COSC 3360 - Quiz 3 (I/O Management and Disk Scheduling, 11 & File Management, 12)

Two issues arise when allowing files to be shared among a number of users:

- Access rights - Management of simultaneous access

File Systems Operations

- Create - Delete - Open - Close - Read - Write

Differences in I/O Devices

- Data Rate - Application - Complexity of Control - Unit of Transfer - Data Representation - Error Conditions

Four terms are commonly used when discussing files:

- Field - Record - File - Database

To manage buffer cache, three lists are maintained:

- Free list - Device list - Driver I/O queue

Categories of I/O Devices

- Human readable - Machine readable - Communications

Five of the common file organizations:

- Indexed file - Direct or hashed file - Pile - Sequential file - Indexed sequential file

Access Rights

- None - Knowledge - Execution - Reading - Appending - Updating - Changing protection - Deletion

User Access Rights

- Owner - Specific users - User groups

Types of operations performed on the directory:

- Search - Create files - Delete files - List directory - Update directory

The basic elements of an access matrix is:

- Subject - Object - Access Right

The actual details of the disk I/O operation depend on the:

- computer system - operating system - nature of the I/O channel and disk controller hardware

Criteria for File Organization

- short access time - ease of update - economy of storage - simple maintenance - reliability

Field

A basic element of data that contains a single value that is of fixed of variable length.

Least Frequently Used (LFU)

A cache replacement policy that evicts whichever block has been used the least often, over some period of time. The block that has experienced the fewest references is replaced. Counter is associated with each block and incremented each time block is accessed.

Volumes

A collection of addressable sectors in secondary memory that an OS or application can use for data storage.

Object

A component in the access matrix that relates to anything to which access is controlled.

Subject

A component in the access matrix which is an entity capable of accessing objects.

Access Right

A component in the access matrix which is the way in which the object is accessed by a subject.

File Allocation Table (FAT)

A data structure that is used to keep track of the portions assigned to a file.

Unbuffered I/O

A device that is simply the DMA between device and process space. Always the fastest method for a process to perform I/O.

Chained Allocation

A file allocation system where allocation is on an individual block basis. Each block contains a pointer to the next block in the chain and the file allocation table needs just a single entry for each file. External fragmentation is not an issue and is best for sequential files.

Indexed Sequential File

A file organization system that is accessed using an index, which is a separate file. Greatly reduces the time required to access a single record. Multiple levels of indexing can be used to provide greater efficiency in access.

Indexed File

A file organization system where an attribute has been extracted from the records and used to build a new file whose purpose is to provide an index to the original file. Used mostly in applications where timeliness of information is critical.

Direct or Hashed File

A file organization system where the file is accessed directly to any block of a known address. Makes use of hashing on the key value. Often used where very rapid access is required, fixed-length records are used, or records are always accessed one at a time. Examples: - Directories - Pricing ables - Schedules - Name lists

RAID 0

A level of RAID that is not a true raid because it does not include redundancy to improve performance or provide data protection. User and system data are distributed across all of the disks in the array. Logical disk is divided into strips.

RAID 5

A level of RAID that is similar to RAID 4 but the parity bits are distributed across all disks. Typical allocation is a round-robin scheme. The loss of any one disk does not result in data loss.

RAID 6

A level of RAID that is similar to RAID 5 but two different parity calculations are carried out and stored in separate blocks on different disks, providing extremely high data availability. Incurs substantial write penalty because each write affects two parity blocks.

RAID 4

A level of RAID where it is similar to RAID 3, but makes use of an independent access technique. Bit-by-bit parity strip is calculated across corresponding strips on each data disk and parity bits are stored in the corresponding bits on the parity disk. Involves write penalty when an I/O write request of small size is performed.

RAID 2

A level of RAID where it makes the use of parallel access technique. Data striping and Hamming code is used. Effective choice where many disk errors might occur.

RAID 3

A level of RAID where it only requires a single redundant disk, no matter how large the disk array. Employs parallel access with data distributed in small strips. Can achieve very high data transfer rates.

RAID 1

A level of RAIDwhere redundancy is achieved by the simple expedient of duplicating all the data. No write penalty and when a drive fails, the data may be still accessed from the second drive. Principal disadvantage is the cost.

Fixed-Length Blocking

A method of blocking where fixed-length records are used and an integral number of records are stored in a block.

