7/2/2015 1 finish proj 3a now! no deadline extension for the rest of quarter project 0 resubmission...

04/19/23 1

Finish Proj 3A NOW! No deadline extension for the rest of quarter

• Project 0 resubmission for autograding : June 1 Project 0 score =max(old score, old score *0.10 +

new score *0.90). Donot print “shell>” prompt.

• Project 3A (May 29). Harness code is released.

• Optional Project 3B (June 4).-- You can use Project 3B to replace midterm OR one of project scores: Project 1, 2, 3A.

• Exercise Set 2 (June 4 Thursday 12:30pm)

File Systems

CS170 Fall 2015. T. Yang

What to Learn?

• File interface review• File-System Structure

File-System Implementation Directory Implementation

• Allocation Methods of Disk Space Free-Space Management Contiguous allocation Block-oriented indexing

– Unix inode structure

Files

• File concept: Contiguous logical address space in a persistent

storage (e.g. disk). • File structure

None - sequence of words, bytes Simple record structure

– Lines – Fixed length– Variable length

Complex Structures: Formatted document• Who decides the structure:

Operating system Program

File Attributes

• Name – only information kept in human-readable form

• Identifier – 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

• 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

File Operations

• Create

• Open(Fi)

search the directory structure on disk for entry Fi

move the content of entry to memory

• Close (Fi) –

move the content of entry Fi in memory to directory structure on disk

• Write• Read• Reposition within file (e.g. seek)• Delete• Truncate

Access Methods

• Sequential Accessread nextwrite next reset

• Direct Accessread nwrite nposition to n

read nextwrite next

rewrite nn = relative block number

File System Abstraction

• Directory Group of named files or subdirectories Mapping from file name to file metadata location

• Path

String that uniquely identifies file or directory Ex: /cse/www/education/courses/cse451/12au

• Links Hard link: link from name to metadata location Soft link: link from name to alternate name

• Mount Mapping from name in one file system to root of another

UNIX File System API

• create, link, unlink, createdir, rmdir Create file, link to file, remove link Create directory, remove directory

• open, close, read, write, seek Open/close a file for reading/writing Seek resets current position

• fsync File modifications can be cached fsync forces modifications to disk (like a memory

barrier)

File System Interface

• UNIX file open is a Swiss Army knife: Open the file, return file descriptor Options:

– if file doesn’t exist, return an error– If file doesn’t exist, create file and open it– If file does exist, return an error– If file does exist, open file– If file exists but isn’t empty, nix it then open– If file exists but isn’t empty, return an error– …

Example of Linux read, write, and lseek

int main() {

int file=0; char buffer[15];

if((file=open("testfile.txt",O_RDONLY)) < -1)

return 1;

if(read(file,buffer,14) != 14)

return 1;

printf("%s\n",buffer);

if(lseek(file,5,SEEK_SET) < 0)

return 1;

if(read(file,buffer,19) != 14)

return 1;

printf("%s\n",buffer);

return 0;

}

$ cat testfile.txt This is a test file

$ ./testing This is a test

is a test file

Protection

• File owner/creator should be able to control: what can be done by whom

• Types of access Read Write Execute Append Delete List

Example in Linux

Access Lists and Groups in Linux

• Mode of access: read, write, execute• Three classes of users

RWXa) owner access 7 1 1 1

RWXb) group access 6 1 1 0

RWXc) public access 1 0 0 1

• Ask manager to create a group (unique name), say G, and add some users to the group.

• For a particular file (say game) or subdirectory, define an appropriate access.

owner group public

chmod 761 game

Attach a group to a file chgrp G game

Windows Access-Control List Management

Directory Structure

• A collection of nodes containing information about all files

F 1 F 2 F 3F 4

F n

Directory

Files

Both the directory structure and the files reside on diskBackups of these two structures are kept on tapes

A Typical File-system Organization on a Disk Partition

Operations Performed on Directory

• Search for a file

• Create a file

• Delete a file

• List a directory

• Rename a file

• Traverse the file system

Directory with single-Level or two-level

• A single directory for all users

• Two -level

Tree-Structured Directories

Directory with acyclic graph structure

• Name Resolution: The process of converting a logical name into a physical resource (like a file) Traverse succession of directories until reach target file Global file system: May be spread across the network

Building a File System

• File System: Layer of OS that transforms block interface of disks (or other block devices) into Files, Directories, etc.

