SSD vs. HDD -


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The debate between SSD vs. HDD is a key debate in current storage. Today’s data centers include a mix of HDDs and SSDs. Legacy storage systems are HDD-only. Newer ones are often hybrid SSD/HDD arrays. And high-performance enterprise data centers will often have at least one all-flash array (AFA). 

However, there is a good deal of confusion in the storage market about which is better for their business: HDDs or SSDs? What are the real differences between SSD and HHD?

Most compelling: what's the price comparison between SDD and HDD?

Note that the enterprise does not have a lock on HDD vs. SSD discussions. An SMB may be a small digital effects firm that needs all flash arrays for rendering CGI effects. In contrast, an enterprise may have invested heavily in SaaS and online backup and archiving and is divesting on-premises arrays.

Ultimately HDD and SSD choices are driven not by the size of the business, but by workloads and storage budgets. Companies of any size will optimize their storage purchases by understanding HDD and SDD differences, and how those differences affect purchasing decisions.

Also see: The Performance and Reliability differences between SSD and HDD.

Jump to:

HDD and SSD: Inside the Technology

Hard disk drives:

Hard-disk drives (HDDs) rely on spinning platters and read/write heads to magnetically store data or retrieve it. They are the typically the default storage option in PCs, servers and storage arrays.

As an HDD's platters spin, read/write heads sweep just above the surface, turning magnetic fields into electrical currents that translate into data, also known as the "read" functionality provided by HDDs. Conversely, read/write heads can use electrical signals to manipulate the magnetic fields on a drive platter to encode or "write" data.

These mechanical parts create bottlenecks that limit how fast data can be read or written. This is somewhat mitigated on some HDDs by the higher rotational speeds, or revolutions per minute (RPM) of the platters.

Today it is common to see 5,400 RPM and 7,200 RPM drives in laptops and desktop PCs. HDDs that spin at 10,000 RPM and 15,000 RPM are typically found in high-performance enterprise storage systems, often shortened to 10K and 15K drives.

Another method hard drive makers use to improve performance is placing a larger disk buffer or drive cache in a hard drive. Working as high-speed memory that temporarily stores data, disk buffers can be found with up to 256MB of memory, as in the case of some Seagate BarraCuda HDDs.

hard disk drive

The hard disk drive uses spinning disks.

Solid State Drives:

Despite these enhancements, HDDs can't hold a candle to SSDs in terms of sheer performance.

Instead of using magnetic platters and drive heads, SSDs use flash chips to store data. Using floating gate transistor or charge trap technologies, SSDs store bits—the ones and zeros that represent digital information, determined by the lack or presence of electrons—within its memory cells.

This lack of moving parts eliminates many of the storage bottlenecks introduced by spinning platters and the actuator arms used to move an HDD's write heads into position. In a PC, an SSD can help slash system boot and application load times to mere seconds. In enterprise server and storage environments, SSDs can yield snappier, more responsive application performance.

Of course, this performance comes at a cost, and in the case of flash storage, that means a higher SSD price tags and the eventual degradation of the flash chips, although most users will likely upgrade their systems well before an SSD begins to fail them.

The lack of mechanical parts in an SSD means that they are less prone to being damaged by vibrations or contaminants.

HDDs are built to strict tolerances and their read/write heads sweep across the surface of rotating platters with a head gap, also called a or flying of floating height, of just a few nanometers. If a read/write heads makes contact with a platter, it can lead to data loss or a damaged drive. Many hard drives have shock or drop sensors that will cause their drive heads to automatically retract if vibrations pose a risk or a fall is detected.

solid state drive

The solid state drive is known for far greater speed than the HDD.

HDDs also use more electricity than SSDs and produce more heat.

The motors used to keep the platters spinning and actuator arms all require more electricity than is required to pump electrons through an SSD's chips. Platters are also susceptible to friction from the surrounding air, although vendors have begun filling some of their drives with helium, a gas that is less dense than air.

Finally, we come to the crux of the HDD and SSD debate: the cost and capacity.

On a per-gigabyte (GB) basis, SSDs are simply more expensive than HDDs. The gap has narrowed significantly over the years, but HDDs prices still remain much less expensive in this regard. Today, IT buyers can snag a 12TB enterprise HDD for a little less than a 1TB SSD hard drive.

It's a state of affairs that is likely to continue for the foreseeable future.

Let's recap the underlying technology of HDD and SDD:


  • Use magnetic platters and moving parts to store data.
  • Are the data storage standard for PCs, servers and enterprise storage arrays.
  • Have lower price tags than SSDs on a per-GB basis.
  • Are slower than SSDs.
  • Use more electricity than SSDs and require more cooling.


  • Use semiconductor chips to store data and have no moving parts.
  • Are gaining popularity but remain a pricey storage alternative for PCs, servers and storage systems.
  • Have higher price tags than HDDs on a per-GB basis.
  • Are much faster than HDDs
  • Use less electricity than HDDs and run cooler.

