大家这都知道存储类型的金字塔模型依次是寄存器，一级缓存，二级缓存，内存，DASD，tape,由于tape的价格相对其它类型的存储类型，比较低廉，在企业实际的应用中是比较常见的。为此我们简单的介绍一下主机的带库系统是如何运行的。
在此之前首先介绍一下后面会用到的术语：
Acronyms
VSM - Virtual Storage Manager
HSC-Host Software Component
VTCS - Virtual Tape Control System
VTSS - Virtual Tape Storage Subsystem
VTV - Virtual Tape Volume
VTD - Virtual Tape Drive
RTD - Real Tape Drive
MVC - Multi-Volume Cartridge
每一个component的功能介绍：
Host Software Component (HSC) is mainframe based software which controls StorageTek tape libraries. This page describes how to change HSC definitions, for example, adding an extra CPU, adding/deleting/defining tape drive addresses, or even adding new libraries.
VTCS
VTCS is an extension to HSC
Major responsibilities
–Influences allocation to virtual tape drives
–Issues “robotic” commands to the VTSS
–Manages the Migration/Recall of Virtual Tape Volumes
–Controls the Use of the VSM-managed Real Tape Drives and
Multi-Volume Cartridges
–Keeps track of Virtual Volume location in CDS
VTSS
VTSS is a modified StorageTek Iceberg
–Based on the RVA T82 manufactured for IBM
Major responsibilities
–Interfaces with the host based software (VTCS)
–Emulates a virtualized robot
–Provides Virtualized Tape Drives
Emulates the complete 3490-E command set
–Provides space for resident Virtual Tape Volumes (VTVs)
Transport mechanical actions occur at electronic speeds
–Contains the migrate / recall engine
Migrate/Recall Engine
A mechanism to move data to and from the VTSS and
the real tape drives
Major responsibilities
–Emulate an ESCON channel
–Execute the 3490-E command set
–Perform error recovery as needed
Migration and Recall Process
Virtual Tape Volumes are migrated and stacked on real tape
cartridges
–The VTV retains it’s original VOLSER
No catalog changes
No TMS changes
Stacked real tape cartridges are called Multi-Volume Cartridges
(MVCs)
Migration events occur in response to customer defined criteria
and disk buffer utilization considerations
Requests for migrated virtual volumes are first moved
from the multi-volume cartridge to the disk cache
Access to the recalled volume occurs through a virtual
tape drive
It looks like a longer mount
整体的布局如下：
带库的工作流程：
When a virtual volume no longer resides in the VTSS (Virtual Tape SubSystem), VTCS (Virtual Tape Control Software) must recall the volume back into the buffer from a tape residing in a library by performing the following steps:
1 Mount a MVC (Multi-Volume Cartridge) on an RTD (Real Tape Drive). This includes the robotic time for the mount and the threading of the tape in the drive.
2 Locating to the start of the VTV on the MVC. On average this is half way down the MVC.
3 Recalling the VTV (reading it off the MVC) back into the VTSS buffer.
4 Mounting the VTV on the requested Virtual Tape Drive.
这幅图左边是virtual 的，右边是physical
这两幅图的不同之处在于第一幅图上有CDS，而第二幅图上没有体现出来，在VTVmigrate到MVC的时候，VTV的信息会写到CDS ，这样在recall的时候，就可以找到对应的MVC了
REF：
%20VSM's%20Concurrent%20Tape%20Recall_Mount.pdf
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