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This is the second of a two-part post about why storage arrays and appliances with SSD drives can be a good idea, here is link to the first post.

So again, why would putting drive form factors SSDs be a bad idea for existing storage systems, arrays and appliances?

Benefits of SSD drive in storage systems, arrays and appliances:

Familiarity with customers who buy and use these devices
Reduces time to market enabling customers to innovate via deployment
Establish comfort and confidence with SSD technology for customers
Investment protection of currently installed technology (hardware and software)
Interoperability with existing interfaces, infrastructure, tools and policies
Reliability, availability and serviceability (RAS) depending on vendor implementation
Features and functionality (replicate, snapshot, policy, tiering, application integration)
Known entity in terms of hardware, software, firmware and microcode (good or bad)
Share SSD technology across more servers or accessing applications
Good performance assuming no controller, hardware or software bottlenecks
Wear leveling and other SSD flash management if implemented
Can end performance bottlenecks if backend (drives) are a problem
Coexist or complemented with server-based SSD caching

Note, the mere presence of SSD drives in a storage system, array or appliance will not guarantee or enable the above items to be enabled, nor to their full potential. Different vendors and products will implement to various degrees of extensibility SSD drive support, so look beyond the check box of feature, functionality. Dig in and understand how extensive and robust the SSD implementation is to meet your specific requirements.

Caveats of SSD drives in storage systems, arrays and appliances:

May not use full performance potential of nand flash SLC technology
Latency can be an issue for those who need extreme speed or performance
May not be the most innovative newest technology on the block
Fun for startup vendors, marketers and their fans to poke fun at
Not all vendors add value or optimization for endurance of drive SSD
Seen as not being technology advanced vs. legacy or mature systems

Note that different vendors will have various performance characteristics, some good for IOPs, others for bandwidth or throughput while others for latency or capacity. Look at different products to see how they will vary to meet your particular needs.

Cost comparisons are tricky. SSD in HDD form factors certainly cost more than raw flash dies, however PCIe cards and FTL (flash translation layer) controllers also cost more than flash chips by themselves. In other words, apples to apples comparisons are needed. In the future, ideally the baseboard or motherboard vendors will revise the layout to support nand flash (or its replacement) with DRAM DIMM type modules along with associated FTL and BIOS to handle the flash program/erase cycles (P/E) and wear leveling management, something that DRAM does not have to encounter. While that provides great location or locality of reference (figure 1), it is also a more complex approach that takes time and industry cooperation.

Figure 1: Locality of reference for memory and storage

Certainly, for best performance, just like realty location matters and thus locality of reference comes into play. That is put the data as close to the server as possible, however when sharing is needed, then a different approach or a companion technique is required.

Here are some general thoughts about SSD:

Some customers and organizations get the value and role of SSD
Some see where SSD can replace HDD, others see where it compliments
Yet others are seeing the potential, however are moving cautiously
For many environments better than current performance is good enough
Environments with the need for speed need every bit of performance they can get
Storage systems and arrays or appliances continue to evolve including the media they use
Simply looking at how some storage arrays, systems and appliances have evolved, you can get an idea on how they might look in the future which could include not only SAS as a backend or target, also PCIe. After all, it was not that long ago where backend drive connections went from propriety to open parallel SCSI or SSA to Fibre Channel loop (or switched) to SAS.
Engineers and marketers tend to gravitate to newer products nand technology, which is good, as we need continued innovation on that front.
Customers and business people tend to gravitate towards deriving greatest value out of what is there for as long as possible.
Of course, both of the latter two points are not always the case and can be flip flopped.
Ultrahigh end environments and corner case applications will continue to push the limits and are target markets for some of the newer products and vendors.
Likewise, enterprise, mid market and other mainstream environments (outside of their corner case scenarios) will continue to push known technology to its limits as long as they can derive some business benefit value.

While not perfect, SSD in a HDD form factor with a SAS or SATA interface properly integrated by vendors into storage systems (or arrays or appliances) are a good fit for many environments today. Likewise, for some environments, new from the ground up SSD based solutions that leverage flash DIMM or daughter cards or PCIe flash cards are a fit. So to are PCIe flash cards either as a target, or as cache to complement storage system (arrays and appliances). Certainly, drive slots in arrays take up space for SSD, however so to does occupying PCIe space particularly in high density servers that require every available socket and slot for compute and DRAM memory. Thus, there are pros and cons, features and benefits of various approaches and which is best will depend on your needs and perhaps preferences, which may or may not be binary.

I agree that for some applications and solutions, non drive form factor SSD make sense while in others, compatibility has its benefits. Yet in other situations nand flash such as SLC combined with HDD and DRAM tightly integrated such as in my Momentus XT HHDD is good for laptops, however probably not a good fit for enterprise yet. Thus, SSD options and placements are not binary, of course, sometimes opinions and perspectives will be.

For some situations PCIe, based cards in servers or appliances make sense, either as a target or as cache. Likewise for other scenarios drive format SSD make sense in servers and storage systems, appliances, arrays or other solutions. Thus while all of those approaches are used for storing binary digital data, the solutions of what to use when and where often will not be binary, that is unless your approach is to use one tool or technique for everything.

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 I)
IT and storage economics 101, supply and demand
Researchers and marketers dont 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.

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)

twitter @storageio

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).

Various SSD packaging options
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)

twitter @storageio

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