While not exactly an SSD tutorial I will try and cover some of the basics of SSD technology, why it’s become so exciting and what it might mean for the average IT guy buying storage. SearchStorage.com just reported that SSDs had a great year in 2010, hitting multi-billion dollar revenues. With all the attention at the CES show and in the press, I thought I’d weigh in on why this technology and why now?
What is an SSD anyway?
Solid state drives, generally referred to as “SSDs”, have the same physical sizes and interfaces as hard disk drives (HDDs), but instead of a spinning disk that holds the data, the information is stored on non-volatile memory chips.
So why is “not spinning” better?
It’s green – Since there are no moving parts, the power consumption for SSDs is much lower than traditional hard drives, and there is no “power up spike” like spinning drives. SSDs also emit less heat, thereby requiring less cooling. And like USB memory drives, SSDs do not need power to retain data beyond what is used to support an on-drive read/write cache.
It’s rugged – With no moving parts, SSDs are not susceptible to the same shock and vibration limitations that can damage or degrade spinning disk drives. And because air density does not affect them (as with HDDs) they can operate at higher altitudes. Additionally, many SSDs are more tolerant of extreme conditions such as dust or moisture than traditional hard disk drives.
It’s speedy – Since access to all data points is immediate, read performance of SSDs can be significantly faster than traditional hard drives. This is especially true in random read applications. SSDs also vastly outperform rotating media where latency is concerned. Now we are getting to it!
If SSDs are so fast, why isn’t everyone using them?
It still comes down to cost per gigabyte. Even after years of falling prices, SSDs can cost much more than even high-performance 15,000 RPM SAS drives, due to the cost of the memory chips used to create them. Even though SSD technology is still too expensive for many applications, there is a lot of excitement about declining prices. Furthermore, SSD technology will gain market share by piggy backing on storage technology trends such as automated storage tiering. And remember, even though SSDs are more expensive on a dollar per gigabyte basis, using other metrics such as dollar per IOP, SSDs can be cheaper than HDDs. For this reason, a “judicious” amount of high performance SSD in a storage array can skew the price/performance ratio in a favorable direction.
Are all SSDs the same?
Of course not! SSDs are available with various interfaces such as SATA, SAS or FibreChannel, and (as with HDDs) SATA is the most cost effective choice, while SAS and FibreChannel drives are higher performing candidates. Also, there are two chip technologies used to create SSDs: SLC (Single Level Cell or 1 bit per cell) and MLC (Multi-Level Cell or 2+ bits per cell). SLC drives are higher performance and have fewer write cycle issues—that is, “hot spots” on the drive. MLC drives offer a higher capacity per chip, delivering a lower cost per gigabyte, but consume more power and sustain fewer writes than SLC. The MLC longevity problem is being addressed by “eMLC” SSD drives (“e” stands for enterprise or extended life, depending who you ask) which typically carry the same five year warranty as SLC drives. And while SLC drives outperform MLC, both types of SSD offer significantly faster performance than even the fastest SAS hard disk drives—random read performance can exceed disk drives up to 100x.
“Hot spot”, nightclub, same thing?
On an SSD, a ‘hot spot’ isn’t nearly as much fun. Memory chips have a much more limited ability to “flip the bit” (write a 0 or 1) than disk drives, and may have a shorter functional life. If specific areas of the drive have too many active writes, these “hot spots” can wear out sections of the chip prematurely. SSD drive vendors are working to fix the “hot spot” problem by improving write cycle management and over-provisioning the drives. In addition, various operating systems (OS) providers are reconsidering the usage of some maintenance activities, such as disk optimization that may intensify the problem.
Who is going to the SSD party?
SSDs are increasingly used in servers as high-speed drives, and in laptops because of their ruggedness and low power consumption. In addition, SSDs are gaining popularity in storage arrays, for several reasons such as performance (high IOPS, low latency), minimal power consumption, environmental conditions warranting ruggedness, demanding enterprise applications, and storage tiering.
Advantages for storage tiering…
In storage arrays, tiering means employing two or more types of drives with differing performance characteristics, and then placing bits onto a particular “tier” based on one or more attributes, thereby gaining a performance advantage. This process can be either manual or automated. An example of manual tiering would be to configure an SSD volume for database metadata while housing the rest of the database on SAS HDDs and archiving onto SATA HDDs. Automated tiered storage (ATS) is also gaining popularity, having trickled down from the enterprise world into upper midrange, soon to gain a foothold in the lower midrange segment. Storage arrays that offer ATS automatically keep “hot” data on fast SSD drives while demoting cooler data to slower, more cost effective drives. As this technology gains ground, so will SSDs, which are an integral part of such solutions.
SSDs sound pretty good so far what are the drawbacks again?
- Firstly, price at least on a dollar per gigabyte basis. SATA HDDs can be acquired for under $1/GB. Currently, high performance SAS or FibreChannel SSDs are still upwards of $100/GB.
- Secondly, longevity although this issue is less of a concern than it was in the past.
- A third “drawback” of SSDs can be unrealistic expectations. For example, even though a particular performance spec such as random I/O may be 10x greater (or more) for a given SSD compared with an HDD, one should not expect overall performance to increase tenfold by upgrading. Similarly, one cannot simply install two dozen SSDs into a storage array and expect performance to be 24x the published drive performance numbers. Numerous other bottlenecks, such as application characteristics, network bandwidth and hardware capabilities of storage controllers, hosts, and switches can and will place limits on the performance of SSDs in storage arrays. There will be limited performance gains beyond a certain number of SSDs and that number will vary significantly depending upon the particulars. That said, SSDs offer advantages that more than offset the additional cost, when deployed in an optimal manner.
So how do I know if SSDs are right for me?
Well, the answer depends. It would be best to ask yourself some questions to help make that determination. Assuming that youre considering SSD for your data center, here is a good starting point for your analysis:
- What does my storage solution look like today?
- What is leading me to consider SSD what am I hoping to get that I dont have now?
- Am I seeking increased performance?
- Do I need ruggedness for shock, vibe, or high altitude?
- Am I looking to decrease power consumption or cooling requirements?
- Do I want to take advantage of a storage tiering solution?
- Are there other factors?
- If performance is a primary consideration, specifically what kind of performance gains am I seeking as compared with my current storage solution?
What is my storage budget and what are my storage capacity needs? I.e., what is my approximate storage budget on a dollar per gigabyte basis?
If you answered “yes” to any of the bold questions and if you are able to quantify (perhaps with some help) what you are looking to gain, SSD may be for you. As for budgetary concerns, remember that in some cases, the right amount of SSD may be just a couple of drives (marginal expense) or might be an array full (significant expense). It pays to do your homework and to talk to the right people…if you can do that, you can probably reap some great benefits with SSDs!
Article Contributed by: Chris Geerlings, Product Marketing Analyst, Dot Hill
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