such SSDs to be used as drop-in replacements for HDDs in computers and other devices. Newer form factors such as mSATA, M.2, U.2, NF1, XFMEXPRESS and EDSFF (formerly known as Ruler SSD) and higher speed interfaces such as NVM Express (NVMe) over PCI Express can further increase performance over HDD performance.

SSDs have a limited number of writes, and will be slower the more filled up they are.

Comparison of NAND-based SSD and HDD

Price per capacity
▸SSDs generally are more expensive than HDDs and expected to remain so into the next decade. SSD price as of first quarter 2018 around 30 cents (US) per gigabyte based on 4 TB models. Prices have generally declined annually and as of 2018 are expected to continue to do so.

▸HDD price as of first quarter 2018 around 2 to 3 cents (US) per gigabyte based on 1 TB models. Prices have generally declined annually and as of 2018 are expected to continue to do so.

Storage capacity
▸In 2018, SSDs were available in sizes up to 100 TB, but less costly, 120 to 512 GB models were more common.

▸In 2018, HDDs of up to 16 TB were available.

Reliability – data retention
▸If left without power, worn out SSDs typically start to lose data after about one to two years in storage, depending on temperature. New drives are supposed to retain data for about ten years. MLC and TLC based devices tend to lose data earlier than SLC-based devices. SSDs are not suited for archival use.

▸If kept in a dry environment at low temperatures, HDDs can retain their data for a very long period of time even without power. However, the mechanical parts tend to become clotted over time and the drive fails to spin up after a few years in storage.

Reliability – longevity
▸SSDs have no moving parts to fail mechanically so in theory, should be more reliable than HDDs. However, in practice this is unclear.
Each block of a flash-based SSD can only be erased (and therefore written) a limited number of times before it fails. The controllers manage this limitation so that drives can last for many years under normal use. SSDs based on DRAM do not have a limited number of writes. However the failure of a controller can make an SSD unusable. 
Reliability varies significantly across different SSD manufacturers and models with return rates reaching 40% for specific drives. Many SSDs critically fail on power outages; a December 2013 survey of many SSDs found that only some of them are able to survive multiple power outages. 
A Facebook study found that sparse data layout across an SSD's physical address space (e.g., non-contiguously allocated data), dense data layout (e.g., contiguous data) and higher operating temperature (which correlates with the power used to transmit data) each lead to increased failure rates among SSDs.
However, SSDs have undergone many revisions that have made them more reliable and long lasting. 
New SSDs in the market today use power loss protection circuits, wear leveling techniques and thermal throttling to ensure longevity.

▸HDDs have moving parts, and are subject to potential mechanical failures from the resulting wear and tear so in theory, should be less reliable than SSDs. However, in practice this is unclear. 
The storage medium itself (magnetic platter) does not essentially degrade from reading and write operations.
According to a study performed by Carnegie Mellon University for both consumer and enterprise-grade HDDs, their average failure rate is 6 years, and life expectancy is 9–11 years. However the risk of a sudden, catastrophic data loss can be lower for HDDs.
When stored offline (unpowered on the shelf) in long term, the magnetic medium of HDD retains data significantly longer than flash memory used in SSDs.

Start up Time
▸Almost instantaneous; no mechanical components to prepare. May need a few milliseconds to come out of an automatic power-saving mode.

▸Drive spin-up may take several seconds. A system with many drives may need to stagger spin-up to limit peak power drawn, which is briefly high when an HDD is first started.

Noise (acoustic)
▸SSDs have no moving parts and therefore are silent, although, on some SSDs, high pitch noise from the high voltage generator (for erasing blocks) may occur.

▸HDDs have moving parts (heads, actuator, and spindle motor) and make characteristic sounds of whirring and clicking; noise levels vary depending on the RPM, but can be significant (while often much lower than the sound from the cooling fans). Laptop hard drives are relatively quiet.

Temperature Control
▸A Facebook study found that at operating temperatures above 40 °C, the failure rate among SSDs increases with temperature. However, this was not the case with newer drives that employ thermal throttling, albeit at a potential cost to performance. In practice, SSDs usually do not require any special cooling and can tolerate higher temperatures than HDDs. High-end enterprise models installed as add-on cards or 2.5-inch bay devices may ship with heat sinks to dissipate generated heat, requiring certain volumes of airflow to operate.

▸Ambient temperatures above 35 °C (95 °F) can shorten the life of a hard disk, and reliability will be compromised at drive temperatures above 55 °C (131 °F). Fan cooling may be required if temperatures would otherwise exceed these values. In practice, modern HDDs may be used with no special arrangements for cooling.

Lowest operating temperature
▸SSDs can operate at −55 °C (−67 °F).

▸Most modern HDDs can operate at 0 °C (32 °F).

Weight and size
▸SSDs, essentially semiconductor memory devices mounted on a circuit board, are small and lightweight. They often follow the same form factors as HDDs (2.5-inch or 1.8-inch) or are bare PCBs (M.2 and mSATA.) The enclosures on most mainstream models, if any, are made mostly of plastic or lightweight metal. High performance models often have heatsinks attached to the device, or have bulky cases that serves as its heatsink, increasing its weight.

▸HDDs are generally heavier than SSDs, as the enclosures are made mostly of metal, and they contain heavy objects such as motors and large magnets. 3.5-inch drives typically weigh around 700 grams (about 1.5 pounds).

Power consumption
▸High performance flash-based SSDs generally require half to a third of the power of HDDs. High-performance DRAM SSDs generally require as much power as HDDs, and must be connected to power even when the rest of the system is shut down. Emerging technologies like DevSlp can minimize power requirements of idle drives.

▸The lowest-power HDDs (1.8-inch size) can use as little as 0.35 watts when idle. 2.5-inch drives typically use 2 to 5 watts. The highest-performance 3.5-inch drives can use up to about 20 watts.

Maximum areal storage density
▸SSD-2.8 Terabits per square inch

▸HDD-1.2 Terabits per square inch

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