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Why SSD based arrays and storage appliances can be a good idea (Part I)

Greg Schulz - Blog

Original Article: 

This is the first of a two-part series, you can read part II here.
Robin Harris (aka @storagemojo) recently in a blog post asks a question and thinks solid state devices (SSDs) using SAS or SATA interface in traditional hard disk drive (HDD) form factors are a bad idea in storage arrays (e.g. storage systems or appliances). My opinion is that as with many things about storing, processing or moving binary digital data (e.g. 1s and 0s) the answer is not always clear. That is there may not be a right or wrong answer instead it depends on the situation, use or perhaps abuse scenario. For some applications or vendors, adding SSD packaged in HDD form factors to existing storage systems, arrays and appliances makes perfect sense, likewise for others it does not, thus it depends (more on that in a bit). While we are talking about SSD, Ed Haletky (aka @texiwill) recently asked a related question of Fix the App or Add Hardware, which could easily be morphed into a discussion of Fix the SSD, or Add Hardware. Hmmm, maybe a future post idea exists there.

Lets take a step back for a moment and look at the bigger picture of what prompts the question of what type of SSD to use where and when along as well as why various vendors want you to look at things a particular way. There are many options for using SSD that is packaged in various ways to meet diverse needs including here and here (see figure 1).

Figure 1: Various packaging and deployment options for SSD
The growing number of startup and established vendors with SSD enabled storage solutions vying to win your hearts, minds and budget is looking like the annual NCAA basketball tournament (aka March Madness and march metrics here and here). Some of vendors have or are adding SSD with SAS or SATA interfaces that plug into existing enclosures (drive slots). These SSDs have the same form factor of a 2.5 inch small form factor (SFF) or 3.5 inch HDDs with a SAS or SATA interface for physical and connectivity interoperability. Other vendors have added PCIe based SSD cards to their storage systems or appliances as a cache (read or read and write) or a target device similar to how these cards are installed in servers.
Simply adding SSD either in a drive form factor or as a PCIe card to a storage system or appliance is only part of a solution. Sure, the hardware should be faster than a traditional spinning HDD based solution. However, what differentiates the various approaches and solutions is what is done with the storage systems or appliances software (aka operating system, storage applications, management, firmware or micro code).
So are SSD based storage systems, arrays and appliances a bad idea?
If you are a startup or established vendor able to start from scratch with a clean sheet design not having to worry about interoperability and customer investment protection (technology, people skills, software tools, etc), then you would want to do something different. For example, leverage off the shelf components such as a PCIe flash SSD card in an industry standard server combined with your software for a solution. You could also use extra DRAM memory in those servers combined with PCIe flash SSD cards perhaps even with embedded HDDs for a backing or preservation medium.
Other approaches might use a mix of DRAM, PCIe flash cards, as either a cache or target combined with some drive form factor SSDs. In other words, there is no right or wrong approach; sure, there are different technical merits that have advantages for various applications or environments. Likewise, people have preferences particular for technology focused who tend to like one approach vs. another. Thus, we have many options to leverage, use or abuse.
In his post, Robin asks a good question of if nand flash SSD were being put into a new storage system, why not use the PCIe backplane vs. using nand flash on DIMM vs. using drive formats, all of which are different packaging options (Figure 1). Some startups have gone the all backplane approach, some have gone with the drive form factor, some have gone with a mix and some even using HDDs in the background. Likewise some traditional storage system and array vendors who support a mix of SSD and HDD drive form factor devices also leverage PCIe cards, either as a server-based cache (e.g. EMC VFCahe) or installed as a performance accelerator module (e.g. NetApp PAM) in their appliances.
While most vendors who put SSD drive form factor drives into their storage systems or appliances (or serves for that matter) use them as data targets for creating LUNs or file systems, others use them for internal functionality. By internal functionality I mean instead of the SSD appearing as another drive or target, they are used exclusively by the storage system or appliance for caching or similar purposes. On storage systems, this can be to increase the size of persistent cache such as EMC on the CLARiiON and VNX (e.g. FAST Cache). Another use is on backup or dedupe target appliances where SSDs are used to store dictionary, index or meta data repositories as opposed to being a general data pool.
Part two of this post looks at the benefits and caveats of SSD in storage arrays.
Here are some related links to learn more about SSD, where and when to use what:
Why SSD based arrays and storage appliances can be a good idea (Part II)
IT and storage economics 101, supply and demand
Researchers and marketers don’t agree on future of nand flash SSD
Speaking of speeding up business with SSD storage
EMC VFCache respinning SSD and intelligent caching (Part I)
EMC VFCache respinning SSD and intelligent caching (Part II)
SSD options for Virtual (and Physical) Environments: Part I Spinning up to speed on SSD
SSD options for Virtual (and Physical) Environments, Part II: The call to duty, SSD endurance
SSD options for Virtual (and Physical) Environments Part III: What type of SSD is best for you?

Ok, nuff said for now, check part II.
Cheers gs
Greg Schulz – Author Cloud and Virtual Data Storage Networking (CRC Press, 2011), The Green and Virtual Data Center (CRC Press, 2009), and Resilient Storage Networks (Elsevier, 2004)

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