Chapter 10: Mass-Storage Structure

1. Chapter 10: Mass-Storage Structure

2. Chapter 10: Mass-Storage Systems • Overview of Mass Storage Structure • Disk Structure • Disk Attachment • Disk Scheduling • Disk Management • Swap-Space Management • Operating System Support • Performance Issues

3. 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

4. Magnetic Disks • Magnetic disks are most common secondary storage of modern computers • Both sides of disk platter covered with magnetic material • All platters of drive connected to spindle, rotate at 60 to 200 times per second (7200rpm = 120 rotations per second) • Read-write headmoves just microns above the disks • Head crash: if it touches the disk; sectors (sometimes entire disk) is ruined • Arm assembly move all heads of drive along radius of disks

5. Magnetic Disks • Each platter is divided in concentric tracks • Each track divided in sectors • A vertical set of tracks among all disks is a cylinder • All disks rotate as one by the spindle and all heads move as one by the arm assembly • Drive reads one track/sector on the entire cylinder at once

6. Magnetic Disks • You want to read track t sector s • Seek time: time needed to move the head to the desired track • Rotational latency: time needed for disk to rotate to desired sector • Positioning time(random access time): seek time + rotational latency (several milliseconds) • Transfer rate: rate to move data between drive and rest of system (megabytes per second)

7. Magnetic Disks • Drive attached to computer via I/O bus • Commands from rest of computers sent to a host controller, through I/O bus to disk controller built into drive • I/O bus transfer much faster than transfer rate: disk controller has cache • Data transfers from disk to cache to I/O bus

8. Other Mass Storage Devices • Punch card • Paper cards, each hole is a bit • Reading head closes a circuit (through the hole) or not (if no hole) • Actually predates computers! • Tape drive • Large-storage-capacity but slow-access-time magnetic tapes • Forward or rewind to desired information • ROM Cartridge • Portable ROM physically connected to system bus and mapped into system’s memory address space • Floppy disk (8-inch, 5¼-inch, 3½-inch, zip) • Miniature flexible portable magnetic disks • Optical disk (CD, DVD, Blu-Ray) • Reflective disks (instead of magnetic): reflection is 1, absorption is 0 • Laser read-write head • Typically read-only or write-once • Solid-state drive (SSD, flash drives) • Electronic (transistor and logic gates) storage (instead of magnetic or reflective) • No moving parts: random access time in microseconds!

9. Disk Structure • Disk sectors are of constant data size (normally 512 bytes) • But outer tracks are physically larger than inner ones • Bit density: number of bits per physical area • We want to keep data transfer rate constant • Constant linear velocity • Keep bit density constant on drive • Result: physically larger tracks hold more sectors • Increase speed for inner tracks • Read same number of sectors per unit of time • Method used in CDs and DVDs • Constant angular velocity • Increase bit density for inner tracks • Result: same number of sectors in outer and inner tracks • Keep rotation speed constant • Method used in HDD

10. Disk Scheduling • Processes make requests for information on disk • OS responsible for using the disk efficiently • Maximize bandwidth: amount of data transferred divided by time needed to complete all requests • Maximized by handling requests as efficiently as possible • Bandwidth has multiple components • Positioning time (seek time + rotational latency) • Read/write time for the head (constant) • Transfer rate (constant) • Maximize bandwidth = minimize seek time • Seek time  seek distance

11. Disk Scheduling • Several algorithms exist to schedule the servicing of disk I/O requests • We can compare them by using a reference string of requests Requested sectors: 98, 183, 37, 122, 14, 124, 65, 67 Initial read-write head position: 53

12. FCFS • First-Come-First-Serve: Service each request in the order it arrives • Easiest to implement • Total seek distance: 640 cylinders

13. SSTF • Shortest-Seek-Time-First: Select the request with the minimum seek time from the current head position • Can cause starvation • Total seek distance: 236 cylinders

14. SCAN • SCAN algorithm (elevator algorithm): move the head back and forth from one end of the disk to the other, and service requests as you get to them • No chance of starvation • Total seek distance: 236 cylinders

15. C-SCAN • Circular-SCAN (C-SCAN) algorithm: move the head from one end of the disk to the other, and service requests as you get to them, then jump back to the beginning and start again • Total seek distance: 382 cylinders • But actually more efficient than SCAN!

16. C-LOOK • C-LOOKalgorithm: variant of C-SCAN that goes to the last request rather than the end and beginning of the disk • Total seek distance: 322 cylinders

17. Disk Scheduling • So which scheduling algorithm is best? • Depends on the number and types of requests… and on the file-allocation method! • Number of requests • Very few requests: SSTF and C-Look reduce to FCFS due to lack of choice, SCAN and C-SCAN are very inefficient • Lots of requests: SCAN and C-SCAN more efficient & avoid starvation • File-allocation method • Contiguous allocation results in lots of nearby requests: FCFS can work • Linked or indexed allocation results in scattered requests: FCFS is pretty bad • Location of directory entries vs. files • FAT: entry at the beginning of disk, file in data region = large disk seek • UFS: inode before each file = contiguous requests

18. Disk Scheduling • Types of requests • OS differentiates between read & writes, pages & files • Swapping pages given priority over file I/O (especially during page faults) • Writing data to files given priority over reading when cache is near full • Default choice given no info? • Typically SSTF or C-LOOK

19. Disk Formatting • When a new magnetic disk is built, it’s just a slab of magnetic material • Low-level formatting(physical formatting) • Dividing a disk into sectors that the disk controller can read and write • A sector has • A data section (256 to 1024 bytes, typically 512) • A header and trailer • An error-correcting code (ECC) that allows the controller to automatically detect and correct a few bits wrong • Logical formatting • Creating the file-system data and structures • Marking all sectors as free space

20. Bad Blocks • All disks have unusable bad blocks • IDE disks on MS-DOS • Bad blocks detected manually by disk formatting or scanning tools (format and chkdsk) • OS writes special value in FAT to make them unusable • Information in sector lost • SCSI disks: sector forwarding • Controller (not OS) handles the bad block list • Low-level formatting sets aside spare sectors not visible to OS • When a bad sector is found, it is substituted for a spare sector • Information is recovered and copied thanks to ECC • SCSI disks: sector slipping • Like forwarding, but move all sectors down one spot to keep sector order • Sector 17 (bad) and sector 200 (spare), so 199 copied to 200, 198 copied to 199, …, 18 copied to 19, 17 copied to 18

21. Swap-Space Management • Middle-term schedule swaps information between memory and disk • Swap-space size • Swap pages to free a few frames (needs several KB to a few MB) • Swap entire processes to free lots of memory (needs several GB) • Solaris estimates swap space as difference between virtual memory and physical memory, Linux as 2x physical memory space • Swap-space location • One large file allocated in the file system • Can be expanded just like any file • Need to access through the file system: slower • Raw partition managed by OS • Managed for speed rather than efficient storage • Can suffer from fragmentation, but ok because data is short-lived • Expanding it requires repartitioning drive

22. Given a magnetic disk with multiple platters, each with its own read-write head, is it possible to read sectors on different cylinders on different platters at the same time? How can the file system used on a disk affect our choice of a scheduling algorithm? Review

23. Exercises • Skip the following sections: 10.3 (Disk Attachment), 10.7 (RAID Structure), and 10.8 (Stable Storage Implementation) • If you have the “with Java” textbook, skip the Java sections and subtract 1 to the following section numbers • 10.1 • 10.2 • 10.3 • 10.5 • 10.9 • 10.10 • 10.11 • 10.14 • 10.15 • 10.16 • 10.21 • 10.22

24. End of Chapter 10