1. Chapter 12: Secondary-Storage Structure

2. 12.2 Silberschatz, Galvin and Gagne ©2005 Operating System Principles 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 Disk Attachment 12.8 Stable-Storage Implementation 12.9 Tertiary Storage Devices Operating System Issues Performance Issues

3. 12.3 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Objectives Describe the physical structure of secondary and tertiary storage devices and the resulting effects on the uses of the devices Explain the performance characteristics of mass- storage devices Discuss operating-system services provided for mass storage, including RAID and HSM (hierarchical storage management)

4. 12.4 Silberschatz, Galvin and Gagne ©2005 Operating System Principles 12.1 Overview of Mass Storage Structure Magnetic disks provide bulk of secondary storage of modern computers Drives rotate at 60 to 200 times per second (3600 to 12000 rpm) Transfer rate is rate at which data flow between drive and computer (400Mb-6Gb/sec) Positioning time (random-access time) is time to move disk arm to desired cylinder (seek time, 4 micro seconds) and time for desired sector to rotate under the disk head (rotational latency, 3 micro seconds) Example: Hitachi Ultrastar C10K300 (http://www.zdnet.com.tw/news/hardware/0,2000085676,201370 89,00.htm) Head crash results from disk head making contact with the disk surface  Normally cannot be repaired

5. 12.5 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Overview Disks can be removable Held in plastic case Drive attached to computer via I/O bus Buses vary, including EIDE, ATA, SATA, USB, FC (Fiber Channel), SCSI Host controller in computer uses bus to talk to disk controller built into drive or storage array using memory- mapped I/O ports Disk controllers usually have built-in cache (MB)

6. 12.6 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Moving-head Disk Machanism

7. 12.7 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Magnetic tape Was early secondary-storage medium 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 rates comparable to disk 20-200GB typical storage Common technologies are 4mm, 8mm, 19mm, ¼ inch, ½ inch, LTO-2 and SDLT

8. 12.8 Silberschatz, Galvin and Gagne ©2005 Operating System Principles 12.2 Disk Structure Disk drives are addressed as large 1-dimensional arrays of logical blocks, where the logical block is the smallest unit of transfer. A logical block is usually of size 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 that cylinder, and then through the rest of the cylinders from outermost to innermost.

9. 12.9 Silberschatz, Galvin and Gagne ©2005 Operating System Principles In practice, it is difficult to convert a logical block number into cylinder, track, sector : defective sectors and the number of sectors per track is not a constant CD-ROM and DVD-ROM increase their rotation speed as the head moves from the outer to inner tracks to keep the same data rate (the density of bits per track is constant, constant linear velocity, CLV) In some disks, the rotation speed is constant, and the density of bits decreases from inner tracks to outer tracks to keep the data rate constant (constant angular velocity, CAV)

10. 12.10 Silberschatz, Galvin and Gagne ©2005 Operating System Principles 12.3 Disk Attachment Host-attached storage accessed through I/O ports talking to I/O busses IDE, ATA, SATA, SCSI, FC SCSI itself is a bus, up to 16 devices on one cable, SCSI initiator requests operation and SCSI targets perform tasks Each target can have up to 8 logical units (disks attached to device controller FC 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 Can be arbitrated loop (FC-AL) of 126 devices

11. 12.11 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Network-Attached Storage (NAS) NAS is storage made available over a network rather than over a local connection (such as a bus) NFS (Unix) and CIFS (Windows) are common protocols Implemented via remote procedure calls (RPCs) between host and storage New ISCSI protocol uses IP network to carry the SCSI protocol so that hosts could treat NAS as directly attached

12. 12.12 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Storage Area Network (SAN) Common in large storage environments becoming more common Multiple hosts attached to multiple storage arrays flexible FC is the most common SAN interconnect InfiniBand is an emerging alternative SAN bus architecture

13. 12.13 Silberschatz, Galvin and Gagne ©2005 Operating System Principles 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 Access time has two major components Seek time is the time for the disk are 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 number of bytes transferred, divided by the total time between the first request for service and the completion of the last transfer

14. 12.14 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Disk I/O System Call The system call for disk I/O specifies This operation is input or output The disk address for the transfer Memory address for the transfer The number of sectors to be transferred For a multiprogramming system, normally, the disk request queue has several pending requests

15. 12.15 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Disk Scheduling Several algorithms exist to schedule the servicing of disk I/O requests. We illustrate them with a disk request queue of requests for blocks on cylinders (0-199): 98, 183, 37, 122, 14, 124, 65, 67 The sequence indicates their order in time Suppose the disk head pointer is initially at cylinder 53

16. 12.16 Silberschatz, Galvin and Gagne ©2005 Operating System Principles First Come First Served (FCFS) Illustration shows total head movement of 640 cylinders.

