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SSD vs. SSHD vs. HDD: Which one is best?

By Brien Posey

Most enterprise storage systems are now based on all-flash architectures, particularly in the case of primary data storage. Yet, in these budget-strapped times, it pays to look at drives that blend flash SSDs with hard disks to provide extra secondary capacity at an affordable price.

Multiple vendors offer these solid-state hybrid drives, which combine a traditional high-capacity HDD with high-speed SSD technology.

What is an HDD?

An HDD, or hard disk drive, is a traditional hard drive with mechanical heads that move across spinning platters. This structure comes with several pros and cons.

HDD advantages

• HDDs offer high-capacity storage at a low cost per gigabyte. Some HDDs can store 20 TB or more on a single disk for a few hundred dollars.
• HDDs have been used for decades. As such, they are an extremely mature technology that is widely supported and quite reliable.

HDD disadvantages

• HDDs depend on mechanical components and are therefore slower than SSDs.
• Because HDDs depend on mechanical parts, they will eventually wear out with enough use.
• HDDs are heavier and bulkier than SSDs. They are also more likely to suffer damage if dropped.
• Because HDDs are mechanical devices, they can be loud and tend to consume more power and give off more heat than SSDs.

What is an SSD?

SSD stands for solid-state drive. Rather than storing data on magnetic platters like HDDs do, SSDs store data in NAND flash memory.

SSD advantages

• Because SSDs don't include any moving parts, they far outperform HDDs, making them the best choice for overall performance.
• SSDs generally consume less power than HDDs, give off less heat and run silently.
• SSDs are available in small form factors, such as M.2 and NVMe.

SSD disadvantages

• Despite their significant capacity gains in recent years, SSDs have a smaller storage capacity than some HDDs.
• SSDs tend to have a higher cost per gigabyte, with prices increasing sharply as the capacity increases.
• The memory cells that SSDs use to store data wear out with use. Early SSDs had relatively short lifespans, but newer models have failure rates that are more in line with HDDs.

What is an SSHD?

A solid-state hybrid drive, or SSHD, is also known as a hybrid hard drive. An SSHD is a device designed to give users the best of both worlds by combining flash memory storage and HDD storage into a single device. SSHDs have solid-state storage built in but also use HDD technology as a way of providing additional storage space. Although these drives are still used, they have become far less common now that multi-terabyte SSDs are available.

SSHDs work by caching frequently used files to flash storage while retaining less commonly used files on spinning media. This means that an SSHD drive initially performs similarly to an HDD, but as the drive learns which files are used most often -- and caches those frequently used files -- it begins to deliver SSD-like performance.

SSHD advantages

• SSHDs are faster than HDDs for frequently accessed files, and they are less expensive than large SSDs.
• SSHDs offer storage capacities that are similar to HDDs.
• Because SSHDs cache frequently used files, they can improve performance for some applications.
• SSHDs deliver similar benefits to storage tiering but with less complexity because they require only a single disk.

SSHD disadvantages

• SSHDs are nearly obsolete since multi-terabyte SSDs have become affordable.
• There is a limit to the amount of SSD cache that is present within an SSHD. The amount of available SSD storage is far less than what you would get with an SSD.
• Performance can be somewhat unpredictable because it is difficult to know for sure which files have been cached.
• SSHDs are subject to the same wear issues as SSDs. They also contain moving parts, like HDDs do, making them vulnerable to shock and mechanical failure.

Before committing to a hybrid drive, it pays to understand the strengths and weaknesses of SSHDs vs. SSDs vs. HDDs.

How do HDDs, SSDs and SSHDs compare?

Generally speaking, HDDs offer the highest capacity with the lowest cost per gigabyte, while SSDs tend to have smaller capacities but far better performance at a higher cost per gigabyte. SSHDs have historically fallen somewhere in the middle.

Even though using an SSHD is still a viable option in many cases, the SSHD market seems to be shrinking. A search on Amazon for the term SSHD reveals far fewer results than a search for SSD or HDD. This is likely because SSDs have become more affordable in recent years and SSD manufacturers have been able to improve storage capacity and drive longevity. As such, it likely makes more sense to simply use an SSD instead of an SSHD.

To put this difference into perspective, Amazon currently sells a 2 TB Seagate FireCuda SSHD for $134.41. This 2.5-inch disk contains 2 TB of HDD storage accelerated with flash storage. According to the manufacturer, the drive's flash-accelerated technology can improve load time by five times over what would be possible without flash memory.

By way of comparison, Amazon sells an all-flash Seagate FireCuda SSD with 2 TB of storage for $164.99. In other words, there is a $30.58 difference between the hybrid model and a comparable all-flash model, though this particular all-flash model is based on the NVMe standard. The all-flash drive offers sequential read/write speeds up to 7,400 MBps; though no read/write speeds are given for the hybrid model, its spinning media cannot match the SSD's speed.

It's worth noting that Seagate's FireCuda products are premium drives marketed toward gamers. It's possible to get a 2 TB all-flash drive for less than the cost of a FireCuda 2 TB SSHD drive. For example, Amazon currently sells a 2 TB Western Digital Black NVMe SSD for $129. The advertised speed for this drive is 7,250 MBps, which is nearly as fast as the FireCuda SSD.

Amazon's current price for Seagate's 2 TB, 7,200 rpm BarraCuda HDD is $66.99. In other words, the all-flash FireCuda drive costs more than twice as much for the same capacity.

	HDD	SSD	SSHD
Cost	Inexpensive	Reasonably priced for disks with a couple of terabytes of storage, though prices increase sharply for larger disks	Prices are on par with SSDs of a similar capacity
Performance	Good	Best	Better
Longevity	Reliable but susceptible to shock and mechanical failures that occur as a result of wear	Memory cells wear out with use, but current-generation SSDs are vastly more durable than early models	Susceptible to both mechanical wear and SSD wear
Noise	Makes a noticeable sound	Silent	Similar sound level to an HDD
Power consumption	Requires more power than an SSD	Requires less power than an HDD	Requires a similar amount of power to an HDD

Which storage type is best?

There is no clear-cut answer as to which type of storage is best. It ultimately comes down to use case.

For laptops?

Most of the time, SSD storage is going to be the preferred storage type for laptops. Nearly all laptops being sold today include SSD storage, with many vendors offering multi-terabyte options.

For desktops?

In the case of a desktop computer, the best storage device often depends on budget and how the device is being used. For instance, for users in need of high performance, consumer-grade SSDs with up to 4 TB of storage come at a price that is roughly on par with their HDD counterparts. For those who require more storage, however, HDDs are still the best option. Many desktop users choose to place the OS on an SSD and then use a separate -- not hybrid -- HDD for data storage.

At one time, hybrid drives offered a compelling alternative to pricey SSDs. Today, however, hybrid drives are nearly extinct, and so organizations would do well to adopt SSDs instead.

Brien Posey is a former 22-time Microsoft MVP and a commercial astronaut candidate. In his more than 30 years in IT, he has served as a lead network engineer for the U.S. Department of Defense and a network administrator for some of the largest insurance companies in America.