分布式共享存储(Distributed shared storage system)系统是将分散的存储系统通过网络连接起来的系统，兼有紧密耦合分布式系统容易编程和松散耦合分布式系统容易扩充规模的优点，程序设计人员不必考虑数据分配，编程工作是单机情况的简单扩充，进程迁移和动态分配实现简单，而且，可以和松散耦合一样扩大系统规模，当增加处理器的数目时不必考虑CPU之间的通信。
随着处理器及存储速度物理极限的渐渐接近，用多处理器来增强计算机的能力变得越来越有吸引力。
目前，紧密祸合的共享存储多处理器和松散祸合的分布式存储多处理器较为流行。
一般情况下，我们将存储分成了4种类型，基于本机的DAS和网络的NAS存储、SAN存储、对象存储。
对象存储是SAN存储和NAS存储结合后的产物，汲取了SAN存储和NAS存储的优点。
我们来了解一下应用是怎么样获取它想要的存在存储里的某个文件信息，并用大家熟悉的Windows来举例，如图1。
所以的知识，比如 cache，database。
学到后面有几个高山是必须要跨过去的。
网络通信
操作系统
编译原理
数据结构与算法
内存知识（磁盘硬件）
$ lscpuIn the early days computers were much simpler.
The various components of a system, such as the CPU, memory, mass storage（大容量存储）, and network interfaces, were developed together and, as a result, were quite balanced in their performance.
For example, the memory and network interfaces were not (much) faster than the CPU at providing data.
It is important to understand commodity hardware because specialized hardware is in retreat（消退）.
Scaling these days is most often achieved horizontally instead of vertically, meaning today it is more cost-effective to use many
smaller, connected commodity computers instead of a few really large and exceptionally fast (and expensive) systems.
There have been many types of RAM over the years and each type varies, sometimes significantly（显著）, from the other.
The older types are today really only interesting to the historians.
We will not explore the details of those.
Instead we will concentrate on（专注于） modern RAM types;
In the section introducing DRAM we saw that DRAM chips multiplex（多路复用） the addresses in order to save resources int the form of address pins（地址引脚）.
We also saw that accessing DRAM cells takes time since the capacitors in those cells do not discharge instantaneously（瞬间放电） to produce a stable signal;
Beside CPUs there are other system components which can access the main memory.
High-performance cards such as network and mass-storage controllers cannot afford to pipe all the data they need or provide through the
CPU.
Instead, they read or write the data directly from/to the main memory (Direct Memory Access, DMA).
CPUs are today much more sophisticated（复杂） than they were only 25 years ago.
In those days, the frequency of the CPU core was at a level equivalent to that of the memory bus.
Memory access was only a bit slower than register access.
ps: 内存访问，仅次于寄存器访问。
Before diving into（一头扎进技术细节） technical details of the implementation of CPU caches some readers might find it useful to first see in some more details how caches fit into the “big picture” of a modern computer system.
