Mass-Storage Systems
Overview of Mass Storage Structure
Magnetic disks（磁盘）
provide bulk of secondary storage of modern computers 提供大量现代计算机的二级存储
- Drives rotate at 60 to 200 times per second 驱动器以每秒60到200次的速度旋转
- Transfer rate is rate at which data flow between drive and computer 传输速率是驱动器和计算机之间的数据流速率
- Positioning time (random-access time) is time to move disk arm to desired cylinder (seek time) and time for desired sector to rotate under the disk head (rotationallatency) 定位时间（随机存取时间）是将磁盘臂移动到所需气缸的时间（寻道时间）和所需扇区在磁盘头下旋转的时间（旋转延迟）
- Head crash results from disk head making contact with the disk surface 磁头与磁盘表面接触导致的磁头碰撞
- That’s bad
- Disks can be removable 磁盘可以移动
- Drive attached to computer via I/O bus 通过I/O总线连接到计算机的驱动器
- Busses vary, including EIDE, ATA, SATA, USB, Fibre Channel, SCSI
- Host controller in computer uses bus to talk to disk controller built into drive or
storage array 计算机中的主控制器使用总线与驱动器中内置的磁盘控制器或存储阵列
固态硬盘（Solid State Drives）
简称固盘，固态硬盘用固态电子存储芯片阵列制成的硬盘，由控制单元和存储单元（FLASH芯片、DRAM芯片）组成。
- 第一只SSD出现在1978年（STK 4305，每MB售价8800美元，DRAM）。
- 全闪存阵列（AFAS）和混合闪存阵列（HFA）呈爆发式增长。
- 现在SSD的容量是15.36TB（SAS）。
- 非易失性、低功耗（只有HDD的三分之一）。
- 无活动部件、可靠性高――位误码率（BER）1 x 1017
- 读取存取时间：0.2毫秒，存取时间比HDD大概快 50倍。
Magnetic tape （磁带）
出货的磁带驱动器中超过85%是LTO（Linear Tape Open）。
磁带驱动器的可靠性、数据传输速率和容量已超过磁盘。
磁带的原生容量为10TB，压缩容量超过25TB。(LTO-10:48TB)
磁带的原生数据传输速率为360MB/s。
LTFS(Liner Tape File System)为磁带提供了一种通用、开放的文件系统。
由于总体拥有成本，云采用磁带解决方案用于归档服务。
对企业级磁带和LTO而言，磁带介质的寿命至少是30年。
Disk Structure
Disk drives are addressed as large 1-dimensional arrays of logical blocks, where the logical block is the smallest unit of transfer 磁盘驱动器作为逻辑块的一维大数组寻址，其中逻辑块是最小的传输单元
The size of the logical block is usually 512 bytes 逻辑块的大小通常是512字节
The 1-dimensional array of logical blocks is mapped into the sectors of the disk sequentially
- Sector 0 is the first sector of the first track on the outermost cylinder 扇区0是最外圆柱体上第一条轨道的第一个扇区
- Mapping proceeds in order through that track, then the rest of the tracks in that cylinder, and then through the rest of the cylinders from outermost to innermost 映射按顺序依次经过该轨迹、该圆柱体中的其余轨迹，然后从最外层到最内层穿过其余圆柱体
Disk Attachment
Host-attached storage 主机连接存储
Host-attached storage accessed through I/O ports talking to I/O busses 通过与I/O总线通信的I/O端口访问
I/O bus like IDE
- a maximum of 2 drives per I/O bus 每个I/O总线最多有2个驱动器
SCSI itself is a bus, up to 16 devices on one cable, SCSI initiator requests operation and SCSI targets perform tasks SCSI本身是一个总线，一根电缆上最多有16个设备，SCSI发起程序请求操作，SCSI目标执行任务
- Each target can have up to 8 logical units (disks attached to device controller) 每个目标最多可以有8个逻辑单元（磁盘连接到设备控制器 ）
FC（Fibre Channel，光纤通道 ） is high-speed serial architecture
- Can be switched fabric with 24-bit address space – the basis of storage area networks (SANs) in which many hosts attach to many storage units 可以是具有24位地址空间的交换结构，是许多主机连接到许多存储单元的存储区域网络（SAN）的基础
- Can be arbitrated loop (FC-AL) of 126 devices 可仲裁126个设备的回路（FC-AL）
Network-Attached Storage 网络连接存储
Network-attached storage (NAS) is storage made available over a network rather than over a
local connection (such as a bus) 通过网络而不是通过本地连接（如总线）
NFS and CIFS are common protocols NFS和CIFS是常见的协议
Implemented via remote procedure calls (RPCs) between host and storage
New iSCSI protocol uses IP network to carry the SCSI protocol
Storage Area Network（SAN） 存储区域网络
Common in large storage environments (and becoming more common)
Multiple hosts attached to multiple storage arrays – flexible 附加到多个存储阵列的多个主机
Virtualization 存储虚拟化技术
SNIA（Storage Networking Industry Association，存储网络联合会）官方对Virtualization（存储虚拟化技术）的定义，如下：
- 是将存储（子）系统内部功能与具体应用、主机及通用网络资源分离、隐藏及抽象的行为。以期达到存储或数据管理的网络无关性。
- 对于存储服务及设备的虚拟化应用，以期达到整合设备功能、隐藏复杂细节以及向已经存在的底层存储资源添加新的应用。
Disk Scheduling
The operating system is responsible for using hardware efficiently — for the disk drives, this
means having a fast access time and disk bandwidth
Access time has three major components
Seek time（寻道时间） is the time for the disk are to move the heads to the cylinder
containing the desired sector
Rotational latency（旋转延迟）is the additional time waiting for the disk to rotate the
desired sector to the disk head
Transfer time (传输时间)
Minimize seek time
Seek time seek distance 寻道时间 寻道距离
Disk bandwidth is the total number of bytes transferred, divided by the total time between the
first request for service and the completion of the last transfer
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