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Flash SSDs - Inferior Technology or Closet Superstar?

BiTMICRO Networks
classic article - February 2004 - by Kelly Cash, BiTMICRO Networks
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Editor:- In recent years improvements in the speed, resilience and price per bit of Flash SSDs have made them a serious contender for use in enterprise acceleration applications. While still not as fast as DRAM they still offer significant performance advantages compared to hard disks. This article looks at the well known weaknesses of flash storage and argues that today's Flash SSDs are up to the job.
There's been a lot of talk in the storage industry recently about solid-state disks (SSDs) and their ability to dramatically speed up a computing environment's performance. One thing that is rarely discussed is the difference between different types of solid-state technology. There are two basic types of memory used by SSDs: DRAM and flash memory. A general perception in the computing industry is that only DRAM is robust enough for enterprise use. That sentiment doesn't give enough credit to flash memory. As with any two different technologies, each has its advantages. This paper explains the differences in effort to help determine which technology is best suited for one's IT environment.

Speed

It's common knowledge that writing to flash memory is much slower than writing to DRAM. Isn't speed the main purpose of installing an SSD in the first place? Yes. However, to say that flash technology is "slower than DRAM" is to sell it short. First, reading data from flash memory is very similar to the speed of reading from DRAM. Second, the better manufacturers of flash SSDs incorporate a DRAM cache in the drives to speed up writes. The best of those manufacturers have algorithms inside the devices which are able to flush that data from cache to flash in the background without impacting performance. If we graph the relative performance of the two types of SSD and a traditional rotating disk, it looks like:
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Megabyte went through his Michelangelo phase. "Somewhere in that lump of rock is a solid state disk..."
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SSDs - the big picture
Editor:- StorageSearch.com was the world's 1st publication to provide continuous editorial coverage and analysis of SSDs (in 1998) and in the 12 years which have followed we've led the market through many interesting and confusing times.
click to read the story about why SSDs are taking up so much time on so many web pages If you often find yourself explaining to your VC, lawyer or non technical BBQ guests why you spend so much time immersed in SSD web pages - and need a single, simple, non very technical reference to suggest - this may be the link they need.
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SiliconDrives from SiliconSystems
2.5" SiliconDrives
from Western Digital
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Fast Purge flash SSDs when "Rugged SSDs" won't do
The need for fast and secure data erase - in which vital parts of a flash SSD or its data are destroyed in seconds - has always been a requirement in military projects.
Fast Purge flash SSDs directory & articles Although many industrial SSD vendors offer products with extended "rugged" operating environment capabilities - and even notebooks SSDs come with encryption - it's the availability of fast data purge which differentiates "truly secure" SSDs which can be deployed in sensitive applications.
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the Problem with Write IOPS

the "play it again Sam" syndrome
Editor:- Flash SSD "random write IOPS" are now similar to "read IOPS" in many of the fastest SSDs.

So why are they such a poor predictor of application performance?

And why are users still buying RAM SSDs which cost 9x more than SLC? - even when the IOPS specs look similar.
the problem with flash SSD  write IOPS This article tells you why the specs got faster - but the applications didn't. And why competing SSDs with apparently identical benchmark results can perform completely differently. ...read the article
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Can you believe the word "reliability" in a 2.5" SSD ad?
Editor:- Reliability is an important factor in many applications which use SSDs.... but can you trust an SSD brand just because it claims to be reliable?

As we've seen in recent years - in the rush for the SSD market bubble - many design teams which previously had little or no experience of SSDs were tasked with designing such products - and the result has been successive waves of flaky SSDs and SSDs whose specifications couldn't be relied on to remain stable and in many products quickly degraded in customer sites.
storage reliability branding article As part of an education series for SSD product marketers - this case study describes how one company - which didn't have the conventional background to start off with - managed to equate their brand of SSD with reliability in the minds of designers in the embedded systems market. ...read the article
figure 1
Typical access times are:
  • DRAM SSD: 20µs
  • Flash SSD: 35-100µs
  • Rotating Disk: 4,000-10,000µs (4-10ms)
. We can see from the above graph that the DRAM-based SSD is indeed faster than the flash-based SSD; it may even be three times as fast. However, we must ask the question: "Is that performance difference significant?" Considering how much faster each SSD technology is than rotating disk, the answer may well be "No." Chances are good that other differentiators below will be more important to many IT environments.

