While SSDs are more expensive than HDDs, solid-state storage yields significant benefits. The decision of storage type often comes down to type of business and specific needs.
SSDs have made inroads in the consumer market, but the common prediction that they would replace HDDs to meet future enterprise storage and application requirements hasn't panned out. SSD expansion hasn't been as all-encompassing in data center and cloud environments, which continue to rely heavily on HDDs for most of their data storage.
Despite the continued reliance on HDDs, both types of drives play important roles in supporting today's workloads. Decision-makers tasked with choosing between them should understand how they work and their differences, considering capacity, performance, durability and other factors.
What is an SSD?
A solid-state drive is a type of non-volatile storage device that stores data on a solid-state integrated circuit. The storage drive has no moving parts and is optimized for high performance and low latency. Most of today's SSDs contain one or more NAND flash memory chips, where the data is stored as electrical charges. The drive also contains a controller that manages data storage, as well as read/write operations.
SSDs are often categorized by the number of bits that can be stored in each cell:
Single-level cell (SLC). Each cell contains only one bit of data.
Multi-level cell (MLC). Each cell contains up to two bits of data.
Triple-level cell (TLC). Each cell contains up to three bits of data.
Quad-level cell (QLC). Each cell contains up to four bits of data.
With increasing bits in each cell, the drive's density grows. Some manufacturers also reduce the size of the cell to squeeze more cells into a drive, further increasing the density. However, increased density can come at the expense of performance and durability. In addition, data is read and written at the page level but erased at the block level, which can further hurt performance and durability.
To help compensate for these issues, the controller carries out operations such as wear leveling, overprovisioning and error correction code. In addition, many vendors now offer 3D NAND chips, which stack the cells on top of each other to create multiple layers, which helps increase density and maintain performance.
SLC, MLC, TLC and QLC flash have different performance levels and price points.
What is an HDD?
A hard disk drive is another type of non-volatile storage device, except it contains moving parts. An HDD uses electromagnetic technologies to store data on spinning disks, or platters, which are coated with magnetic material. The platters are enclosed in a protective case, along with a controller that manages how the data is stored, accessed and written across the platters. Data is stored on an HDD in blocks that its actuator heads can randomly read.
HDDs come in many form factors. Enterprises use them for both primary and secondary storage. They also support specific interface types, such as SATA and SAS.
An HDD might be installed within a computer or in an external storage unit. In enterprise settings, it's often part of a storage array for improved performance and reliability. HDDs can vary significantly in terms of storage capacity and performance. Even the most advanced enterprise HDD, though, cannot deliver the same level of performance as an SSD.
Comparison of SSD vs. HDD by capabilities
When choosing between SSDs and HDDs, decision-makers should understand how the two technologies compare across capacity, performance, ease of use, durability and cost.
Capacity
Many of today's SSDs offer far greater densities than HDDs. For example, multiple vendors now offer drives that can store over 60 TB. In fact, Nimbus Data sells an SSD that supports up to 100 TB. The greater densities translate to a smaller footprint and less demand on data center resources.
On the other hand, enterprise HDDs, such as Seagate's Exos Mozaic 3+, currently top out at 32 TB, and they require more space and resources to operate.
Speed
Performance is the primary reason that organizations are willing to pay the extra cost per terabyte for SSDs. HDDs might be the workhorses of the data center, but they cannot compete with SSDs when it comes to faster speeds and lower latencies, an important consideration for mission-critical applications.
For instance, Micron's 9550 NVMe SSD promises up to 14 GBps sequential reads and 10 GBps sequential writes. Compare this to Seagate's Mach.2 Exos 2X14, a dual-actuator HDD that can deliver a maximum sustained transfer rate of 524 MBps.
Ease of use
SSDs perform better than HDDs, enabling end users to resolve issues quicker and achieve greater productivity. Administrators also benefit from the increased durability in SSDs.
SSDs are quieter, which helps decrease noise levels in their work environments, and they produce less heat, which reduces cooling requirements. In addition, SSDs don't need to be defragmented like HDDs, although they still require a certain level of maintenance, such as updating the firmware and monitoring the drive's health.
Durability
SSDs have a big advantage over HDDs because they have no moving parts. At its core, an HDD is a mechanical device with components that continuously spin, turn or reposition themselves. The components wear out, and the storage drive is more susceptible to physical damage. For example, knocking over an HDD could misalign the actuator arms, causing the drive to malfunction.
HDDs also generate more heat, which can hurt durability. Admins understand HDD technology well, however, and they can take steps to reduce risks. Besides, an SSD's memory cells can also break down over time, impacting the drive's life span. As a result, SSD controllers perform operations such as wear leveling or overprovisioning, which help to extend the life of the cells.
Cost
The traditional HDD continues to dominate in data centers because the cost per terabyte remains significantly lower than SSDs. For example, Micron's 9300 Pro NVMe SSD costs about $1,700 for 15.36 TB, which comes to $111/TB. In contrast, Seagate's Exos X18 HDD runs $299 for 16 TB, which is less than $19/TB.
But cost estimates need to consider more than just the initial investment. For instance, an SSD uses less physical storage space and electricity. It is also more durable than the HDD, resulting in a longer life span.
When to choose between SSD vs. HDD
As a result of performance, durability and energy benefits, organizations turn to SSDs to support mission-critical applications that demand high-speed data access. Business types include online banking, stock trading, airline reservations and real-time analytics.
HDDs are often a good fit whenever data quantities make SSDs too costly.
Data centers and other enterprise settings still extensively use HDDs, though, with lower cost a major reason. They are well suited to warm and cold data, such as surveillance videos or media libraries, as well as backup and archival data. This type of data often comes in large volumes, although enterprises don't need to access it as frequently as hot data. In fact, HDDs are often a good fit whenever data quantities make SSDs too costly.
Both SSDs and HDDs can play a role in AI, which requires a vast amount of data to achieve meaningful results. For example, HDDs might store the large sets of raw data and support massive data lakes, providing a central repository as data is ingested from multiple sources. SSDs can then transform the data, build large language models and carry out AI training.
Alternative storage options
Discussions about enterprise data storage often revolve around SSDs and HDDs, but organizations also have tape storage as a third option. Data centers have been using tape storage for decades and, despite the claims of its demise, continue to do so. Tape offers an ideal medium for storing large amounts of cold and inactive data, such as for backups or archives, as well as for protecting data from cyberattacks.
Tapes can be formatted in different ways, but today's most common standard is LTO, which is now in its ninth generation. A single LTO-9 tape cartridge offers up to 18 TB of native storage capacity or up to 45 TB compressed.
Organizations use tape for their cold storage because it is cheaper than disk-based storage and requires fewer resources to maintain. Tape can also retain data much longer than other media and provides greater resiliency against environmental factors. In addition, businesses can store tape without connecting to a network, which makes it more immune to ransomware and uses less energy. LTO-9 supports encryption and write-once, read-many capabilities, further improving data protection.
Tape storage comes with several challenges, though. Organizations must invest the upfront costs necessary to deploy a tape storage system. Tapes also do not support random data access -- only linear access -- so retrieving data is a slow process compared to disk storage.
Robert Sheldon is a freelance technology writer. He has written numerous books, articles and training materials on a wide range of topics, including big data, generative AI, 5D memory crystals, the dark web and the 11th dimension.
