Moving-head Disk Mechanism

1. Moving-head Disk Mechanism Rotation Speeds: 60 to 200 rotations per second Head Crash: read-write head makes contact with the surface

2. Disk Structure • Cylinder: the set of tracks that all the heads are currently located at. • Track: A ring on a disk where data can be written • Sector: The smallest transfer unit of data accessed in a block • Cluster: A group of sectors the operating system treats as a unit • Organization Choices • Sector mapping (One dimension array of logical blocks) • 0 is first sector, track 0 of the outermost cylinder. • Subsequent sectors map through tracks, through cylinders, in an outer to inner direction. • Sector counts and density • fixed sectors per track: varying densities • Varied sectors per track: outer tracks have more sectors; constant density • Bad block management • Sector sparing: replace bad sectors with spares in the same cylinder • Sector slipping: copy all sectors down to the next spare

3. Magnetic Tape • Early secondary-storage medium of choice • Persistent, inexpensive, and has large data capacity • Very slow access due to sequential nature • Used for backup and for storing infrequently-used data • Kept on spools • Transfer rates comparable to disk if read write head is positioned to the data • 20-200GB are typical storage capacities

4. Storage Attached to a Host System • Basic: Connections are accessed through I/O ports communicating through busses • Small Computer System Interface (SCSI) protocol • Connects to sixteen devices • One SCI initiator and up to fifteen targets • Each target controls up to 8 logical units • Logical units can be RAID arrays or a CD jukebox • Fibre Channel (FC) is high-speed architecture • Private high speed switched network to many attached hosts • Storage can be dynamically allocated to each hosts • Other interface Standards: USB, EIDE, ATA

5. Network-Attached Storage (NAS) • Storage made available over a network • Protocols • NFS is a common protocols • iSCSI embeds SCSI protocol in IP. • Implementation: remote procedure calls (RPCs)

6. Storage Area Network • Popular for large storage environments • Private high speed network • Flexible for attaching many hosts to multiple storage arrays

7. Basic Storage System Concepts • The OS presents a one of two virtual storage abstraction • Raw device: an array of data blocks (a partition). • File system: OS schedules interleaved application requests • Goal: optimize disk access time and throughput • Access time (seek time + rotational latency) • Seek time: moving the heads to the cylinder with the data • Rotational latency: rotate the disk head to the desired sector • Transfer rate: data flow speed between drive and computer • Operating systems attempts to minimize seek time; hardware optimizes rotational latency • Definitions • Throughput: bytes transferred per unit time • Disk bandwidth: total bytes transferred / (time from first request to time of last transfer)

8. First Come First Serve (FCFS) Service the first request first This Illustration shows a total head movement of 640 cylinders.

9. Shortest Seek Time First (SSTF) Service the request closest to the current head position • SSTF scheduling is similar to SJF scheduling • Starvation of some requests is possible • This Illustration shows total head movement of 236 cylinders.

10. SCAN (Elevator) and C-Scan SCAN: Service the first request encountered in the current head direction C-Scan: Services in one direction; SCAN: Services in both directions C-Scan: Return to the beginning after each pass (circular list) • C-Scan: Provides a more uniform wait time than SCAN. • Repositioning to the beginning is faster than repositioning in small pieces because of acceleration and deceleration. Head going left Head going right Movement = 236 cylinders Movement = 382 cylinders

11. Look and C-Look Similar to SCAN and C-SCAN except don’t go to end of disk on each pass Head going right Movement = 322 cylinders C-Look Algorithm

12. Which Disk Scheduling Algorithm? • SSTF applies if starvation is not an issue (light loads) • SCAN and C-SCAN perform better with heavy loads • Requests for disk service can be influenced by the file-allocation method • The disk-scheduling algorithm • Normally a separate background running module of the operating system • It can easily be replaced if desired

13. Disk Management • Factory Low-level formatting • Divide disk into sectors for access by a disk controller • OS Partitioning • Divide disks into separate groups of ‘raw’ cylinders • Sector sparing is a technique to recover from “bad” blocks • User Logical formatting • Initialize the file system MS DOS Structure

14. Disk Booting in Windows 2000 • The ROM bootstrap reads the boot sector, which then proceeds to load the OS

15. Swap-Space Management • Swap-space: A disk extension of main memory • Implementation can be: • a normal part of the file system (slower but flexible) • in a separate disk partition (faster but less flexible) • Examples • 4.3 BSD • Allocates swap space when process starts • holds text segment (the code) and data segment • Solaris 2: allocates swap space only when a page is forced to disk

16. Virtual memory extension of main memory Swap Space BSD Program (Text) Segments BSD Data Segments

17. Redundant Array of Inexpensive Disks (RAID) • Goal: achieve reliability through redundancy or increase speed through parallelism • Seven Raid Organizations 0: Non-redundant striping: Parts of data stored on different disks. Group of disks acts as a single storage device 1: Mirrored data: stores (shadows) duplicates of each disk 2: Error correction codes: Parity is spread over a group of dedicated disks 3: Bit interleaved parity: Parity is written to a dedicated disk 4: Block interleaved parity: Parity stored on a disk separate from the data 5: Block-interleaved distributed parity: stripes data and parity across the RAID 6: P + Q redundancy: Parity and data are written, both with redundancy Parity: Information to reconstruct data in case of failure

18. RAIDExamples • Top: striped and then mirrored • Bottom: mirrored and then striped

19. Stable Storage Definition: Storage that carries a “guarantee” of reliability • Transactions and write-ahead logs require stable storage • To implement stable storage: • Replicate information on more than one nonvolatile storage media with independent failure modes. • Update information in a controlled manner to ensure that we can recover the stable data after any failure that happens during data transfer or recovery. • After failure, copy incomplete copies from the media that has successfully completed its operation.

20. Tertiary (Removable) Storage • Characteristic: Low cost, high capacity • Examples • Removable magnetic disks: nearly as fast as hard disks, but less reliable • A magneto-optic disk: records on rigid platter coated with magnetic material • The read write heads are further from the head, resistant to head crash • lasers used to amplify magnetic material for write and detect • Optical disks: Altered by laser light. Very durable and reliable. • Tape: Falling price gap between tapes and disks, makes these less popular • Hierarchical Storage Management (HSM) • Extends primary and secondary storage to include tertiary storage • Found in supercomputing centers that have enormous volumes of data • Usually implemented as a jukebox of removable disks • Disk-resident files are archived to a tape or disk library for low cost storage. • Files are automatically staged back (slowly) to disk for active use Note: fixed drives tend to be more reliable than removable ones

21. Cost Price/Megabyte Hard DiskFrom 1981 to 2000 Price/MB DRAMFrom 1981 to 2000 Price/Mb Tape, From 1984-2000