A collection of information stored on a computer using a hard disk as a carrier

Allocate memory space

Create an entry for the new file in the directory, which records the file name, location in the file system, etc.

Execute system call

A write pointer to the file must be maintained

Execute system call

A read pointer to the file must be maintained

file addressing

Use the open system call to add the attributes of the specified file (including the location of the external memory) to an entry in the open file table of the memory, and return the number (index pointer) to the user.

The OS maintains a table containing all open files (open-file table)

Precautions

Related information

Such as binary files or character files

Only through exhaustive search

i. sequence file

ii. index file

iii. index sequence file

iv. direct file or hash file

A file that organizes all file directories

Purpose

File Directory

basic structure

Precautions

Fundamental

shortcoming

Master File Directory (MFD)

User File Directory (UFD)

shortcoming

shortcoming

advantage

(Quick search speed)

Files can be deleted if and only if count=0

Pathname to store accessed shared files

1. Only the file owner has a pointer to the index node

2. After the file owner deletes the file, other users only fail to access it according to the path, without any other impact.

3. The problem is: after the original file is deleted by the user, and another user creates a file with the same name in its directory, the sharing user of the original file will access the wrong file.

User access rights

file properties

the easiest

Can't have the same name

Find quickly

Simple insertion and deletion

Hash table length is fixed

Dependence of hash function on table length

How files are distributed

File storage space management

file name

start position

length

("As much as you want, give as much as you want")

Resolved external debris

1. implicit link

2. explicit link

Put all disk numbers of the file together to form an index table (block)

The address of the index block is stored in the directory table

First get a piece of free space

Then write its address to the index table

random access

No external debris

Easy to add and delete

Index table takes up memory

The index block should be as small as possible, but too small to handle large files

link scheme

multi-level index

hybrid index

Create a free disk fast table for all free areas, each free area corresponds to a free table entry

All free areas are arranged in ascending order by starting address

Methods such as first adaptation can also be used

When recycling, the issue of merging the front and rear free areas should also be considered.

The first free disk fast number

Number of free disk blocks

Link in units of disk blocks

Link in units of panels

A bitmap is actually a two-dimensional table

When positioning, pay attention to whether rows and columns are numbered starting from 0 or 1.

Such as UNIX

i. (cylinder, disk, sector)

ii. reason

Move the head to the specified track

Ts=m*n s

Locate from track to target sector

Tr=1/(2r)

actual reading time

Can be optimized by OS

fair

No "magnetic wall adhesion", the following three algorithms are available

No hunger

Process the track closest to the current position each time

have hunger

The magnetic head reaches the maximum value of the requested track number (not necessarily the two ends of the track) and then returns to the other maximum value of the requested track number.

Not conducive to access requests far away from the head end

After the magnetic head reaches one of the maximum values ​​of the requested track number, it immediately returns to the other maximum value.

Divide new disk into sectors

Use special data structures for each sector

Divide the disk into one or more cylinder partitions (C drive, etc.)

Logically format a physical partition

Keep only a small bootloader in ROM

Disk with boot partition

OS cannot be repaired