Learn the pros and cons of choosing an SSD over an HDD, and how SSDs can speed up your computer storage for everything from business solutions to gaming.
SSD vs HDD
HDD is a conventional storage device that reads and writes data using rotating disks.
Compared to HDDs, SSDs are faster and use less power.
Compared to HDDs, SSDs are faster and use less power.
While SSD prices are declining, HDDs are still less expensive.
Choosing the right storage for you is not just about comparing capacity and cost. The type of storage you use for your computer affects performance, including power usage and reliability. Solid-state drives (SSDs) and hard disk drives (HDDs) are the two main storage options to consider. This article provides a brief introduction to how to best use each and compare them.
What is HDD?
A hard disk drive (HDD) is a data storage device found inside a computer. It has a spinning disk inside, where data is stored magnetically. HDDs have an arm with multiple heads (transducers) that read and write data on the disk. It works similarly to how a turntable record player works, with an LP record (hard disk) and a needle (transducer) attached to the arm. The arm moves the head over the surface of the disk to access different data.
HDDs have been around longer than SSDs and are considered a more traditional technology. HDDs are generally low-cost and practical for data that does not need to be accessed frequently, such as photos, videos, and backups of business files. HDDs come in two common form factors: 2.5-inch (for notebooks) and 3.5-inch (for desktops).
What is an SSD?
The name SSD, or “solid state,” comes from the use of solid-state elements inside. In SSDs, all data is stored on integrated circuits. This difference from HDDs has many important aspects, especially in size and performance. Because SSDs do not require spinning disks, they can be as small as a stick of chewing gum (known as the M.2 form factor) or even as small as a postage stamp. They also vary in capacity (amount of data they can store), making them more flexible for small devices such as thin laptops, convertibles, and 2-in-1s. Also, SSDs do not need to wait for the platters to start spinning, which significantly reduces access times.
SSDs are more expensive per gigabyte (GB) or terabyte (TB) of storage capacity than HDDs, but the price gap is narrowing as SSD prices are falling faster than HDD prices every year.
SSD vs HDD: Speed
The reason SSDs are becoming more popular is their speed. Overall, SSDs operate faster than HDDs because they use electrical circuits and have no physical moving parts. This results in less latency during boot-up and less delays when launching apps and performing heavy computing tasks. For example, the Intel® SSD D5-P5316 is a 15.36TB enterprise-grade SSD with a bandwidth of over 7000 MB/s. Meanwhile, a compatible HDD, the 14TB Seagate Exos 2×14, only has a bandwidth of up to 500 MB/s. That’s a whopping 14 times the difference.1
These speed improvements translate into better performance in several scenarios, such as logging in, waiting for apps and services to launch, and performing storage-intensive tasks like copying large files. While HDDs experience a significant drop in performance, SSDs allow other tasks to continue.
Speed is also affected by the interface that connects to other computer systems when sending and receiving data. You may have heard of interfaces such as SATA and PCI Express (PCIe). SATA is an old, slow, and conventional technology, while PCIe is newer and faster. SSDs with PCIe interfaces are usually much faster than SATA HDDs because they have more channels for data transfer. It’s the same reason why more cars can travel on a four-lane highway than on a one-lane country road. No
one ever complains that their computer is too fast, but there are times when HDDs are enough. If the files you need to store are in the terabytes in size, HDDs are still the low-cost option, but SSDs are becoming cheaper and newer NAND technologies are becoming more and more popular that offer higher bit density per NAND die. Computer storage can be easily determined by thinking of data as cold or hot. An example of “cold” data would be years’ worth of photos that you don’t look at every day and don’t need quick access to, but you still want to store them on your laptop. HDDs are a good, cost-effective choice for cold data. Conversely, if your business does real-time trading, video and photo editing and requires fast access to databases of files, video clips or models, or the data required to run your operating system is called “hot”, the fast performance of an SSD makes it the best choice when quick access to data is paramount.
SSD vs HDD: Durability
Because data is written in pages but erased in blocks, the amount of wear caused by writing to a NAND SSD also depends on the state of the data already on the drive. When writing sequential data to a relatively new SSD, the data can be efficiently written to contiguous free pages on the drive. However, when a small block of data needs to be updated (a document revision or a numeric change), the old data is read into memory, revised, and written back to a new page on disk. The old pages containing the obsolete data are marked as invalid. When there are no more free pages, these “invalid” pages are made free for use by a background process called “defragmentation” or “wear leveling.” All existing valid pages in a particular block must first be copied to other free locations on the drive, so that the original block contains only invalid, obsolete pages. The original block can then be erased to free up space for new data to be written. Due to
internal NAND maintenance processes such as wear leveling, write amplification occurs, causing the total amount of internal writes in the SSD to be greater than the amount of writes required simply to store new data on the drive. Write amplification is one of the main causes of wear, as each write slightly degrades each NAND cell. Due to internal processes, NAND SSDs distribute wear evenly across the drive. However, it is important to note that write-heavy workloads, especially random writes, cause high write amplification and therefore wear out NAND SSDs faster than other input/output (I/O) patterns. Fortunately, SSD drive
endurance levels are always specified with the worst-case random write pattern in mind. For example, if a drive is rated to be able to be written once per day, that means that you can write at least the full capacity of the drive with daily random write usage for the life of the drive’s warranty (usually five years).
Showdown: SSD vs HDD
In terms of capacity, SSDs for computers come in capacities from 120GB to 30.72TB, compared to 250GB to 20TB for HDDs. In terms of cost per capacity, HDDs win, but as SSD prices fall, they will likely become less of a differentiator for HDDs in the future. However, SSDs do much less work per server, so fewer devices are needed to get the same output as HDDs. The result? SSDs have a lower TCO (total cost of ownership).
Reliability means whether data is stored as intended and without corruption. SSDs are generally more reliable than HDDs. Again, because they have no moving parts. Because they have no moving parts, SSDs are not affected by vibrations and associated thermal issues. SSDs
generally have much faster data access and consume less power and last longer on batteries because the device is idled more often. HDDs have spinning disks, so they require more power at startup than SSDs.
Cost savings of SSDs over HDDs
We’ve seen that SSDs perform much better than HDDs. We’ve seen that SSDs have a lot more reliability advantage. Given these inherent advantages, SSDs don’t require replication for performance, and they generally require much less replication for reliability. The increased performance of SSDs also contributes to much more efficient data reduction methods than HDDs. The data reduction ratio is the percentage of host data that is stored on the required physical storage. 50% equates to a data reduction ratio of 2:1. Data reduction allows users to store more data than is available on the physical hardware, resulting in increased effective capacity. Compression and deduplication techniques can significantly reduce the raw storage capacity required to meet “usable capacity” requirements.
Modern algorithms are optimized for SSDs and leverage their performance to achieve high Data Reduction Ratios (DRR) and boost application performance. For example, Facebook’s Zstandard compression algorithm compresses and decompresses at speeds much faster than HDD reads and writes, allowing the algorithm to run in real time on SSDs2. Additionally, VMware vSAN only offers compression and deduplication in all-flash configurations.