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SSDs replacing HDDs?
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that's not exactly the way it happened

by Zsolt Kerekes, editor -

It irks me to see articles on the web which talk glibly about "SSDs replacing hard drives" - because that's a gross simplification. It's wrong - and shows ignorance about the rational and compelling reasons that people buy SSDs.

For example SSDs can replace servers - not just replace storage (SSD CPU Equivalency) and since 2014 (key SSD idea #3 - random access memory doesn't have to be RAM) we've seen growing evidence that flash SSDs will also replace most of the traditional demand for DRAM too.

What has actually been happening in the history of the SSD market is that in some applications the user value propositions for using SSDs provided different compelling reasons to adopt SSDs in some market segments (particularly in the enterprise) years before SSD adoption made sense in others. (See who's who in storage market research - for more data.)

Server acceleration and ruggedized environments (industrial and military) are the oldest reasons - for SSD adoption going back more than 2 decades in "getting the job done at any cost" applications.

But it wasn't a smooth ride for early SSD pioneers.

Instead the SSD accelerator market was patchy and discontinuous throughout the 1980s and 1990s - with fast SSD products randomly appearing and soon after disappearing - because the other ways of speeding up apps got better too - and scaled better than early SSDs. (Those early SSD killers were faster CPUs, commodity parallel processing - which later became multi-core CPUs, wider CPU and RAM busses - 8, 16, 32, 64, 128, and 256 bit wide RAM busses, arrays of disks - better known today as RAID, and even faster RPM hard disks - until 2000.)

The economic arguments for SSD server acceleration started changing 10 years ago from going faster at high cost to actually costing less - by using less servers in some high-end SSD accelerated apps. Then, through the 2000 to 2005 years as flash got cheaper and HDDs got bigger (but not any faster) we saw the floor price factor sweep low capacity flash SSDs into embedded systems - because low capacity SSDs actually did cost less than the lowest capacity hard drives.

Reliability in long life systems was another factor which opened business doors to SSDs in the 2005 or so era - because the cost of replacing failed cheap HDDs outweighed the initial higher cost of reliable SSDs.

From where we are today - there is high confidence in the SSD industry that 100% of new online storage drives in the enterprise will be solid state in the datacenter in the 2016 to 2020 period.

The slowest switch-off from hard drives will take place in the consumer PC market - simply because many common PC entertainment apps - don't need the performance of SSDs - whereas they do need high capacity. And although we're now 10 years into the modern era of the notebook SSD market - many early products were disappointing - which has made consumers wary of SSD promises. Eventually all storage will be solid state - but not because solid state storage media hits cost parity with magnetic media. It will happen even if hard disk storage is given away free - because hard drives won't be able to deliver the application performance or the densities needed in future systems.

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(this classic article below was published here at this url in August 2005)
War of the Disks: - Hard Disk Drives vs Flash SSDs

by - Wilson WeiSheng Wang
& - Joanne De Peralta BiTMICRO
... BiTMICRO Networks click for profile
Much has been written about solid state disks (SSDs) becoming the next big thing in the IT industry. But as to whether or not we have enough knowledge of how it will revolutionize the industry is a different question. How much do we really know about SSDs? If we are to ask people on the streets, it wouldn't come as a surprise that they don't know much of what an SSD is. Even if they do know, most probably it would be limited to a USB pen drive, a CompactFlash or a Secure Digital card, which are more on the consumer side of the storage industry.

Technically speaking, they are not wrong. Most web definitions describe an SSD as a high-performance plug-and-play storage device that contains no moving parts. Therefore, given that most of the aforementioned mobile disk storage devices contain no movable parts, they can certainly be categorized as SSDs.

But then again, there's more to SSDs than being a non-volatile device. The purpose of this article is to give readers a clearer picture of what an SSD is, its usage as well as its difference when pitted against the predominant data storage device at present - the hard disk drive (HDD). More than its non-volatility, this article will reveal how much potential SSDs have in optimizing the performance of the computing system.

SSD Defined

Here are several ways the SSD is defined.

From the article in the SPARC Product Directory - Solid State Disks - intro, applications & FAQs

"A solid state disk (SSD) is electrically, mechanically and software compatible with a conventional (magnetic) hard disk or winchester. The difference is that the storage medium is not magnetic (like a hard disk) or optical (like a CD) but solid state semiconductor such as battery backed RAM, EPROM or other electrically erasable RAMlike chip. This provides faster access time than a disk, because the data can be randomly accessed and does not rely on a read/write interface head synchronising with a rotating disk. The SSD also provides greater physical resilience to physical vibration, shock and extreme temperature fluctuations. The only downside is a higher cost per megabyte of storage."