Variable-Length Spanned Blocking

A method of blocking where variable-length records are used and are packed into blocks with no unused space.

Variable-Length Unspanned Blocking

A method of blocking where variable-length records are used but spanning is not employed.

Dynamic Allocation Policy

A policy that allocates space to a file in portions as needed.

Preallocation Policy

A policy that requires the maximum size of a file be declared at the same time of the file creation request. Tends to be wasteful because users and application programmers tend to overestimate size.

First-In, First-Out (FIFO)

A process scheduling method that is fair to all processes and processes in sequential order.

Shortest Service Time First (SSTF)

A process scheduling method that selects the disk I/O request that requires the least movement of the disk arm from its current position. Always chooses the minimum seek time.

SCAN

A process scheduling method where the arm moves in one direction only, also known as the elevator algorithm. Favors jobs whose requests are for tracks nearest to both innermost and outermost tracks.

FSCAN

A processing scheduling method where two sub-queues are used. When scan begins, all requests are in one of the queues, with the other empty. During scan, all new requests are put into the other queue. Service of new requests deferred until all of the old requests have been processed

Character Queue

A queue that is used by character oriented devices.

Least Recently Used (LRU)

A replacement scheme in which the block replaced is the one that has been unused for the longest time. Most commonly used algorithm in disk cache replacement.

Two-Level Scheme

A scheme where there is one directory for each user and a master directory. The master directory has an entry for each user directory providing address and access control information. Each user directory is a simple list of the files of that user.

Redundant Array of Independent Disks (RAID)

A set of physical disk drives viewed by the operating system as a single logical drive. Redundant disk capacity is used to store parity information, which guarantees data recoverability in the even of a disk failure. Data distributed across the physical drives of an array in a scheme known as striping.

Contiguous File Allocation

A system where files are stored in sequential block/cluster addresses. A single contiguous set of blocks is allocated to a file at the time of file creation. Preallocation strategy is used with variable-size portions. It is the best form the point of view of the individual sequential file.

Chained Free Portions

A system where the free portions may be chained together by using a pointer and length value in each free portion. Negligible space overhead because there is no need for a disk allocation table. Suited to all file allocation methods. - Leads to fragmentation - Every time you allocate a block, you nee to read the block first to recover the pointer to the new first free block before writing data to that block

Circular Buffer

A system where 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.

Double Buffer

A system where two system buffers are used instead of one. Process can transfer data to or from one buffer while OS empties or fills the other buffer, also known as buffer swapping

Bit Tables

A table that uses a vector containing one bit for each block on the disk. (0=free, 1=in use) + Works well with any file allocation method + It is as small as possible

Block-Oriented Single Buffer

A type of buffer where the input transfers are made to the system buffer. Reading ahead and anticipated input. Provides speedup compared to lack of system buffering. - Complicates logic in the OS - Swapping logic is also affected

N-Step-SCAN

A variation of the SCAN scheduling strategy for direct access storage devices that's used to optimize seek times. Segments disk request queue in N sub-queues. Subqueues processed one at a time, using SCAN. If fewer than N requests avilable at the end of a scan, all of them are processed within next scan.

Basic File SYstem

Also referred to the physical I/O level. It is the primary interface with the environment outside the computer system and deals with blocks of data that are exchanged with disk or tape systems. Concerned with the placement of blocks on the secondary storage device. Concerned with buffering blocks in main memory. Considered part of the operating system.

Machine Readable I/O Device

An I/O device that is suitable for communicating with electronic equipment. Such as disk drives, USB keys, sensors, and controllers.

Communication I/O Device

An I/O device that is suitable for communicating with remote devices. Such as modems or digital line drivers.

Human Readable I/O Device

An I/O device that is suitable for communicating with the computer user. Such as terminals, displays, keyboard, or mouse.

Interrupt Driven I/O

An I/O scheme that employs interrupts to indicate to the processor that an I/O device needs attention. Signals blocking or non-blocking.

Indexing

An approach that treats free space as a file and uses an index table as it would for file allocation. This approach provides efficient support for all of the file allocation methods.

Record

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

Database

Collection of related, organized data where relationships among elements of data are explicit. Designed for use by a number of different applications and consists of one or more types of files.

File

Collection of similar records and treated as a single entity. May be referenced by name and access control restrictions usually apply at the file level.

