To show the contrast between what is kept on SSD and what is managed by FAST, EMC states that it has sold over 24 petabytes of flash, but that the total amount of information under FAST management is 1.3 exabytes. The ratio of flash sold to storage managed is over 50 to 1, which indicates that a small amount of cache goes a long way. That underlines just how successful EMC has been at selling the use of flash.
Now, any number of vendors realized that simply installing SSD as a pseudo-HDD did not allow those devices to achieve their full potential. Why? Because a disk array controller managing I/O access treats all storage devices the same, even those with physical capabilities that affect I/O performance.
Developing new approaches that made better use of SSD's primary qualities was a particularly good move for smaller companies, since it allowed them to compete more effectively against larger vendors.
The approach several came up with (which is typically flash although some combination of flash and DRAM is possible) is offered as a layer known as flash cache. The two principal (but not necessarily exclusive) locations for this are as a server-based flash cache (which is housed within the server rack itself), and the server-network flash cache (which is housed in the network between the servers and storage (i.e., Ethernet) rather than the SAN storage network (i.e., Fibre Channel). Of course, SSDs can also be used as a cache on the array itself, for example, FAST Cache on CLARiiON and VNX arrays.
Though this approach has primarily (but not exclusively) been the province of smaller companies, large vendors (including EMC, but not limited to EMC) are poised to strike back. Which brings us to EMC's Project Lightning.
In essence, EMC's VFCache is 300 GB of SLC (Single-Level Cell) flash memory (not MLC (Multi-level Cell flash memory) on a PCIe (Peripheral Component Interconnect Express) card. Since PCIe is a popular computer expansion bus standard which supports hardware I/O virtualization, it seems a good place to put the flash memory (and also supports EMC's long-held strategy of integrating its entire solutions stack with VMware virtualization). EMC states that being more closely coupled to a server gives at least an order of magnitude improvement of performance over SSDs incorporated as disk drives within a storage array. Note that EMC feels that terabytes of data are not needed as in one competitive solution that is primarily direct-attached storage; VFCache is strictly a cache and can cater to a much larger application (remember the over 50 to 1 ration mentioned earlier). Note also that EMC feels that SLC's characteristics, such as endurance, make it a better choice at this time for enterprise-class solutions, although MLC may very well serve a role down the road.
VFCache is storage-agnostic which simply means that it can work with storage arrays of any sort, including those made by EMC's competitors. However, EMC can and will incorporate VFCache as a new tier of FAST storage where the hottest data resides on PCIe flash. This would, in effect, tightly couple VFCache to EMC storage, which is most likely the way that EMC will position the new solution to customers.
