In my last two columns I've written about All-Flash Arrays, the challenges they need to overcome and what the market looks like. One type of alternative we discussed was Hybrid Arrays. In this column, we will focus on a  type of All-Flash device that serves as an alternative to All-Flash Arrays – the SSD Appliance. SSD Appliances, also known as Flash Appliances, have a similar trait to All-Flash Arrays in that they are 100% solid state storage. Unlike All-Flash arrays however, SSD Appliances typically include only basic storage management feature sets and tend to focus more on performance.

Any storage management or data service feature consumes some storage processor time. Thin provisioning, for example, has to dynamically allocate additional volume space as data is being written to the volume. Snapshots have to track what blocks need to be preserved and which blocks can be safely over written because they are under snapshot protection. In order to deliver a more attractive price point, All-Flash Arrays sacrifice potentially more performance by leveraging deduplication and/or compression to mask their cost premium compared to hard drive based systems.

Hard Drive based storage systems, because of the latency of the hard disk, can provide many of these features without any serious impact to performance. These services essentially give the storage controller something to do while drive platters are rotating into place. Systems that are 100% solid state do not have that luxury. There is no latency, at least at the device level.

The impact of data services and storage optimization can be masked by leveraging efficient code or adding additional storage processing at the controller level. But this adds either cost or complexity to the system to compensate. In theory, Flash Appliances should be able to offer higher performance at a lower cost point than All-Flash Arrays because they are not burdened with providing data services and storage optimization.

Most Flash Appliances instead have general data protection capabilities which take the form of a specialized implementation of RAID designed specifically for flash. They also include basic storage sharing capabilities like LUN provisioning, masking and networking. However, unlike All-Flash Array suppliers (which tend to be focused on software innovation) Flash appliance manufacturers tend to be more focused on advancements in hardware. Lastly, Flash Appliances tend to compliment traditional disk based storage arrays instead of serving as all out replacements.

Flash Appliance Innovations

As mentioned above, while Flash Appliances tend to be more innovative at the hardware layer, All-Flash Arrays tend to focus on software innovation. A common area of innovation for Flash Appliances is to implement custom designed flash memory components rather than off the shelf drive form factor solid state disks (SSD). This allows them to maximize storage density, reduce power requirements and lower hard costs.

Most Flash Appliance vendors also design their own controller technology which includes enhanced flash protection and endurance optimization. Most All-Flash Array vendors leverage the controller technology that is already on the SSD from the OEM. A few Flash Appliance vendors have focused on reducing networking bottlenecks. This includes optimizing network cards that support Flash Appliance I/O streams or directly integrating network switching on to the Flash Appliance itself.

One area of weakness for Flash Appliances has been high availability as most did not have complete redundancy built into their system. Originally Flash Appliances were used in environments where the data it held was temporal in nature and the added cost of high availability (HA) was not required. In the past year, however, most Flash Appliance vendors have added an HA option to their offering. While an HA option raises the overall cost of the system, it significantly expands its viability for deployment across many more use cases.

Flash Appliance Use Cases

In general, Flash Appliances and All-Flash arrays are deployed for many of the same use cases. Flash Appliances, however, hold more appeal in data center environments where storage services are not needed or are already being supplied by the existing storage infrastructure.

For example, if the appliance in a database environment is utilized to store undo or redo logs then the advanced capabilities of All-Flash Arrays (like auto-data migration) are not required. Instead this function has typically been delegated to database administrators whom have the knowledge and skill sets to determine which sub-set of their applications can benefit from Flash Appliance data placement.

Up until recently, database acceleration has been the primary use case for Flash Appliances, but two changes in the past year have greatly expanded the use cases for Flash Appliances. The first is the ability to leverage Storage Virtualization Appliances to deliver storage services like thin provisioning, snapshots and replication universally across any storage platform in the data center. In effect, the Storage Virtualization Appliance can aggregate all storage resources, regardless of the underlying disk manufacturer, as a common pool of disk and enable the IT administrator to apply the feature sets to the corresponding disk assets as required. Flash Appliances become one of the storage resources to be managed. 

Another methodology for delivering storage services is through software, otherwise referred to as “software defined storage”. Rather than running on a physical hardware appliance, storage services are instead incorporated into the operating system, hypervisor or as a virtual machine. We examined this concept in the article entitled, "What is the Storage Hypervisor?" Storage software that requires a stand-alone appliance is not included in this category, since that it is essentially storage virtualization as described above.

While the ability to "pour services on top" made flash appliances immediately appealing, a critical shortcoming preventing widespread adoption was the lack of standard high availability features. This was especially true when the only workaround to the lack of HA was incurring the expense of mirroring two solid state systems. Now with the inclusion of HA capabilities on Flash Appliances, IT decision makers can reap the speed of flash with the added feature set value and with the comfort of knowing their data is being stored on a device that will reliably provide access.

Flash Appliance or Flash Array?

Like the choice between a Hybrid Arrays or an All-Flash Array, the decision to leverage Flash Appliances is largely dependent on the application environment and its corresponding performance needs. With the increasing popularity of storage virtualization and software defined storage, Flash Appliances, especially in light of their recent addition of high availability features, should no longer be just considered for niche use cases. They could be a perfect performance tier compliment in a virtualized infrastructure or shared database environment.

previous entry: “The Value Of HBA SSD Caching”