Moving-head Disk Mechanism

Rotation Speeds: 60 to 200 rotations per secondHead Crash: read-write head makes contact with the surface

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

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

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

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)

Storage Area Network

• Popular for large storage environments

• Private high speed network

• Flexible for attaching many hosts to multiple storage arrays

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)

First Come First Serve (FCFS)

This Illustration shows a total head movement of 640 cylinders.

Service the first request first

Shortest Seek Time First (SSTF)

• SSTF scheduling is similar to SJF scheduling• Starvation of some requests is possible• This Illustration shows total head movement of 236 cylinders.

Service the request closest to the current head position

SCAN (Elevator) and C-Scan

• 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.

SCAN: Service the first request encountered in the current head directionC-Scan: Services in one direction; SCAN: Services in both directionsC-Scan: Return to the beginning after each pass (circular list)

Movement = 236 cylinders Movement = 382 cylinders

Head going left Head going right

Look and C-LookSimilar 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

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

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

Disk Booting in Windows 2000

The ROM bootstrap reads the boot sector, which then proceeds to load the OS

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

Swap Space

BSD Program (Text) Segments

BSD Data Segments

Virtual memory extension of main memory

Redundant Array of Inexpensive Disks (RAID)

• Goal: achieve reliability through redundancy or increase speed through parallelism

• Seven Raid Organizations0: Non-redundant striping: Parts of data stored on different disks. Group of disks acts as a single storage device1: Mirrored data: stores (shadows) duplicates of each disk2: Error correction codes: Parity is spread over a group of dedicated disks3: Bit interleaved parity: Parity is written to a dedicated disk4: Block interleaved parity: Parity stored on a disk separate from the data5: Block-interleaved distributed parity: stripes data and parity across the RAID6: P + Q redundancy: Parity and data are written, both with redundancy

Parity: Information to reconstruct data in case of failure

RAIDExamples

• Top: striped and then mirrored

• Bottom: mirrored and then striped

Stable Storage

• 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.

Definition: Storage that carries a “guarantee” of reliability

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

Cost

Price/MB DRAMPrice/MB DRAMFrom 1981 to 2000From 1981 to 2000

Price/Megabyte Hard DiskPrice/Megabyte Hard DiskFrom 1981 to 2000From 1981 to 2000

Price/Mb Tape, From 1984-2000Price/Mb Tape, From 1984-2000