chapter 12 - file management

Question 1

3 desirable properties of files

Answer 1

Long-term existence: kept on secondary storage
Sharable between processes: for shared access
Structure: some file systems manage the internal structure of a file, which can be organized in a way suitable for particular applications

Question 2

what are the 6 file operations?

Answer 2

create, delete, open, close, read, write

Question 3

what is the file structure?

Answer 3

field, record, file, database

Question 4

what is a field? (file structure)

Answer 4

basic element of data, characterized by its length and data type, contains single value

Question 5

what is a record? (file structure)

Answer 5

collection of related fields
ex. employee record
can be variable length

Question 6

what is a file? (file structure)

Answer 6

collection of similar records, treated as single entity

Question 7

what is a database? (file structure)

Answer 7

collection of related file(s)

Question 8

what are Unix files?

Answer 8

streams of bytes, don’t have internal structure as far as OS is concerned

Question 9

what is a file management system?

Answer 9

set of system software providing file services to users and applications

Question 10

7 objectives for a file management system?

Answer 10

Meet the data management needs and requirements of the user

Guarantee that the data in the file are valid.

Optimize performance.

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

Provide I/O support for multiple users.

Question 11

what is involved in a file system architecture?

Answer 11

Device driver: communicates directly with hardware device.

Basic file system: handles physical I/O.

Basic I/O supervisor: responsible for file I/O initiation and termination, device selection, I/O scheduling, buffer allocation.

Logical I/O: Enables users and applications to access records. Provides record-oriented I/O capability.

Access method: access files based on their structure and method of access.

Question 12

what are the 5 file management functions?

Answer 12

identify and locate a selected file

enforce user access control (shared system)

employ access method on the records

provide blocking for file I/O

manage secondary storage

Question 13

5 criteria for choosing a file organization?

Answer 13

Rapid Access
Ease of update
Economy of storage
Simple maintenance
Reliability

Question 14

5 fundamental file organizations?

Answer 14

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

Question 15

what is a pile? (fundamental file org)

Answer 15

data collected in the order it arrives
Purpose is to accumulate a mass of data and save it
Records may have different structures or no structure.
Data access is by exhaustive search
May be useful to store data prior to processing

Question 16

what is sequential file? (fundamental file org)

Answer 16

Fixed format used for records.
Records are the same length.
All fields the same (order and length).
Field names and lengths are attributes of the file.
One field is the key field
- Uniquely identifies the record
- Records are stored in key sequence
New records are placed in a log file or transaction file.
Log file is periodically merged with the master file.
Useful in batch processing.

Question 17

what is an indexed sequential file? (fundamental file org)

Answer 17

File is like a sequential file with key and data.
Index provides a lookup capability to quickly reach the vicinity of the desired record.
Greatly reduces the time required to access a single record, while still providing sequential processing.
Additions can be handled by an overflow file, which can be periodically merged with the main file.

Question 18

what is an indexed file? (fundamental file org)

Answer 18

Uses multiple indexes for different key fields.
Abandons sequential format with single key.
Records are accessed only by their index.
Records can be variable length.
Exhaustive index has pointers to every record.
Partial index only has pointers to records having the field of interest.
Useful in applications such as an airline reservation system that need to access particular records quickly, but rarely need to process them sequentially.

Question 19

what is a direct/hash file? (fundamental file org)

Answer 19

Turn key value into an address to support direct access of records in the file.
Gives very rapid access to data.
Useful if records are only accessed one at a time.

Question 20

what is a B-tree?

Answer 20

indexes often implemented as this.

has large branching factor leading to tree of low height -> results in fewer disk accesses to traverse the tree in a search

Question 21

what is a directory?

Answer 21

contains info about file (attributes, location, ownership)

is a file itself, owned by OS, managed by file system

Question 22

2 issues that arise from file sharing

Answer 22

access rights and simultaneous access

Question 23

2 approaches for simultaneous access

Answer 23

1. lock entire file when it needs to be updated
2. lock individual record during update

Question 24

when does record blocking occur? when does unblocking occur?

Answer 24

when writing data to file

when reading data in file

Question 25

what are the 3 blocking methods (record blocking)

Answer 25

Fixed: fixed-length records. May cause internal fragmentation.

Variable-length spanned: variable-length records which may span blocks. Permits unlimited record size. May require multiple I/O’s.

Variable-length unspanned: variable-length records which do not span blocks. May cause internal fragmentation.

Question 26

2 management issues in secondary storage management

Answer 26

allocating blocks to files and maintaining a list of free blocks

Question 27

what are the 2 methods of allocation portion size in a file

Answer 27

Large, variable-length contiguous portion.
Good performance.
May cause fragmentation.
Allocation table is simple, just need pointer to beginning and length.

Small fixed-sized blocks.
Good flexibility.
Requires more complicated allocation table to keep track of non-contiguous blocks.

Question 28

3 file allocation methods

Answer 28

contiguous, chained, indexed

Question 29

contiguous allocation

Answer 29

a single contiguous set of blocks is allocated to a file at the time of file creation.

is a preallocation method of a variable-size portion.

May require periodic compaction due to fragmentation.

Good for sequential file access.

Question 30

chained allocation

Answer 30

allocate individual blocks

each block contains a pointer to the next block

Question 31

indexed allocation

Answer 31

an index specifies the location of each block

Question 32

what does a disk allocation table do

Answer 32

keeps track of available space

Question 33

what are the 4 techniques for free space management

Answer 33

Bit tables: each bit represents a block
Chained free portions: free portions are chained together.
Indexing: index keeps track of every free portion as if it was a file.
Free block list: blocks are numbered and list of free block numbers is maintained.