Operating Systems CMPSCI 377 Lecture 15: File Systems I

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science

Emery BergerUniversity of Massachusetts, Amherst

Operating SystemsCMPSCI 377

Lecture 15: File Systems I

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science 2

Course Outline Processes & Threads

CPU Scheduling Synchronization & Deadlock

Memory Management File Systems & I/O Distributed Systems


Files: OS Abstraction Files: another OS-provided abstraction

over hardware resources

OS Abstraction Hardware Resource

ProcessesThreads

CPU

Address space MemoryFiles Disk


File System Abstraction Applications operate on files in a file

system Device-independent interface open(), close(), link(), read(), write(), rename()

Device-level interface Manage disk in terms of sectors, tracks, &

blocks seek(), readblock(), writeblock()

OS converts these to operations on raw hardware (disk)


User Expectations on Data Persistence

Data lives across jobs, power outages, crashes

Speed Quick access to data

Size Ability to store lots of data

Sharing/protection Users can share or restrict access to data

when appropriate Ease of use

User can easily find, examine & modify data


Hardware/OS Support for Data

Persistence: disks: non-volatile memory

Speed: random access devices Size: disk capacity growing fast

Persistence: redundancy, fault-tolerance

Sharing/protection: UNIX privileges Ease of use:

Names associated with data (files) Hierarchical directories Transparent mapping of devices

hardware

operatingsystem


Files Named collection of related information

recorded on secondary storage Logical unit of storage on a device e.g., helloworld.cpp, resume.doc

Can contain programs (source, binary) or data

Can be structured or unstructured IBM mainframe OS: series of records (structured) UNIX: file = stream of bytes (unstructured)

Files have attributes: name, type, location, size, protection, creation

time...


User Interface to File System Data operations:

create(), delete() open(), close() read(), write(), seek()

Naming operations: get & set attributes (ownership,

protection) hard link, soft links, rename


OS File-Related Data Structures Open file table – shared by all processes

with open file open count file attributes (ownership, protection, etc.) location(s) of file on disk pointers to location(s) of file in memory

Per-process file table – one for each file Pointer to entry in open file table Current position in file (offset) Mode in which process accesses file (r, w, r/w...) Pointers to file buffer


File Operations: Creating a File Create(name)

Allocates disk space Checks disk quotas, permissions, etc.

Creates file descriptor for file (name, location on disk, attributes) Adds file descriptor to directory that contains file

May mark file with type attribute (esp. Mac) Advantages: error detection, launch appropriate

app Disadvantages: not supported everywhere,

complicates file system and OS, less flexible UNIX: no types, Windows: file extensions

indicate type


File Operations: Deleting a File Delete(name)

Find directory containing file Free disk blocks used by file Remove file descriptor from

directory


File Operations:Opening Files fd = open(name, mode)

Check if file is already open by another process If not:

Find file Copy file descriptor into system-wide open file

table Check protection of file against requested

mode If not OK, abort

Increment open count Create entry in process’s file table pointing to

entry in system-wide table Initialize current file pointer to start of file


File Operations: Closing Files close (fd)

Remove entry for file in process’s file table

Decrement open count in system-wide file table

If open count == 0: Remove entry from system-wide file

table


OS File Operations: Reading File Random access: read(fd, from, size, buf)

OS reads “size” bytes from file position “from” into “buf”

for (i = from; i < from + size; i++) buf[i – from] = file[i];

Sequential access: read(fd, size, buf) OS reads “size” bytes from current file

position “from” into “buf”; increments file position

for (i = 0; i < size; i++) buf[i] = file[fp + i];

fp += size;


OS File Operations write

Similar to read, but copies from buffer to file

seek Just updates fp

memory mapping file map portion of virtual address space to file read/writes to that memory = OS

reads/writes from corresponding location in file

avoids need for explicit reads/writes


File Access Methods Common file access patterns from programmer’s

perspective Sequential: data processed in order

Most programs use this method Example: compiler reading source file

Keyed: address block based on key table Example: database search, hash table, dictionary

Common file access patterns from OS perspective Sequential: pointer to next byte; update on

read/write Random: address any block directly given offset in

file


Naming & Directories Need method of retrieving files

from disk! OS uses numbers (inodes) for files People prefer names... OS provides directory to map names

to file descriptors


Flat File Systems One level directory

One namespace for entire disk, every name unique

Directory contains (name, index) pairs Used by Apple, CP/M, DOS 1.0, first

MacOS Two level directories

Separate directories for each user

Not a problem for 140K disks, but...


Hierarchical File Systems Tree-structured name space

Used by all modern operating systems Directory becomes special file on disk

Marked by special flag bit User programs may read directories, but

only system may manipulate directories Each directory contains (name, inode)

pairs, names need not be unique in file system

Distinguished root directory (UNIX) or drive names (Windows)


Referential Naming Hard links (UNIX: ln command)

Allows multiple links to single file Example: “ln A B”:

initially: A -> file # 100 after: A, B -> file # 100

OS maintains reference counts Deletes file only after last link to it is deleted

Problem: users could create circular links with directories Reference counting can’t reclaim cycles

Solution: can’t hard link directories


Referential Naming Soft (symbolic) links (UNIX: ln -s)

Makes symbolic pointer from one file to another “ln -s A B”:

Initially, A -> file #100 After, A -> file #100, B -> A

Removing B does not affect A Removing A: dangling pointer

Problem: Circular links can cause infinite loops E.g., list all files in directory & its subdirectories

Solution: (lame) Limit number of links traversed


Directory Operations Search for file: locate entry Create file: adds directory listing Delete file: removes directory

listing List directory: list all files (ls) Rename file Traverse file system


Protection OS must allow users to control access to files Grant or deny access to file operations

depending on protection information Access lists and groups (Windows)

Access list for each file with user name and access type

Lists can become large & tedious to maintain Access control bits (UNIX)

Three categories of user (owner, group, world) Three types of access privileges (read, write,

execute) One bit per operation (111101000 = rwxr-x----)


Summary File systems provide

Naming Protection Persistence Fast access

Next time: file system implementation

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Operating Systems CMPSCI 377 Lecture 15: File Systems I

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