From the webopedia SSD article

"Abbreviated SSD, a solid state disk is a high-performance plug-and-play storage device that contains no moving parts. SSD components include either DRAM or flash memory boards, a memory bus board, a CPU, and a battery card. Because they contain their own CPUs to manage data storage, they are a lot faster (18MBps for SCSI-II and 44MBps for UltraWide SCSI interfaces) than conventional rotating hard disks ; therefore, they produce highest possible I/O rates. SSDs are most effective for server applications and server systems, where I/O response time is crucial. Data stored on SSDs should include anything that creates bottlenecks, such as databases, swap files, library and index files, and authorization and login information."

For the rest of this particular article, we will compare the HDDs and Flash-based SSDs. The latter are the most popular type of SSDs employed by the military, aerospace, industrial and embedded systems industries.

(Editor's note - RAM based SSDs - used for server acceleration are an alternative technology - and can run faster than flash - but are unsuitable for mobile or embedded applications.)

Mortality Rate

Hard disk failure is a common occurrence in this age of desktops and notebooks. A lot of horror stories have been told, passed around and even published about how valuable data was lost and the agony of trying to retrieve or at least save a portion of the crashed hard drive. It is during this time that people wonder, why do hard disks fail?

The reason is simple - wear and tear. Just like any good old machine with mechanical components, HDDs tend to deteriorate over time. HDDs consist of rotating, magnetically coated disks, known as platters, that are used to store data. This rotating motion of the mechanical arms results in much wear and tear after long periods of use. The operational lifespan of a computer HDD is typically over three years .

A Flash-based SSD is different. Although the most common Flash chips have around 300,000 write cycles, the best Flash chips are rated at 1,000,000 write cycles per block. On top of that, Flash SSD manufacturers employ different ways to increase the longevity of the drives. In some cases, they use a "balancing" algorithm to monitor the number of times each disk block has been written, which greatly extends the operational lifespan of the drive. Furthermore, these manufacturers also designed special "wear-leveling" algorithms where once a certain percentage threshold for a given block is reached, the SSD controller will swap the data in that block with the data in another block that has exhibited a "read-only-like" characteristic in the background. This reduces performance lag and avoids further wearing off of the blocks and thrashing of the disk. Even with usage patterns of writing/reading gigabytes per day, a Flash-based SSD should last several years, depending on its capacity. Add to that the inclusion of a DRAM cache in the disk architecture that further enhances its operational capabilities as well as lifespan.

(Editor's note see also:- article:- Increasing Flash Solid State Disk Reliability - which discusses the effectiveness of different wearout reduction algorithms built into SSD products).

Speeding Up

With the very fast paced lifestyle these days, most businesses are time-bound and cannot afford a slowing down in their transactions. This makes speed of HDDs a crucial point in technological purchases. The typical access time for a Flash based SSD is about 35 - 100 micro-seconds, whereas that of a rotating disk is around 5,000 - 10,000 micro-seconds. That makes a Flash-based SSD approximately 100 times faster than a rotating disk.

This however raises another point - what's the benefit of a high-speed SSD when the entire system cannot support it? The evolution of CPUs in terms of performance has far surpassed the development of the data storage system. The HDD is actually limiting the potential of a computer system when they are not able to keep up with the performance of the other components, particularly the CPU. If an HDD is replaced by a high-speed SSD, the performance would significantly improve. The resulting system would be a high-powered set-up that can go beyond the demands of high-speed business or military environments.

Size Matters

To be a true plug-in replacement for HDDs, Flash-based SSDs are manufactured in standard 2.5" and 3.5" form factors. 2.5" SSDs are normally used in laptops or notebooks while the 3.5" form factors are used in desktops, or in JBOD or other SAN/NAS/DAS configurations. They can often be used to replace traditional disks in storage arrays or in a server's internal disk bays. Flash-based SSDs certainly fit the storage needs of most embedded systems that require much smaller form factors.


In terms of reliability, conventional HDDs pale when compared to SSDs. The absence of mechanical arms and spinning platters is the reason behind its reliability. In demanding environments, SSDs provide the type of ruggedness required for mobile applications. Unlike the HDD, SSD's can withstand extreme shock and vibration with data integrity and without any danger of data loss. This feature is very important in industrial applications where exposure to highly combustible materials and electromagnetic radiation are typical. Their ability to deliver unnerving performance in extreme conditions also makes SSD play a vital role in military operations, be it in defense, aerospace or aviation applications. Military applications require, in most cases, an operating temperature range of -60°C to +95°C. Shock, vibration, and temperature ratings of HDDs cannot comply with military standards, only SSDs can.

Power Play

In addition to speed, reliability and mortality rate being positively affected by the SSD's non-volatility, they also consume much less power than traditional HDDs. No additional power is required to activate the platters or the mechanical arms present in most HDDs. Its power consumption is practically only a fraction of a hard disk drive.

Heat Dissipation

Along with the lower power consumption, there is also much lesser heat dissipation for systems using Flash-based SSDs as their data storage solution. This is due to the absence of heat generated from the rotating/movable media. This certainly proves to be the one of the main advantages of Flash-based SSDs relative to that of a traditional HDD. With less heat dissipation, it serves as the ideal data storage solution for mobile systems such as PDAs, notebooks, etc. Users can do away with large cooling fans, internal power supply and batteries for large storage arrays in storage area networks.