• File System Components Disk Management: collecting disk blocks into files Naming: Interface to find files by name, not by blocks Protection: Layers to keep data secure Reliability/Durability: Keeping of files durable despite

crashes, media failures, attacks, etc• User vs. System View of a File

User’s view: Durable Data Structures System call interface:

– Collection of Bytes (UNIX) System’s view (inside OS):

– Collection of blocks (a block is a logical transfer unit, while a sector is the physical transfer unit on disk)

– Block size sector size; in UNIX, block size is 4KB

Kubiatowicz’s cs162 UCB

Translating from User to System View

• What happens if user says: give me bytes 2—12? Fetch block corresponding to those bytes Return just the correct portion of the block

• What about: write bytes 2—12? Fetch block Modify portion Write out Block

• Everything inside File System is in whole size blocks For example, getc(), putc() buffers something

like 4096 bytes, even if interface is one byte at a time• From now on, file is a collection of blocks

FileSystem

Kubiatowicz’s cs162 UCB

File System Design

• Data structures Directories: file name -> file metadata

– Store directories as files

File metadata: how to find file data blocks Free map: list of free disk blocks

• How do we organize these data structures? Device has non-uniform performance

Design Challenges

• Index structure How do we locate the blocks of a file?

• Index granularity What block size do we use?

• Free space How do we find unused blocks on disk?

• Locality How do we preserve spatial locality?

• Reliability What if machine crashes in middle of a file system op?

File System Workload

• Studying application workload and characteristics can help feature prioritization or optimization of design

• What should be considered? File sizes

– Are most files small or large?– Which accounts for more total storage: small or large files?

File access pattern– Small file, large file?– Random access vs sequential access?


• File sizes Are most files small or large?

– SMALL

Which accounts for more total storage: small or large files?

– LARGE


• File access Are most accesses to small or large files? Which accounts for more total I/O bytes: small or

large files?


• File access Are most accesses to small or large files?

– SMALL

Which accounts for more total I/O bytes: small or large files?

– LARGE


• How are files used? Most files are read/written sequentially Some files are read/written randomly

– Ex: database files, swap files

Some files have a pre-defined size at creation Some files start small and grow over time

– Ex: program stdout, system logs

Designing the File System: Access Patterns

• Sequential Access: bytes read in order (“give me the next X bytes, then give me next, etc.”) Most of file accesses are of this flavor

• Random Access: read/write element out of middle of array (“give me bytes i—j”) Less frequent, but still important, e.g., mem. page from swap file Want this to be fast – don’t want to have to read all bytes to get to the

middle of the file

• Content-based Access: (“find me 100 bytes starting with JOSEPH”) Example: employee records – once you find the bytes, increase my

salary by a factor of 2 Many systems don’t provide this; instead, build DBs on top of disk

access to index content (requires efficient random access)

A. Joseph UCB CS162. Spr 2014

Designing the File System: Usage Patterns• Most files are small (for example, .login, .c, .java files)

A few files are big – executables, swap, .jar, core files, etc.; the .jar is as big as all of your .class files combined

However, most files are small – .class, .o, .c, .doc, .txt, etc

• Large files use up most of the disk space and bandwidth to/from disk May seem contradictory, but a few enormous files are

equivalent to an immense # of small files

• Although we will use these observations, beware! Good idea to look at usage patterns: beat competitors by

optimizing for frequent patterns Except: changes in performance or cost can alter usage

patterns. Maybe UNIX has lots of small files because big files are really inefficient?


File System Design

• For small files: Small blocks for storage efficiency Concurrent ops more efficient than sequential Files used together should be stored together

• For large files: Storage efficient (large blocks) Contiguous allocation for sequential access Efficient lookup for random access

• May not know at file creation Whether file will become small or large Whether file is persistent or temporary Whether file will be used sequentially or randomly

File System Goals

• Performance and Flexibility

Maximize sequential performance

Efficient random access to file

Easy management of files (growth, truncation, etc)

• Persistence and Reliability

File-System Implementation

• Directories and index structure Special root block at a specific location contains

the root directory Directory structure organizes the files

– Given file name, find a file number– Given a file number which contains the file structure

info, locate blocks of this file.

• Per-file File Control Block (FCB) contains many details about the file Called i-node in Linux/Unix

A Typical File Control Block

Layered File System

• Virtual File Systems (VFS) provide an object-oriented way of implementing file systems.

• VFS allows the same system call interface (the API) to be used for different types of file systems.

• The API is to the VFS interface, rather than any specific type of file system.

Schematic View of Virtual File System

Directory Implementation

• Linear list of file names with pointer to the data blocks. simple to program time-consuming to execute

• Hash Table – linear list with hash data structure. decreases directory search time collisions – situations where two file names hash

to the same location• Search tree

How do we actually access files?

• All information about a file contained in its file header File control block: UNIX calls this an “inode”

– Inodes are global resources identified by index (“inumber”, or inode number)

Once you load the header structure, all blocks of file are locatable

• the maximum number of inodes is fixed at file system creation, limiting the maximum number of files the file system can hold.