Priority (PRI)

Control of scheduling is outside the control of disk management software. Goal is not to optimize disk utilization but to meet objectives. Short batch jobs and interactive jobs are given higher priority. Provides for good interactive response time. Terrible policy for database systems. Longer jobs may have to wait an excessively long time.

Logical I/O

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

Free Space Management

File system maintains free space list to track available blocks/clusters.

Long-term Existence

Files are stored on disk or other secondary storage and do not disappear when a user logs off

Structure

Files can be organized into hierarchical or more complex structure to reflect the reationships among files

Shareable Between Processes

Files have names and can have associated access permissions that permit controlled sharing

Rotational Delay

In a disk drive, the time required for the disk to revolve until the correct sector is under/over the read/write heads. See also the definition for "seek time".

Pile

Least complicated form of file organization. Data are collected in the order they arrive and each record consists of one burst of data. Purpose is to simply accumulate the mass of data and save it. Record access is by exhaustive search.

Access Method

Level of the file system closest to the user. Provides a standard interface between applications and the file systems and devices that hold the data.

Device List

List of all buffers currently associated with each disk

Free List

List of all slots in the cache that are available for allocation.

Driver I/O Queue

List of buffers that are actually undergoing or waiting for I/O on a particular device

Access Control Lists

Lists that contains users and their permitted access rights. The access matrix can be decomposed by columns, resulting in this list.

Sequential File

Most common form of file structure and organization. Fixed format is used for records and key field uniquely identifies the record. Typically used in batch applications and the only organization that is easily stored on tape as well as disk.

Single Buffer

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

Buffering

Perform input transfers in advance of requests being made and perform output transfers some time after the request is made

Basic I/O Supervisor

Responsible for all file I/O initiation and termination. Controls structures that deal with device I/O, scheduling, and file status are maintained. Concerned with scheduling disk and tape accesses to optimize performance.

C-SCAN

Restricts scanning to one direction only. When last track has been visited in one direction, the arm is returned to the opposite end of the disk and the scan begins again.

B-Trees

Self-balancing tree similar to a binary tree, but not limited to a branching factor of 2. Standard method of organizing indexes for databases. Commonly used in OS file systems and provides for efficient searching, adding, and deleting of items.

Disk Cache

Storage space on a computer hard disk used to temporarily store downloaded data. Used to apply memory that is smaller and faster than main memory and that is interposed between main memory and the processor. Reduces average memory access time by exploiting principle of locality. Contains a copy of some of the sectors on the disk.

Utility of Buffering

Technique that smooths out peaks in I/O demand. Buffering can increase efficiency of the OS and performance of individual processes.

Programmed I/O

The Processor issues an I/O command, on behalf of a process, to an I/O module; that process then busy waits for the operation to be completed before proceeding.

File Organization

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

Access Time

The sum of the seek time and the rotational delay equals the _________, which is the time it takes to get into position to read or write.

Seek Time

The time it takes for the hard drive's read/write heads to move over the surface of the disk to the correct track.

Blocks Portions

This portion size has small fixed portions to provide greater flexibility. May require large tables or complex structures for their allocation. Contiguity has been abandoned as a primary goal and blocks are allocated as needed.

Variable large contiguous portions

This portion size will provide better performance. The variable size avoids waste and the file allocation tables are small. however the space is hard to reuse.

Capability Tickets

This specifies authorized objects and operations for a user. Decomposition of the access matrix can yield this.

Stream-Oriented Single Buffer, Byte-at-a-time Operation

Used on forms-mode terminals, when each keystroke is significant.

Stream-Oriented Single Buffer, Line-at-a-time Operation

User input is one line at a time with a carriage return signaling at the end of a line. Output to the terminal is similarly one line at a time.

Device Drivers

Utility software used by the operating system to communicate with peripheral devices or their controllers or channels. Responsible for I/O operations on a device and processes the completion of an I/O request. Considered to be part of the OS.

Direct Memory Access (DMA)

When a device controller transfers an entire block of data from its own buffer storage to memory without CPU intervention. A DMA module controls the exchange of data between main memory and an I/O module.

No Buffer

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

Block-oriented Device

stores information in blocks that are usually of fixed size, and transfers are made one block at a time.

Stream-oriented Device

transfers data in and out as a stream of bytes with no block structure