RAM: Random access memory (random access memory, RAM), also known as "random access memory", is an internal memory that directly exchanges data with the CPU, also called main memory (memory). It can be read and written at any time, is fast, and is often used as a temporary data storage medium for the operating system or other running programs.
Memory is very fast, but at the same time it is volatile, and RAM cannot retain data when the power is turned off. If data needs to be saved, they must be written to a long-term storage device (eg, a hard disk).
Hard drives, although non-volatile, are very slow. Therefore, there is a lot of non-volatile storage between the hard disk and the memory, which has been developing. Here is a brief summary of the common ones today:
SSD :
Solid State Drives (Solid State Drives), referred to as solid state drives, solid state drives (Solid State Drives) are hard drives made of solid-state electronic storage chip arrays, consisting of a control unit and a storage unit (FLASH chip, DRAM chip).
NVDIMM
non-volaitle dual inline memory module, a memory module that integrates ordinary DDR RAM + non-volatile FLASH chip. When the system is powered off abnormally, the NVDIMM uses its backup super capacitor as a power source to put data into the flash chip in a short time, thereby permanently saving the data in the memory. Compared with non-volatile memory of other media, NVDIMM has gradually entered the mainstream server market, and foreign memory manufacturers such as micron, viking, and AGIGA have launched their own NVDIMM.
3D Xpoint:
3D Xpoint ditched the transistor at the heart of the NAND chip. NAND works by moving electrons back and forth across a transistor called its "floating gate" to represent the zeros and ones of the binary code. One problem with this technique is that it cannot refresh a single bit of data at a time. Larger blocks of information need to be wiped and then rewritten to incorporate changes.
3D XPoint works fundamentally different from NAND. NAND captures different numbers of electrons through insulating floating gates to achieve bit value definition, while 3D XPoint is a resistance-based memory technology achievement that distinguishes 0s from 1s by changing the cell resistance level.
3D XPoint的结构非常简单。它由选择器与内存单元共同构成，二者则存在于字线与位线之间(因此才会以‘交叉点’来定名)。在字线与位线之间提供特定电压会激活单一选择器，并使得存储单元进行写入(即内存单元材料发生大量属性变化)或者读取(允许检查该存储单元处于低电阻还是高电阻状态)。我猜测，写入操作要求具备较读取更高的电压，因为如果实际情况相反，那么3D XPoint就会面临着上在读取存储单元时触发大量材料变化(即写入操作)的风险。
上图是对比各种介质之间的延迟，3d xpoint的延迟在10ns级别，总的来说，而3D XPoint架构其实是一种大容量存储技术，虽然比DRAM要慢，但它比DRAM要便宜，比NAND要快，但是比NAND要贵，最重要的是它是非易失性的。所以，断电之后数据不丢失。
就基于3D XPoint的产品来讲，其最为立竿见影的应用方式就是在DRAM与SSD之间充当新的存储层。在计算科学发展的历史长河当中，存储与处理器之间的其它层级一直在不断出现——芯片内多级缓存、芯片外缓存、以及SSD缓存等等——而3D XPoint内存将充当这一体系当中的另一种新型存储介质，从而弥合DRAM与现有高速非易失性存储方案之间的空白区域。通过将3D XPoint作为另一种缓存层，这项技术将被应用于未来的高速应用程序（最典型就是数据库应用，缓存应用）当中，从而克服目前内存容量或者存储延迟给这类应用造成的拖累。
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