Operating Systems Chapter 11 and 12

Scan

Arm moves in one direction only. Satisfies all requests until it reaches the last track then is reversed. Favors jobs on the inner and outermost tracks

RAID 3

Bit-interleaved parity. Requires only a single redundant disk, no matter how large the disk array.

RAID 5

Block-level distributed parity. Similar to RAID-4 but distributes the parity bits across all disks

RAID 4

Block-level parity. Makes use of an independent access technique. Involves a write penalty when an I/O write request of small size is performed

Database

Collection of related data. Relationships among data are explicit. Consists of one or more types of files

Record

Collection of related fields that can be treated as a unit by some application program. Fixed or variable length

File

Collection of similar records. Treated as a single entity. Referenced by name. Access control restrictions usually apply at the file level

PRI: Priority

Control of the scheduling is outside the control of the disk management software. Goal is NOT to maximize disk utilization. Short batch jobs and interactive jobs given higher priority. Poor policy for DBS

File operations

Create Delete Open Close Read Write

The Pile

Data collected in order they arrive. just gathers data. No structure or organization or standard record length or keys

Files

Data collections created by users. Desirable properties: long-term existence, sharable between processes, structure

Generality Design Objectives

Desirable to handle all devices in a uniform manner. Applies to the way process view I/O devices and the way the OS manages I/O devices

RAID 6

Dual Redundancy. Two different parity calculations are carried out and stored in separate blocks on different disks

Logical I/O

Enables users and applications to access records. Provides general purpose record I/O capability. Maintains basic data about file

Hierarchical Design

Functions of the OS should be separated according to their complexity, their characteristic time scale, and their level of abstraction. Leads to an organization of the OS in a series of layers

Block-oriented Single Buffer

Input transfers are made to the system buffer. Disadvantages: Complicates the logic in the OS, swapping logic is also affected.

Access Method

Level of file system closest to the user. Standard interface between applications and the file system and devices that hold the data.

Device Drivers

Lowest level. Communicate directly with peripheral devices. Responsible for starting I/O operations.

Efficiency Design Objectives

Major effect in I/O design. Important because I/O ops often form a bottleneck. Most I/O devices are very slow

RAID 1

Mirrored. Redundancy is achieved by the simple expedient of duplicating all the data. Disadvantage is the cost.

Sequential File

Most common. Fixed format used for records, key field uniquely identifies the record. Length and position of each field is known

Do the records in an indexed file have to be sorted?

NO

RAID 0(Zero)

Not a true raid. Data distributed across all drives. non-redundant

Single Buffer

OS assigns a buffer in main memory for an I/O request

Buffering

Performs input transfers in advance of requests being made and performs output transfers some time after the request is made.

FIFO: First In First Out

Processes in sequential order. Fair to all processes. Approximates random scheduling in performance if there are many processes competing for the disk

Indexed File

Records accessed through indexes. Variable length records. Used mostly in applications where timeliness of information is critical and where data are rarely processed exhaustively

RAID 2

Redundancy through Hamming Code. Makes use of a parallel access techniques.

Basic I/O Supervisor

Responsible for all file I/O initiation and termination. Selects the device on which file I/O is to be performed, based on the particular file selected. It is also concerned with scheduling disk and tape accesses to optimize performance

C-SCAN: Circular Scan

Restricts scanning to one direction only. When the last track is visited in one direction, the arm is returned to the appropriate side of the disk

What advantage is realized by a disk that spins 10,000 rpm over one that spins 7,200 rpm?

Rotational delay will be reduced when doing data transfer. Seek time will not be affected. Data transfer time will be improved.

What distinguishes between I/O devices?

SPEED

Operations Performed on a Directory

Search Create Files Delete Files List directory Update directory

What constitutes the time it takes to read OR write data (to a hard-drive)?

Seek time(time it takes to move head) Rotational Delay(how fast HD is spinning) Data Transfer(actual transfer of electrons transferring data)

N-Stop-Scan

Segments the disk request queue into sub queues of length n. Processed one at a time using Scan. New requests must be added to another queue while the current one is processed.

SSTF: Shortest Service Time First

Select the disk I/O request that requires the least movement of the disk arm from its current position

Criteria for file organization

Short access time Ease of update Economy of storage Simple maintenance Reliability

Machine Readable

Suitable for communicating with electronic equipment. Disk drives, USB drives, sensors

Programmed I/O

The processor issues an I/O command on behalf of a processor to an I/O module. That process then busy waits for the operation to be completed.

Interrupt-driven I/O

The processor issues and I/O command on behalf of a process. Could be blocking or non-blocking.

Circular Buffer

Two or more buffers are used. Each individual buffer is one unit in a circular buffer. Used when I/O operation must keep up with process operation

Double Buffer

Use two system buffers instead of one. A process can transfer data to or from one buffer while the OS empties or fills the other buffer. Also known as buffer swapping.

Parity Bit

Used as the simplest form of error detecting code. AKA Check bit(added to end of string in binary code that indicates whether the number of bits in the string with the value 1 is even or odd)

Disk Cache Memory

Used to apply to a memory that is smaller and faster than main memory and that is interposed between main memory and the processor. Reduces average access time

Stream-Oriented Single Buffer

Uses line at a time or byte at a time operation

F-Scan

Uses two sub queues. When a scan begins all the requests are in one of the queues with the other one empty. During Scan all requests are placed in a new queue

No Buffer

Without a buffer the OS directly accesses the device when it needs it

Do the records in an indexed sequential file have to be sorted?

YES

Field

basic element of data. contains a single value. fixed or variable length

File organization

logical structuring of the records determined by the way in which they are accessed

Communication

suitable for communicating with remote devices. Modems, digital line drivers

Human Readable

suitable for communicating with the computer user. Printer, terminals, video display

Block Oriented Devices

transfer blocks of data at a time, usually a fixed size. Reference by block #. Disks and USB keys

Stream Oriented Devices

transfer data at a time as a stream of bytes. Terminals, printers

File Management System Objectives(7)

• Meet the data management needs and requirements of the user, which include storage of data. • To guarantee, to the extent possible, that the data in the file are valid • To optimize performance, both from the system point of view in terms of overall throughput and from the user's point of view in terms of response time • Provide I/O support for a variety of storage device types • Minimize or eliminate the potential for lost or destroyed data • Provide a standardized set of I/O interface routines to user processes • Provide I/O support for multiple users, in multiple-user systems

Minimal User Requirements(7)

1. Each user should be able to create, delete, read, write, and modify files. 2. Each user may have controlled access to other users' files. 3. Each user may control what types of accesses are allowed to the user's files. 4. Each user should be able to restructure the user's files in a form appropriate to the problem. 5. Each user should be able to move data between files. 6. Each user should be able to back up and recover the user's files in case of damage. 7. Each user should be able to access his or her files by name rather than by numeric identifier.

Direct Memory Access

A DMA module controls the exchange of data between main memory and an I/O module

RAID: Redundant Array of Independent(or inexpensive) Disks

A way of organizing and using your HD. Set of physical drives viewed by the OS as a single logical drive. Data is distributed across the physical drives of an array in a scheme known as striping.

Basic File System

AKA Physical I/O level. Deals with blocks of data that are exchanged with disk or tape systems. Buffering blocks in main memory.

Direct/Hashed File

Access directly any block of known address. Hashes o a key value. Used where rapid access is required. fixed-length records used, records always accessed at one time

Indexed Sequential FIle

Adds an index to the file to support random access, overflow file added. Multiple levels of indexing can be used. Greatly reduces time required to access a single record