Because both companies are still flying under most people's radars, I can't reveal the deep nitty-gritty about how their technologies work, but I'm sufficiently impressed with both to tease you with a brief overview.
XtremIO is building an all-solid-state scale-out storage array. and like some other all-solid-state players, including SolidFire and Pure Storage, the system has data deduplication cooked into its very DNA and data layout.
XtremIO's engineers--who hail from outfits including IBM Storage Research, NetApp, Voltaire, Exanet and VMware--have even managed something I long thought was impossible: They've built a scale-out system that uses peer nodes like Equallogic's or Isilon's that can support Fibre Channel as well as iSCSI connections. The only other vendor with anything that could be reasonably called scale out for Fibre Channel is Kaminario, which uses a hierarchical architecture with dedicated I/O director nodes and storage nodes. XtremIO's approach is way cool, but you'll have to wait a while for a full description as I'm currently sworn to secrecy.
While XtremIO is building storage arrays, Proximal Data is joining EMC's VFcache/Project Lighting, Fusion-IO's IOturbine and what seems like a cast of thousands, including SANdisk/Flashsoft, OCZ/SANrad, NEVEX and VeloBit, in the server-side flash caching market. Fusion-IO made a big splash with its PCIe flash cards, but since PCIe SSDs are only available to the server they're mounted in, simply plugging in a PCIe SSD doesn't work for use cases like virtual server hosting, where shared storage is required.
Server-side caching promises the best of both worlds: the low latency and high IOPS of PCIe flash without the limitations inherent in storing data in a single server. Where most of the other players have great solutions for database servers and the like, they haven't quite got all their ducks in a row for virtual server hosting, especially under VMware. They implement caching in the guest virtual machine or require specialized guest configuration, which complicates the installation and management of the flash cache and in many cases doesn't support vMotion or the more sophisticated vSphere features that themselves rely on vMotion, such as DRS (dynamic resource scheduling), FT (fault tolerance) or HA (high availability).
Despite the fact that VMware ESXi, unlike Windows or Linux, doesn't provide an API for filter drivers, the Proximal Data boffins have figured out how to install a module into ESXi on the host that inserts itself between the hypervisor and storage drivers. All an administrator has to do is install an SSD, PCIe if ultimate performance is desired, and the Proximal Data software. It will then use the SSD as a read cache, speeding up all the guest VMs and reducing the load on the back-end storage array.
I'm very interested in the server side flash market and will be following Proximal and the other players closely.