(Editor's note:- an earlier article by BiTMICRO shows how the size and power advantages of flash SSDs could benefit Blades in server farms.)

Cost Considerations

Of course, everything comes with a price. Based on a cost-benefit logic, it is but expected that something with very impressive features be priced higher than something that offers fewer benefits. SSDs as being a bit more pricey than the traditional hard disk drive may be one of the compelling reasons why up to now it hasn't gained as much popularity as it should be getting with all its brilliant functions.

While the use of SSDs today have mostly been limited to industrial and military implications, enterprise are now looking closely at SSDs for implementing tiered storage and optimizing their storage architectures.

To quote from the Solid State Disks Buyers Guide published by

"In fact the SSD is replacing servers and software licenses. SSDs can be used either to speed up the response time of existing applications as an alternative to buying more servers, or to reduce the number of servers and software licenses deployed. The economics can be compellingly in favor of an SSD deployment".

The industry is also expecting more breakthroughs as higher memory capacities and faster performance are now being produced for a wider array of choices fit for any mission-critical application.

Analysts foresee a rapidly closing gap between HDD and flash SSD pricing (see related article by Marius Tudor, BiTMICRO Networks - "Are Flash Solid-State Disks Ready for the Enterprise" ).

This vision is also bolstered by a recent announcement (July 2005) of electronic giant Samsung that they will invest heavily in improving their flash and DRAM production lines and flash SSD equipped laptops. Only time will tell if we will witness widespread commercial adoption of SSDs. ...BiTMICRO Networks profile

Editor's afterword:- the above article was published here in August 2005 but remains popular.

For more SSD articles take a look at these directories.
"One petabyte of enterprise SSD could replace 10 to 50 petabytes of raw HDD storage in the enterprise - and still get all the apps running faster."
the enterprise SSD software event horizon
SSD ad - click for more info
the varied futures of enterprise data storage?
Here are some articles written at different times and with different contextual assumptions.
... ...
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from Cactus Technologies
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Price of SSDs factors and examples
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how fast can your SSD run backwards?
SSDs are complex devices and there's a lot of mysterious behavior which isn't fully revealed by benchmarks, datasheets and whitepapers.

Underlying all the important aspects of SSD behavior are asymmetries which arise from the intrinsic technologies and architecture inside the SSD.
SSD symmetries article Which symmetries are most important in an SSD? That depends on your application. to read the article

Surviving SSD sudden power loss
Why should you care what happens in an SSD when the power goes down?

This important design feature - which barely rates a mention in most SSD datasheets and press releases - has a strong impact on SSD data integrity and operational reliability.

This article will help you understand why some SSDs which (work perfectly well in one type of application) might fail in others... even when the changes in the operational environment appear to be negligible.
image shows Megabyte's hot air balloon - click to read the article SSD power down architectures and acharacteristics If you thought endurance was the end of the SSD reliability story - think again. the article

SSD Jargon

Do you understand all the ingredients in the SSD news headlines?

This might help.
... SSD Jargon
SSDs and HDDs - how things changed in the 10 years from 2000 to 2010

  • Hard disk capacity grew 5x in the period 2000 to 2009. 3.5" HDDs went from 180GB to 2TB.
  • flash SSD random IOPS - (in 2.5" / 3.5" drives) grew 100x in the period 2000 to 2010 while HDD random IOPS remained exactly the same.
"Now whether a server company hates or loves Fusion-io - they have all become hostages to the idea that they absolutely have to offer some kind of supported SSD style of acceleration. They no longer have any choice in the matter."
hostage to the fortunes of SSD
the flash SSD story - survival of the fittest?
The emerging size of the flash SSD market as you see it today was by no means inevitable. It owes a lot to 3 competing storage media competitors which failed to evolve fast enough in the Darwinian jungle of the storage market in the past decade.

One of these 3 contenders is definitely on the road to extinction - but could one of the other 2 still emerge to threaten flash SSDs?

A recently published article - SSD's past phantom demons explores the latent market threats which hovered around the flash SSD market in the past decade. They seemed real and solid enough at the time.
SSD past phantom demons image - click to read the article Getting a realistic perspective of flash SSD's past demons (which seemed very threatening at the time) may help you better judge the so-called "new" generation of nv memory contenders - which are also discussed in the article. the article
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Calling for an End to Unrealistic SSD vs HDD IOPS Comparisons
"...One size does not fit all in the SSD world. ... SSDs are not simply a faster version of HDDs. If SSDs performance characteristic doesn't match data access patterns, SSDs will not perform as expected and sometimes can be slower than HDDs."
...Eric Kao, Chairman and CEO - Memoright - in his presentation - the Age of Application-Specific SSDs (pdf) at the Flash Memory Summit (August 2011)

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