Long-term Information Storage Must store large amounts of data Information stored must survive the termination of the process using it Multiple processes must be able to access the information concurrently. In short:

Long-term Information Storage Files: Good!

Long-term Information Storage Files: Good! No Files: Bad!

File System Operating system determines how files are: Structured Named Accessed Used Protected Implemented Most important aspect to users is how files appear to them: naming convention, available operations, protection, etc. (Not implementation!!).

File Naming Unix: Windows: Case sensitive. Allows, but does not require, extensions (e.g., prog.c). Assigns no meaning to extensions. Add as many extensions as desired (e.g., prog.back.stupid.c). Does not allow spaces in name (unless “\ “) ; Windows: Not case sensitive. Allows 1-3 character extensions. Extensions have meaning (to other application codes, not to the OS) Allows spaces in file name. (ever tried to copy “my work” to Unix?)

Typical file extensions. File Naming Typical file extensions.

File Structure None - sequence of words, bytes Simple record structure Lines Fixed length Variable length Complex Structures Formatted document Relocatable load file

File Structure Three kinds of files byte sequence (i.e., no structure). record sequence Tree (e.g., data base)

File Structure Can simulate last two with first method by inserting appropriate control characters Who decides: Operating system Program (i.e., programs can support any model they want) Unix and Windows support only the sequence of bytes functionality.

An executable file (Unix)

File Access Sequential access Random access read all bytes/records from the beginning cannot jump around, could rewind or back up convenient when medium was mag tape Random access bytes/records read in any order essential for data base systems read can be … move file marker (seek), then read or … read and then move file marker

File Attributes Name – only information kept in human-readable form Identifier (file descriptor) – unique tag (number) identifies file within file system Type – needed for systems that support different types Location – pointer to file location on device Size – current file size

File Attributes Protection – controls who can do reading, writing, executing Time, date, and user identification – data for protection, security, and usage monitoring Information about files are kept in the directory structure, which is maintained on the disk (although generally cached).

File Operations Create Write Read Reposition within file Delete Truncate

File Operations in Unix int fd = open(Fi) – search the directory structure on disk for entry Fi, and move the content of entry to memory fd is a file descriptor (integer). close (fd) – move the content of entry Fi in memory to directory structure on disk seek() // change pointer to current location in file. read(fd, buf, num_bytes)

Open Files Several pieces of data are needed to manage open files: File pointer: pointer to last read/write location, per process that has the file open Open-file count: counter of number of times a file is open – to allow removal of data from open-file table when last processes closes it Disk location of the file: cache of data access information Access rights: per-process access mode information

Open Files Unix maintains an open-file table for each process and for the whole system. File descriptor is used as an index into the process open-file table. Entries are items that have to do with that particular process (e.g., file pointer, access rights, etc.). A pointer to the system-wide open-file table is also in the process open-file table. System-wide open-file table holds process-independent information (e.g., location on disk, last access time, file size, count of the number of processes using the file).

Open File Locking Provided by some operating systems and file systems Mediates access to a file Mandatory or advisory: Mandatory – access is denied depending on locks held and requested Advisory – processes can find status of locks and decide what to do

Directory Both the directory structure and the files reside on disk A collection of data structures containing information about files Directory Files F 1 F 2 F 3 F 4 F n Both the directory structure and the files reside on disk Backups of these two structures are kept on tapes

Operations Performed on Directory Search for a file Create a file Delete a file List a directory Rename a file Traverse the file system

Organize the Directory (Logically) to Obtain Efficiency – locating a file quickly Naming – convenient to users Two users can have same name for different files The same file can have several different names Grouping – logical grouping of files by properties, (e.g., all Java programs, all games, …)

Single-Level Directory Naming problem Grouping problem

Two-Level Directory Separate directory for each user Path name Can have the same file name for different user Efficient searching No grouping capability

Tree-Structured Directories

Tree-Structured Directories (Cont) Efficient searching Grouping Capability In Unix, a directory is a file that contains meta-data about the files it contains.

Tree-Structured Directories (Cont) Most OS support absolute and relative path names. Unix has two pre-defined relative path names: . Represents current directory .. Represents parent directory Current directory (working directory) cd /spell/mail/prog or cd .. (relative to CD)

Path Names A UNIX directory tree

To Open dict, the absolute path name is: /usr/lib/dict.

Relative Path Name Assume Current Directory is /usr/jim. Then .. is /usr, . is /usr/jim To access dict: ../lib/dict.

rmdir removes an entire directory (and all sub-directories). Unix: mkdir creates a new sub-directory below the current working directory. rmdir removes an entire directory (and all sub-directories). rm deletes a file If someone suggests that you try out a cool command called rm –r * don’t do it!!

File system containing a shared file Shared Files (1) File system containing a shared file

Links This is termed a hard link. Both directory entry pointing to the same inode. (a) Situation prior to linking (b) After the link is created (c) After the original owner removes the file

Symbolic Links Provide the path name of the target file in the linked file. Other processes do not have access to the inode (i.e., directory structure). What happens when file deleted by owner?

Implementing Directories (1) (a) A simple directory fixed size entries disk addresses and attributes in directory entry (b) Directory in which each entry just refers to an i-node

File Control Block

Accessing a File

Allocation of File Blocks Contiguous allocation Linked-list allocation FAT Indexed (inodes).

Directory Structure with Contiguous Allocation of File Blocks

Implementing Files: Contiguous Allocation (a) Contiguous allocation of disk space for 7 files (b) State of the disk after files D and E have been removed

Linked-list Allocation

File Allocation Table

Entry 4 bytes. Blocks 1K. 20 Million Entries (not files Entry 4 bytes. Blocks 1K. 20 Million Entries (not files!) == 80 MB for table.

