Glossary
Persistent memory
Persistent memory is memory that a processor reads and writes in bytes, at close to DRAM speed, and that keeps its contents when power is lost. It sits between volatile DRAM and block-based flash storage, and its main use in storage systems is making small writes durable the moment they arrive.
Three layers compared
| Layer | Addressed in | Typical response | Survives power loss |
|---|---|---|---|
| DRAM | Bytes | Nanoseconds | No |
| Persistent memory | Bytes | Close to DRAM | Yes |
| NAND flash | Pages and blocks | Tens of microseconds | Yes |
| Hard disk | Sectors | Milliseconds | Yes |
The distinction from an SSD is the access method as much as the speed. Persistent memory sits on the memory bus and is written in the same way as RAM, while flash is reached as a storage device and handles data in whole pages.
Which writes it helps
Small writes that must be safe before the system can continue gain the most. A storage system updating metadata or appending to a journal has to confirm each record is durable. On flash, a record of a few bytes still forces a whole page to be handled, which costs time and adds wear, as described under flash storage endurance. In persistent memory, the record lands directly, is acknowledged at once and is moved to flash or disk later in large batches. Large sequential streams gain little. Because the protected layer is small, the design question is how much of the write path it covers: metadata alone, the journal, or every incoming write until it has been stored in its protected form on the drives below.
After the main product line
The best-known persistent memory product line was discontinued by its maker in 2022, leaving designs that depended on it without a supplier. The pattern continued in other forms. NVDIMM modules pair DRAM with flash and a backup power source, saving their contents when power fails. Many all-flash arrays and storage servers use battery- or capacitor-protected memory as a write cache with the same purpose. Memory attached over CXL, an open interconnect standard, is the direction the industry points to next. Unlike a flash cache, which speeds reads by holding copies, each of these exists to make writes durable sooner.
Frequently asked questions
Is persistent memory the same as an SSD?
No. Persistent memory is addressed in bytes on the memory bus. An SSD is a storage device reached through a storage interface such as NVMe and works in pages and blocks.
What is CXL?
Compute Express Link, an open interconnect standard that lets processors attach and share memory beyond the slots on the motherboard. It is the most likely route for future persistent memory.