Longevity/Lifespan

Unlike DRAM, flash memory chips have a limited lifespan. Further, different flash chips have a different number of write cycles before errors start to occur. Flash chips with 300,000 write cycles are common, and currently the best flash chips are rated at 1,000,000 write cycles per block (with 8,000 blocks per chip). Now, just because a flash chip has a given write cycle rating, it doesn't mean that the chip will self-destruct as soon as that threshold is reached. It means that a flash chip with a 1 million Erase/Write endurance threshold limit will have only 0.02 percent of the sample population turn into a bad block when the write threshold is reached for that block. The better flash SSD manufacturers have two ways to increase the longevity of the drives: First, a "balancing" algorithm is used. This monitors how many times each disk block has been written. This will greatly extend the life of the drive. The better manufacturers have "wear-leveling" algorithms that balance the data intelligently, avoiding both exacerbating the wearing of the blocks and "thrashing" of the disk: When a given block has been written above a certain percentage threshold, the SSD will (in the background, avoiding performance decreases) swap the data in that block with the data in a block that has exhibited a "read-only-like" characteristic. Second, should bad blocks occur, they are mapped out as they would be on a rotating disk. With usage patterns of writing gigabytes per day, each flash-based SSD should last hundreds of years, depending on capacity. If it has a DRAM cache, it'll last even longer.

Data Integrity

Most flash SSD makers employ error-checking algorithms and are able to correct a few bytes in a 512-byte block. Some of the less-robust error-checking will miscorrect three byte errors about 20% of the time. The best flash SSD providers can correct six random byte errors (and detect nine) in a 512-byte block. They will also never miscorrect a three-byte error. This level of error-checking gives security that the data integrity of the drive will last much longer than we as IT professionals will have to worry about it.

Volatility

Unlike DRAM, flash is inherently non-volatile. There's an old axiom which states that "a computer's attention span is only as long as its power cord." This definitely holds true for DRAM as well. While flash memory will retain its data beyond 10 years without power, little more than 10 milliseconds without power will give DRAM a most annoying case of amnesia. To prevent this, DRAM-based SSD makers must add batteries and disks to keep the data from being lost during a power failure. Though rechargeable, these batteries must be maintained (replaced) on a regular basis (maintenance cycles vary; consult the SSD's manufacturer) to ensure their ability to completely backup the data in the SSD. The batteries maintain power to the memory and disk(s) long enough to transfer the data from DRAM to the non-volatile storage. Two things to consider are: Some power failures happen in rapid succession- This may cause the backup operation of the SSD to start over, which essentially drains the batteries prematurely. This may mean that the batteries will not retain enough power to complete a backup cycle. Second, backup and restoration of the data takes time. It can take 30 to 60 minutes or more to backup and restore the data. The backup time usually isn't painful, but the restoration can cause extended downtime. Consider the scenario of a power failure and successful data backup to disk. When power returns, the server(s) can be up and ready long before the SSD's data is restored from its backup disk. This can mean that the server will be unavailable for an extra hour or so. Depending on the application, this could range from a mere annoyance to a business-threatening outage.

Form Factor

Most DRAM-based SSDs are large, rack-mount devices. They require large internal power supplies, fans, batteries and disk drives to provide non-volatility. In comparison Flash-based SSDs are much smaller, usually the same form factor as a conventional disk. Flexibility Because the form factor of flash-based SSDs is so much smaller, they are inherently more flexible in their use. They can often be used in place of traditional disks in storage arrays or in a server's internal disk bays. Embedded applications or mobile systems often require the much smaller footprint of a flash-based SSD.

Reliability

Both types of SSD are quite reliable since there are few, if any, moving parts. Even the backup disks of the DRAM-based SSDs are typically spun down during normal operation. This means that both types of SSDs are much more reliable than a traditional disk. However, for more demanding environments the smaller, more rigid flash-based SSDs are often more desirable. They typically withstand greater vibration and temperature ranges than DRAM-based SSDs. Some flash-based SSDs are even considered "ruggedized" by NASA and the U.S. Military. These drives will withstand intense extremes that would reduce a rack-mount box to rubble.

Power Consumption/Heat Dissipation

One benefit of flash memory is that is uses much less power than DRAM chips. Because of this, flash-based SSDs generate much less heat than their DRAM counterparts. This also means that they don't need cooling fans, whereas the DRAM-based SSDs do. Again, fans take space and require power themselves, which in turn generates heat and noise.

Cost

No IT department would purchase a solution without looking at its price tag. While DRAM chips and flash memory chips are similar in price, the overall cost per megabyte is generally lower for flash-based SSDs. This is due to the simpler design, and the lack of need for backup batteries and disks, and the enclosures in which to hold them. Some of the cost of the DRAM-based SSDs is the extra sheet metal for holding the batteries and disks, as well as the labor involved in assembling it all.

Conclusion

Now that we've taken an in-depth look at the different functionality and features, it should be clear to see that flash memory is useful for more than just consumer devices- it's also well-suited for the enterprise. Of course, one type of SSD will likely be better suited to the needs of your applications and IT environment than the other. Clearly though, it can be seen that flash memory has quite a list of capabilities that make it a "superstar" technology for many IT organizations. ...BiTMICRO Networks profile

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