Reprinted from: http://blog.jcix.top/2019-02-18/pm-nvm/#NVMPM_SCM
In recent years, whether in academia or industry, NVM is a relatively cutting-edge technology. In fact, the term NVM is not new, because the emergence of the concept of PM (or SCM) related to NVM makes the term NVM "fire" again. In related literature in recent years, NVM is also often mixed with PM (SCM), which is confusing. This article examines the relationship between them.
NVM
To be precise, NVM (non-volatile memory) refers to non-volatile physical storage media, such as NAND flash, PCM, 3D XPoint, etc. are all considered NVM, these NVM can be made into SSD form, can also be made into PM (or called SCM).
NVM存储介质
|
V
NAND Flash ---+
|
+-- PCM ---+--> SSD
+--+ |
| +-- 3D XPoint ---+
V
PM
(SCM)The relationship between NVM and SSD
Our common SSD, whether it is a SATA interface or PCIe interface, is basically based on NAND Flash; of course there are exceptions. Intel ’s Optane SSD launched two years ago is based on 3D XPoint instead of NAND Flash. Regardless of whether the underlying media is Flash or 3D XPoint, these media belong to NVM in a broad sense. They are all storage devices, block devices, and IO devices. In addition to performance, user software can treat them as SSDs without distinction.
Relationship between NVM and PM (SCM)
Now, new NVMs such as PCM and 3D XPoint are also used to make DIMM interface memory modules, which are called PM or SCM.
In fact, PM (persistent memory) or SCM (storage class memory) is basically synonymous, and refers to a byte addressable NVM device with a memory DIMM interface. The new NVM storage media is easier to achieve fine-grained reading and writing than NAND Flash, and is more suitable for byte-addressable memory. And, because of the higher density, the PM based on the new NVM media can achieve several times the capacity of DRAM memory; also because the new NVM media is not as good as the performance of DRAM, the read and write speed of PM may be several times slower than that of DRAM memory.
What is NVDIMM?
"Non-volatile memory sticks" have not appeared in recent years. Long time ago, there were non-volatile memory sticks based on NAND Flash (commonly known as NVDIMM), but such NVDIMMs are usually made into "battery + Flash +" "DRAM" form: when power is turned on, it still relies on DRAM to work. When power is turned off, the battery power is used to flash the DRAM data back to Flash. Flash cannot replace DRAM directly because the read and write granularity of Flash is inconsistent, and there are requirements such as erasure and copy before writing, which is not conducive to direct fine-grained access. This design results in that NVDIMM may only be more expensive than general DRAM memory, and it is impossible to do with a larger capacity density than DRAM memory.
to sum up
Although it is easy to be ambiguous, in recent years, there are still many literatures called NVM based on PM or SCM. Therefore, when reading the literature, we can think that: NVM can refer to storage media such as Flash and PCM in a narrow sense; NVM can also refer to storage devices such as SCM and PM in a broad sense. In fact, in the early years, SSD was also called NVM.
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