Can flash live up to the hype?

Doug Rainbolt, vice president of marketing, Alacritech | June 27, 2012
The popularity of flash memory has soared over the last year because flash has definite advantages over conventional media.

This vendor-written tech primer has been edited by Network World to eliminate product promotion, but readers should note it will likely favor the submitter's approach.

The popularity of flash memory has soared over the last year because flash has definite advantages over conventional media. It often isn't clear however, what distinguishes one flash offering from another. Here is a review of four common flash design implementations, each of which has strengths and weaknesses.

Let's start with the use of PCIe flash memory cards in servers coupled with software that treats flash as an extension of system memory. Applications that depend upon high performance database accesses where low latency is very important can benefit from the use of these cards.

Data is generally moved as blocks closer to the application given the need for very high performance. Compared to traditional disk I/O, latency is far lower and the cost per IOPS is low. Because NFS is not the primary protocol being used for data access, customers that prefer this option are primarily SAN minded folk that are very sensitive to latency.

The cons associated with this approach are first, it's not a shared storage model; servers that benefit must be furnished with the flash cards. Second, it consumes inordinate amounts of CPU because the wear leveling and grooming algorithms require a great amount of processor cycles. Third, for some customers, consuming PCIe slots is a concern. All of these factors need to be factored into how servers are provisioned, assuring adequate processor and PCIe slot support.

The second design approach is to build storage arrays purely from flash memory. These constitute shared storage targets that often sit on a SAN. You wouldn't purchase these systems to accelerate or displace NAS, but you can include support for NFS caching so long as one flash memory array sits alongside an NFS gateway server. The added latency associated with including such a gateway make it less than ideal in performance-sensitive environments. The pure SAN model has gained significant traction displacing conventional storage from incumbent suppliers in latency sensitive environments, such as the financial markets.

Despite the raw performance, the storage management tools tend to lag. One of the major disadvantages with these systems is the processor utilization in the storage array. This will likely be the bottleneck that limits scalability. And once the processors hit 100%, it doesn't matter how much more flash memory is installed; the system will be incapable of generating incremental I/O. A better approach might be to apply flash to the data that needs it and make use of less expensive media for the data that doesn't. Aged or less important data doesn't require the same IOPS as hot data.

The third design approach has taken on chameleon-like qualities. It can function as either a write-through caching appliance that offloads NAS or file servers, or just as a file server. As a file server, it is positioned as an edge NAS that delivers performance to users. There is still a back-end NAS that sits behind this device where everything is stored. Active data isn't moved to the Edge NAS, it's copied to it, and this option makes use of faster media to increase performance for users.

The products come in the form of nodes that can make up a cluster. Nodes are configured with DRAM, NVRAM and either higher-performance SAS hard drives or flash memory. The nodes form a high performance cluster that can be managed as a Global Name Space.

Data can be pushed to edge NAS nodes in different clusters in an effort to reduce latency associated with the WAN. Written data can be flushed immediately to the back end filers, hence the write-through caching model, or "stored" on the cluster, and at set intervals, the written blocks as they exist, are flushed back. There is no form of WAN optimization, either de-duping or compression, that takes place in this model.

Some of the pros associated with this design approach are that it can generate incremental performance for users when the back-end filers lack the ability to generate the IOPS. It could also develop into what could be a strong full featured scale-out NAS offering over time. But the use of the back-end NAS in this model is temporary.

The major downsides with this design are it's not optimized to be purely a caching solution, and when it is used as a File Server, it's intrusive to the existing NAS. If it holds on to Writes in this mode, the back-end NAS can't receive them and execute snaps or replication. For those looking for a cost effective cluster mode scale-out, this might well be an insurance policy, assuming that these products do evolve into full-fledged NAS appliances that don't rely upon other back-end storage.

This takes us to the fourth and final design, an NFS acceleration appliance. In this design, flash is used as cache, not as storage. The appliance consists of DRAM and flash memory that act as cache for active data based upon LRU and data acceleration technology that consists of both 10GbE acceleration hardware in silicon and custom software.

This acceleration technology is optimized for processing NFS requests and getting data on and off the wire with minimal latency and CPU utilization. The appliance simply sits alongside the NAS already in place and acts as a performance Read and NFS Metadata cache and accelerator. Given typical NFS mixes, this model has the appliance absorbing approximately 90% of the NFS traffic so the filer doesn't have to.

In this model, the appliance can support a number of filers, not just one, so it's both sharable and scalable. The intent is to provide cycles back to the NAS so it can do its job in delivering performance to applications and have its life extended.

The pros are simplicity and cost-effective performance. The cons include not being able to cache CIFS traffic today and the dependency on the back-end NAS to handle non-cacheable operations. If there are bottlenecks on the filer, such as too few drives with limited IOPS to handle a burst in un-cached reads, there could be a temporary spike in latency. But for many working sets in environments where many clients are accessing much of the same data, as the cache warms, this isn't an issue.

In summary, there are many flash products available in the market today. They are diverse enough to where they shouldn't all be put in a bucket labeled as flash storage. Hopefully this review of Flash solutions' architectural differences, benefits and shortcomings has helped illuminate the fact that more informed choices can be made.