17. 12.17 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Shortest Seek Time First (SSTF) Selects the request with the minimum seek time from the current head position SSTF scheduling is a form of SJF (shortest job first) scheduling; may cause starvation of some requests. Illustration shows total head movement of 236 cylinders.

18. 12.18 Silberschatz, Galvin and Gagne ©2005 Operating System Principles SCAN The disk arm starts at one end of the disk, and moves toward the other end, servicing requests until it gets to the other end of the disk, where the head movement is reversed and servicing continues. Sometimes called the elevator algorithm. Illustration shows total head movement of 208 cylinders.

19. 12.19 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Circular SCAN (C-SCAN) Provides a more uniform wait time than SCAN. The head moves from one end of the disk to the other. servicing requests as it goes. When it reaches the other end, however, it immediately returns to the beginning of the disk, without servicing any requests on the return trip. Treats the cylinders as a circular list that wraps around from the last cylinder to the first one.

20. 12.20 Silberschatz, Galvin and Gagne ©2005 Operating System Principles C-SCAN

21. 12.21 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Look and C-LOOK Version of SCAN and C-SCAN Arm only goes as far as the last request in each direction, then reverses direction immediately, without first going all the way to the end of the disk. LOOK or C-LOOK?

22. 12.22 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Selecting a Disk-Scheduling Algorithm SSTF is common and has a natural appeal SCAN and C-SCAN perform better for systems that place a heavy load on the disk. Performance depends on the number and types of requests. Requests for disk service can be influenced by the file- allocation method. The disk-scheduling algorithm should be written as a separate module of the operating system, allowing it to be replaced with a different algorithm if necessary. Either SSTF or LOOK is a reasonable choice for the default algorithm.

23. 12.23 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Selecting a Disk-Scheduling Algorithm Difficult to schedule for improved rotational latency Disk manufacturers have implemented disk scheduling algorithms in the controller hardware In practice, OS has other constraints Demand paging may take priority over application I/O Writes are more urgent than reads if cache is running out of free pages Sometimes the order of writes must be kept to make the file system robust in case of system crashes

24. 12.24 Silberschatz, Galvin and Gagne ©2005 Operating System Principles 12.5 Disk Management Low-level formatting, or physical formatting — Dividing a disk into sectors that the disk controller can read and write. Header and trailer of sector contain information used by the disk controller, such as a sector number and an error-correcting code (ECC) Most hard disks are low-level formatted at the factory To use a disk to hold files, the operating system still needs to record its own data structures on the disk Partition the disk into one or more groups of cylinders  Each partition is treated as a separate disk Logical formatting or “creation of a file system”  Maps of free and allocated space and an empty directory

25. 12.25 Silberschatz, Galvin and Gagne ©2005 Operating System Principles To increase efficiency, most file system group blocks together into clusters Disk I/O is via blocks, but file I/O is via clusters Some OS’s allow raw-disk Example: DBMS Boot block initializes system The bootstrap is stored in ROM. Only Bootstrap loader program is stored A disk with a boot partition is called a boot disk or system disk

26. 12.26 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Booting from a Disk in Windows 2000 Master Boot Record (MBR)

27. 12.27 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Bad Blocks Bad blocks are handled manually on simple disks, such as disks with IDE controllers Example: chkdsk in MS-DOS Sophisticated disk controller, like SCSI, maintains a list of bad blocks sector sparing: the controller could replace each bad sector logically with a spare sector  May invalidate optimization of disk-scheduling  Solution: provide spare sectors in each cylinder and a spare cylinder as well sector slipping: remaps all sectors from the defective one to the next available sector by moving them all down one spot