Comparing SSD and HDD: Side by Side

·  Read and write speeds. SDDs are significantly faster than HDDs. HDD platters spin from 7,5000 rpm to 15,000 rpm. The read/write heads position themselves over the spinning platters to read or write data. Sequential reads and writes are efficient, but when discs are crowded with data the heads must access multiple sectors – a operation called fragmenting. SSDs are not subject to fragmenting because read/write operations access cells simultaneously. This makes SSDs much faster even then 15K RPM enterprise drives.

·  Capacity. SSD capacity has overtaken HDDs. Nimbus recently displayed a 100TB 3D NAND flash SSD in a 3.5-inch form factor. The vendor will release the drive summer 2018. HDD manufacturers are still working to increase areal density. Toshiba recently introduced a 14TB 3.5-inch form factor HDD that uses conventional magnetic recording, as opposed to higher capacity shingled magnetic recording. The Toshiba HDD encloses nine 7mm disks.

·  Encryption. Software-based encryption works on both HDDs and SDDs. However, this type of encryption is password-protected. Data passes through an algorithm that encrypts data as it writes to disk, and de-encrypts data upon read. The function is simple and inexpensive, but passwords are vulnerable to storage system hacks. Software-based encryption also puts a heavy load on CPU resources. A separate processor enables hardware-based encryption and is more secure than password-protected software. The dedicated device takes fewer cycles from the CPU. SSDs were initially a challenge for hardware-based encryption, but AES encryption administered by a cryptoprocessor has had success. The device is located on a chip or microprocessor in the SSD.

·  Pricing. Tape is the cheapest form of storage followed by HDDs, followed by SSDs. HDD prices lower year after year. So do enterprise SSDs, which also steadily increase their capacity and performance. Over the past several years, the prices of flash chips have decreased about 30% per year. Today enterprise HDD prices are roughly commensurate with enterprise SSDs at about $.25-$.27 per GB. Lower performance/higher capacity HDDs at 7200 RPM are much lower at $.02-$.03 per GB.

·  Reliability. Flash drives under 2-3 years in age have a significantly lower ARR (Annual Replacement Rate) than hard drives. HDDs measure their reliability by running clusters of disk models and families and use the resulting numbers to produce mean time between failures (MTBF) or annualized failure rates (AFR).

·  Durability. As the SSD ages, the story changes. In some tests 20% of flash drives developed uncorrectable errors in a four-year period, which is considerably higher than hard drives. Additional tests over a 32-month period concluded that 30%-80% of flash drives developed bad blocks during their lifetime, while HDDs developed only 3.5% bad sectors. Since hard drive sectors are smaller than flash drive blocks, HDD sector failures impact less data. Why such a large range of 30%-80% for SSDs?

This is partially due to three different vendor measurements for SSD reliability and durability: standard age, total written TBs, and the average amount of TBs written to the drive within a specified time like a week or day. Three very different measurements, along with the difficulty of accurately measuring the results, makes it challenging to predict actual wear over time. A joint study between Google and the University of Toronto reached some interesting conclusions. The authors wrote in the paper’s abstract, “We see no evidence that higher-end SLC drives are more reliable than MLC drives within typical drive lifetimes." When comparing traditional hard disk drives and flash drives, flash drives have a significantly lower replacement rate in the field, however, they have a higher rate of uncorrectable errors.”

·   Workloads. SSDs are ideal for high performance processing, whether they reside in an AFA or as Tier 0/Tier 1 media in hybrid storage arrays. Traditionally companies reserved SSDs for high performance applications and HDDs for high capacity. But there are a growing number of applications that require both capacity and performance, such as cloud-based video and media streaming. HDD value is high capacity nearline tiers and long-term retained data retention environments.

SSD vs. HDD Drawbacks and Benefits

ssd vs. hdd

The Future for SDD and HDD

Some analysts believe that archiving will be the single largest use case for hard drives, and that production environments will adopt all flash over the next few years. I don’t doubt that for tier 0 and tier 1 data in transactional processing environments.

We will see more SSDs in nearline and active archives that are subject to analysis. Aging unstructured data comprises at least 80% of data in most corporate environments, and that is a low estimate. Much of this data presently exists in nearline, archival and big data environments and is important for historical and trending business analysis. Since analytics operations are performance-intensive, they do well on high-performance SSDs.

However, SSD’s deep inroads into traditional HDD storage is not the death knell it may appear to be for HDDs. Hard disk drives remain strong sellers for two major use cases in the data center and the cloud: nearline storage and long-term data retention.

·  Nearline SATA disk. HDD per-disk capacity is not growing as quickly as SSD capacity. However, high capacity SATA drives are extremely inexpensive per GB, and present an excellent economy/capacity balance for large nearline storage environments. Such environments primarily serve large enterprise and hyperscale applications.

·  Long-term data retention. Compliance and litigation are major drivers for long-term storage in cold storage tiers. Long-term data retention is a poor use case for SSDs, thanks to a higher rate of uncorrectable errors over time than HDDs. Even though corporations are shifting archival and backup data to cloud-based cold storage, cloud vendors buy massive farms of hard drives to reliably retain their customers’ cold storage data.

Despite all the market energy around SSD, all-flash data centers are years away. Choosing your optimal media takes balancing reliability, durability, cost, performance, capacity and workloads.

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