计算机基本概念

1 SCSI

    小型计算机系统接口（英语：Small Computer System Interface; 简写：SCSI），一种用于计算机和智能设备之间（硬盘、软驱、光驱、打印机、扫描仪等）系统级接口的独立处理器标准。 SCSI是一种智能的通用接口标准。它是各种计算机与外部设备之间的接口标准。
SCSI Adapter (SCSI适配器)
    SCSI非常灵活，它不仅是磁盘接口，还是个系统级的接口，允许连接许多不同种类的设备。SCSI是一条总线，可以支持总共7个或15个设备。如果存在多通道适配器，每个通道可以支持高达7或15个设备。

  SCSI控制器称为主适配器，其作用就像SCSI总线和PC系统总线之间的网关。总线上的每一个设备有一个内置的控制器。SCSI总线不直接与如硬盘之类的设备通信，而与驱动器中内置的控制器通信。
    单个SCSI总线可以支持多达8或16个物理单元，通常称为SCSI-ID。这些单元中的一个就是PC中的SCSI主适配器，另外7或15个可以是其他的外设，可将硬盘、磁带驱动器、CD-ROM驱动器、图形扫描仪或其他设备（可达7或15个设备）连接到一个单个SCSI主适配器上，大多数系统可支持4个主适配器，每一个最多连接15个设备，总共可连接60个设备！
    如果使用的是双通道适配器，连接的设备数可以加倍。
    SCSI外部设备通常包括设备、SCSI控制器和SCSI适配器，只有这样设备才是完整的，可以直接插到SCSI总线上。这种类型的驱动器通常称为内嵌式SCSI设备（SCSI接口是内置的）。例如，大多数SCSI硬盘驱动器除了将附加的SCSI总线适配器电路（通常是单个芯片）加到控制板上外，在技术上与IDE驱动器是一样。人们并不需要知道在SCSI驱动器内部是那一种控制器，因为系统并不直接与控制器通信，虽然看起来好像与标准IDE驱动器一样插在系统总线上。实际上是与装在系统总线上的SCSI主适配器进行通信。只有在SCSI协议下才能访问驱动器。
    总的来说,SCSI就是服务器.


2热插拔

热插拔（hot-plugging或Hot Swap）即带电插拔，热插拔功能就是允许用户在不关闭系统，不切断的情况下取出和更换损坏的、电源或板卡等部件，从而提高了系统对灾难的及时恢复能力、扩展性和灵活性等，例如一些面向高端应用的系统都可以提供磁盘的热插拔功能。具体用学术的说法就是：热替换（Hot replacement）、热添加（hot expansion）和热升级（hot upgrade）.


3 LPAR
  Logical partition (virtual computing platform)(wikipedia)  A logical partition, commonly called an LPAR, is a subset of computer's hardware resources, as a separate computer. In effect, a physical machine can be partitioned into multiple logical partitions, each hosting a separate .
  IBM developed the concept of ( in and ) and in 1972 provided it for the as Virtual Machine Facility/370. IBM introduced the Start Interpretive Execution (SIE) instruction as part of 370-XA on the 3081, and VM/XA versions of to exploit it. is a type-1 Hypervisor based on the CP component of that runs directly on the machine level and allocates system resources across LPAR's to share physical resources. It is a standard feature on and machines.
Hardware partitioning

Logical partitioning divides hardware resources. Two LPARs may access from a common memory chip, provided that the ranges of addresses directly accessible to each do not overlap. One partition may indirectly control memory controlled by a second partition, but only by commanding a process in that partition. may be dedicated to a single LPAR or shared. While on Amdahl's MDF(Multiple Domain Feature) it was possible to configure an LPAR with both shared and dedicated CPUs. This is no longer possible with any mainframes flavours currently in the market.

On IBM mainframes, LPARs are managed by the facility. IBM mainframes operate exclusively in LPAR mode, even when there is only one partition on a machine. Multiple LPARs running z/OS can form a or , whether on one machine or spread across multiple machines.

On IBM , LPARs are managed by the Power . The Hypervisor or acts as a virtual switch between the LPARs and also handles the virtual traffic between LPARs. supports 10 times as many LPARs as processors with fractional allocations. It was introduced with the processor. All IBM and systems may be partitioned. Note that a full system partition may be defined where all resources are consumed by a single partition. System P servers with PowerVM enabled allow LPARs with shared CPUs to delegate their unused cycles into the shared pool. Dedicated processors are not available for sharing. Unused cycles become available for other partitions and are governed by the parameters specified when the LPAR is defined. Changes to a running partition can be made dynamically up to the maximum value set, and down to the minimum value set in the active profile. The changing of resource allocations without restart of the logical partition is called . IBM (formerly known as Advanced POWER Virtualization or ) is the licensed/purchased feature that enables the virtualization features on p4,5,6,7 series servers.

LPARs safely allow combining multiple test, development, quality assurance, and production work on the same server, offering advantages such as lower costs, faster deployment, and more convenience. IBM mainframe LPARs are Common Criteria certifiable, equivalent to physically unconnected servers, so they support the highest security requirements, including military use. Nearly all IBM mainframes run with multiple LPARs with the and supporting up to 60 LPARs.