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3/25/2011
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George Crump
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Understanding SSD Vendor Talk

If you are either evaluating or getting ready to evaluate investing in solid state storage for your data center you are going to be faced with learning a new language, confronted with a new set of specs and a new set of debate around what features are most important. This will be the first entry in a series that will give you the decoder ring to understanding what Solid State Disk (SSD) vendors are talking about and what statistics are most important.

If you are either evaluating or getting ready to evaluate investing in solid state storage for your data center you are going to be faced with learning a new language, confronted with a new set of specs and a new set of debate around what features are most important. This will be the first entry in a series that will give you the decoder ring to understanding what Solid State Disk (SSD) vendors are talking about and what statistics are most important.In our upcoming webinar "How to Compare SSDs" and in future entries we will cover the terms and explain what the different benchmark results mean. Before we do that though it makes sense to explain what all the SSD confusion is about. First, SSD has evolved into a generic term that covers all of solid state storage (SSS). The problem with using SSD instead of SSS is that there are many variations of solid state storage available to you. SSS is really a better term for the category. SSD is a better term for a certain variant of that technology, SSS that is in the same form factor as a hard drive so that it can be more easily integrated into existing storage system. SSS can come in many other forms in addition to being packaged in the same form factor as a hard drive.

All SSS is memory that is designed to store data in the same way that a hard drive does. As you know there is a price premium using memory but for that premium you get dramatically better performance. There are two memory types used in SSS, DRAM and Flash. DRAM is faster and more expensive than Flash memory and it does not persistently store data. In other words when power is removed so is your data. Flash memory is slower (still faster than a hard drive), less expensive and does retain the data when power is removed. Flash's persistence and affordability have propelled it to the top of the SSS heap, to the point that DRAM is almost a forgotten cousin. That said DRAM based SSS still has a role to play and something we will take a deeper dive in a future entry. In short don't write DRAM SSS off just yet.

The affordability and persistent nature of Flash based SSS comes with a penalty that can impact performance and reliability if not handled correctly. The good news from a user perspective is that many vendors are addressing those issues. However one of the key differentiators and why comparing SSS can get so confusing is understanding how vendor workarounds impact the fast storage that you were hoping for.

The key issue for Flash is how it handles writes. When a Flash memory cell needs to be written to it needs to be erased first. This erase is done essentially by a "write" that reprograms the cell. Then the cell can be written to. Also Flash memory cells have a distinct number of times that they can be written to before they wear out. As a result SSS vendors have to make sure that the cells are pre-conditioned to handle inbound writes and that those writes are written to evenly across the Flash memory storage area. These two techniques are called Garbage Collection and Wear Leveling. We will dive deeper into both of these in our next entry.

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George Crump is lead analyst of Storage Switzerland, an IT analyst firm focused on the storage and virtualization segments. Find Storage Switzerland's disclosure statement here.

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