Indexed Allocation

Unix inode

Unix Directory Entry File Name Inode Tester 15

Unix File System Unix File System: 1 inode for each file/directory. B Inode list Data blocks Boot area superblock

Pointing to file data blocks Pointing to file data blocks File Attributes Direct blocks Pointing to file data blocks Pointing to file data blocks Single Indirect Double and Triple Indirect blocks not shown Points to data block whose data is the addresses of data blocks belonging to the file

Inode 0 Inode 1 Inode 2 Inode 3 File Attributes 100 400 File Attributes Direct block 0 Direct block 1 File Attributes 180 Direct block 1 File Attributes 253 Direct block 1 Inode 0 Inode 1 Inode 2 Inode 3 Root inode Problem: Open file /usr/pmd

Inode 0 Inode 1 Inode 2 Inode 3 File Attributes 100 400 File Attributes Direct block 0 Direct block 1 File Attributes 180 Direct block 1 File Attributes 253 Direct block 1 Inode 0 Inode 1 Inode 2 Inode 3 Root inode Open file /usr/pmd Step 1: Fetch inode for / (root, always inode 0)

Inode 0 Inode 1 Inode 2 Inode 3 File Attributes Direct block 0 Direct block 1 File Attributes 180 Direct block 1 File Attributes 253 Direct block 1 Inode 0 Inode 1 Inode 2 Inode 3 Root inode Open file /usr/pmd File Attributes 100 400 Inode 0

Inode 0 Inode 1 Inode 2 Inode 3 File Attributes 100 400 File Attributes Direct block 0 Direct block 1 File Attributes 180 Direct block 1 File Attributes 253 Direct block 1 Inode 0 Inode 1 Inode 2 Inode 3 Root inode Step 2: Grab data block 100 (it had better be a directory!!)

Inode 0 Inode 1 Inode 2 Inode 3 File Attributes 100 400 File Attributes Direct block 0 Direct block 1 File Attributes 180 Direct block 1 File Attributes 253 Direct block 1 Inode 0 Inode 1 Inode 2 Inode 3 Root inode bin 8 include 6 usr 2 data block 100 (directory listing for /)

Inode 0 Inode 1 Inode 2 Inode 3 File Attributes 100 400 File Attributes Direct block 0 Direct block 1 File Attributes 180 Direct block 1 File Attributes 253 Direct block 1 Inode 0 Inode 1 Inode 2 Inode 3 Root inode bin 8 include 6 usr 2 Step 3: Grab inode 2. data block 100 (directory listing for /)

Inode 0 Inode 1 Inode 2 Inode 3 File Attributes 100 400 File Attributes Direct block 0 Direct block 1 File Attributes 253 Direct block 1 Inode 0 Inode 1 Inode 2 Inode 3 Root inode File Attributes 180 Direct block 1 Step 4: Grab Data block 180 inode 2

Inode 0 Inode 1 Inode 2 Inode 3 File Attributes 100 400 File Attributes Direct block 0 Direct block 1 File Attributes 180 Direct block 1 File Attributes 253 Direct block 1 Inode 0 Inode 1 Inode 2 Inode 3 Root inode Step 4: Grab Data block 180

Inode 0 Inode 1 Inode 2 Inode 3 File Attributes 100 400 File Attributes Direct block 0 Direct block 1 File Attributes 180 Direct block 1 File Attributes 253 Direct block 1 Inode 0 Inode 1 Inode 2 Inode 3 Root inode weew 8 Pops 6 pmd 3 data block 180. Directory listing for /usr

Inode 0 Inode 1 Inode 2 Inode 3 File Attributes 100 400 File Attributes Direct block 0 Direct block 1 File Attributes 180 Direct block 1 File Attributes 253 Direct block 1 Inode 0 Inode 1 Inode 2 Inode 3 Root inode Step 5: Grab inode 3 weew 8 Pops 6 pmd 3 Data block 180. Directory listing for /usr

Inode 0 Inode 1 Inode 2 Inode 3 File Attributes 100 400 File Attributes Direct block 0 Direct block 1 File Attributes 180 Direct block 1 Inode 0 Inode 1 Inode 2 Inode 3 Root inode File Attributes 253 Direct block 1 inode 3

Inode 0 Inode 1 Inode 2 Inode 3 File Attributes 100 400 File Attributes Direct block 0 Direct block 1 File Attributes 180 Direct block 1 Inode 0 Inode 1 Inode 2 Inode 3 Root inode Step 6. Fetch Data block 253. File Attributes 253 Direct block 1 inode 3

Inode 0 Inode 1 Inode 2 Inode 3 File Attributes 100 400 File Attributes Direct block 0 Direct block 1 File Attributes 180 Direct blbock 1 File Attributes 253 Direct block 1 Inode 0 Inode 1 Inode 2 Inode 3 Root inode Step 6. Fetch Data block 253. Orc 8 Poo9 6 wimp 3 Data block 253. Directory listing for /usr/pmd

The steps in looking up /usr/ast/mbox The UNIX File System The steps in looking up /usr/ast/mbox

Exam Comprehensive Will cover essentially the same concepts from previous tests. Will cover virtual memory and Unix file system organization and searching. VM: Understand page replacement algorithms. What they are based on. Work through optimal, LRU, clock, counter as done in class.

Understand demand paging. Page faults. What causes a page fault and what happens! When exam asks to solve a problem using approach shown in class, please do so.