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.