Before persistent memory, the "gap" between disk storage and DRAM was very large. Persistent memory, however, is much closer to DRAM performance than disk or flash performance, so the gap between disk storage and persistent memory is very large. This technology will change the data storage hierarchy.
Persistent memory is bit addressable, which means you can perform I/O to it as if it were memory. In terms of capacity, persistent memory will be roughly an order magnitude larger than DRAM.
Using persistent memory for storage is basically the same idea as using a RAM disk, with one very important difference: When the power is turned off, the data is not lost. To the operating system, the persistent memory looks like conventional block storage, and a filesystem can be built using that block storage.
The performance of persistent memory is always under discussion, particularly in the case of 3D XPoint, because it is so close to release. In this case, performance has always been discussed in general terms: