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1、LI Wensheng, SCST, BUPT Chapter 12 Mass-Storage Structure Teaching hours: 2h Strong points: Disk Structure Disk Scheduling Disk Management Wensheng Li BUPT 2 Chapter Objectives Describe the physical structure of disk and disk scheduling algorithms Describe the policies of disk management Wensheng Li
2、 BUPT 3 Contents 12.1 Overview of Mass Storage Structure 12.2 Disk Structure 12.3 Disk Attachment 12.4 Disk Scheduling( ) 12.5 Disk Management 12.6 Swap-Space Management 12.7 RAID Structure(*) 12.8 Stable-Storage Implementation (*) 12.9 Tertiary-Storage Structure (*) Wensheng Li BUPT 4 1. Overview o
3、f Mass Storage Structure Magnetic disks provide bulk of secondary storage of modern computers Drives rotate at 60 to 200 times per second Transfer rate is rate at which data flow between drive and computer Positioning time (random-access time) is the time to move disk arm to desired cylinder (seek t
4、ime) and the time for desired sector to rotate under the disk head (rotational latency) Head crash results from disk head making contact with the disk surface Thats bad, cannot be repaired, the entire disk must be replaced Disks can be removable Drive attached to computer via I/O bus Busses vary, in
5、cluding EIDE, ATA, SATA, USB, Fiber Channel, SCSI Host controller in computer uses bus to talk to disk controller built into drive or storage array Wensheng Li BUPT 5 Moving-head Disk Machanism Wensheng Li BUPT 6 Overview of Mass Storage Structure (Cont.) Magnetic tape Was early secondary-storage me
6、dium Relatively permanent and holds large quantities of data Access time slow Random access 1000 times slower than disk Mainly used for backup, storage of infrequently-used data, transfer medium between systems Kept in spool and wound or rewound past read-write head Once data under head, transfer ra
7、tes comparable to disk 20-200GB typical storage Common technologies are 4mm, 8mm, 19mm, inch, inch, named according to technology, LTO-2 and SDLT Wensheng Li BUPT 7 12.2 Disk Structure Disk drives are addressed as large 1- dimensional arrays of logical blocks, where the logical block is the smallest
8、 unit of transfer. (block size is usually 512 bytes) 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. Mapping proceeds in order through that track, then the rest of the tracks in
9、that cylinder, and then through the rest of the cylinders from outermost to innermost. Constant Linear Velocity (常数线性周转率常数线性周转率) 位密度相同位密度相同, CD-ROM, DVD-ROM Constant Angular Velocity (常数角周转率常数角周转率) 位密度不同位密度不同, hard disk Wensheng Li BUPT 8 12.3 Disk Attachment Computer access disk storage in two ways
10、 Via I/O ports (host-attached storage) Via a remote host in a distributed file system (NAS) Wensheng Li BUPT 9 Host-attached storage Is accessed through I/O ports talking to I/O busses IDE, ATA, support maximum of 2 drives per I/O bus SATA（Serial ATA ） SCSI itself is a bus, up to 16 devices on one c
11、able, SCSI initiator (controller card in the host) requests operation and SCSI targets (storage devices) perform tasks A SCSI disk is a common SCSI target Each target can have up to 8 logical units (disks attached to device controller) FC is a high-speed serial architecture, can operate over optical
12、 fiber or over a 4-conductor copper cable, two variants: 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 Can be arbitrated loop (FC-AL，Fibre Channel Arbitrated Loop) of 126 devices Wensheng Li BUPT 10 Network
13、-Attached Storage Network-attached storage (NAS) is storage made available over a network rather than over a local connection (such as a bus) NFS and CIFS（Common Internet File System ） are common protocols, RPC interface Implemented via remote procedure calls (RPCs) between host and storage New iSCS
14、I protocol uses IP network protocol to carry the SCSI protocol Wensheng Li BUPT 11 Storage Area Network Private network, connecting servers and storage units Common in large storage environments (and becoming more common) Multiple hosts attached to multiple storage arrays flexible FC is the most com
15、mon SAN interconnect. Wensheng Li BUPT 12 12.4 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. To read or write, the disk head must be positioned at the desired track and at the beginning
16、 of the desired sector. Access time has two major components: Seek time is the time for the disk arm to move the heads to the cylinder containing the desired sector. Rotational latency is the additional time waiting for the disk to rotate the desired sector to the disk head. Minimize seek time Seek time seek distance Disk bandwidth is the total numbe