• A typical allocation heuristic for inodes in a file system is one percent of total size.

• The inode number indexes a table of inodes in a known location on the device

i-node number

Question: how does the user ask for a particular file?

One option: user specifies an inode by a number (index).– Imagine: open(“14553344”)

Better option: specify by textual name– Have to map nameinumber

Another option: Icon– This is how Apple made its money. Graphical user

interfaces. Point to a file and click


Named Data in a File System

Directories Are Files

Directory Layout

Directory stored as a fileLinear search to find filename (small directories)

Large Directories: B Trees

Large Directories: Layout

Recursive Filename Lookup

How many disk accesses to resolve “/my/book/count”?

• Read in file header for root / (fixed spot on disk) • Read in first data block for root /

Table of file name/index pairs. Search linearly – ok since directories typically very small

• Read in file header for “my”• Read in first data block for “my”; search for “book”• Read in file header for “book”• Read in first data block for “book”; search for “count”• Read in file header for “count”

• Current working directory: Per-address-space pointer to a directory (inode) used for resolving file names Allows user to specify relative filename instead of absolute

path (say CWD=“/my/book” can resolve “count”)


• Open system call: Resolves file name, finds file control block (inode) Makes entries in per-process and system-wide tables Returns index (called file descriptor or file handle ) in

open-file table

In-Memory File System Structures

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 File-open 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 file locking is provided by some systems

Mediates access to a file

• Read/write system calls: Use file handle (descriptor) to locate inode Perform appropriate reads or writes

In-Memory File System Structures

Allocation of Disk Blocks

• An allocation method refers to how disk blocks are allocated for files: Contiguous allocation Linked allocation Indexed allocation

Contiguous Allocation of Disk Space

Contiguous Allocation

• Each file occupies a set of contiguous blocks on the disk

• Advantages: Simple – only starting location (block #) and

length (number of blocks) are required Fast Random access

• Disadvantages: Not easy to grow files. Waste in space (e.g. external fragmentation)

Linked Allocation

• Each file is a linked list of disk blocks: blocks may be scattered anywhere on the disk.

Microsoft File Allocation Table (FAT)

• Linked list index structure Simple, easy to implement Still widely used (e.g., thumb drives)

• File table: Linear map of all blocks on disk Each file is a linked list of blocks

FAT

• Pros: Easy to find free block Easy to append to a file Easy to delete a file

• Cons: Small file access is slow Random access is very slow Fragmentation

– File blocks for a given file may be scattered– Files in the same directory may be scattered– Problem becomes worse as disk fills

One-level Indexed Allocation

• Place all direct data pointers together into the index block• Example

Nachos file

control block

has 32 data

block pointers:

128 bytes/block

index table

Example of One-level Indexed Allocation

One-level Indexed Allocation (Cont.)

• Advantages Support random access No external fragmentation.

• Disadvantages: Space overhead: need 1 block for index table

• Maximum file size? Assume each block is 4KB index block holds 1024 entries (4B/entry) 1024x block size = 4MB Maximum fie size for Nachos file system

– 32x128 bytes = 4KB.

Two-level Indexed Allocation: Single indrection

Level 1index

Indirect pointers index table:

Direct pointersFile data

Maximum size ?4GB

1K entries 1K entries 4KB data

Hybrid multi-level scheme: UNIX file system

• Key idea: efficient for small files, but still allow big files

• File header contains 13-15 pointers called an “inode” in UNIX

• File Header format: First 10-12: direct data pointers 1 “indirect block” 1 “doubly indirect block” 1 triple indirect block

Berkeley UNIX FFS (Fast File System)

• i-node metadata File owner, access permissions, access times, …

• Each file block: 4KB• 15 pointers

Set of 12 direct data pointers– With 4KB blocks => max size of 48KB files

1 indirect block pointer– Indirect block: 4KB contains 1K entries data blocks => 4MB

(+48KB)

1 double indirect pointer– 1K*1K blocks

1 triple indirect pointer– 1K*1K*1K blocks

• Maximum size: 4TB + 4GB + 4MB + 48KB

Free-Space Management

• Bitmap (n blocks)

…

0 1 2 n-1

bit[i] = 0 block[i] free

1 block[i] occupied

Block number calculation(number of bits per word) *(number of 0-value words) +offset of first 1 bit

Performance Optimization

• Disk cache – separate section of main memory for frequently used blocks

• Read-ahead (prefetching)– techniques to optimize sequential access

• improve PC performance by dedicating section of memory as virtual disk, or RAM disk

• Q1: True _ False _ inumber is the id of a block

• Q2: True _ False _ inumber is a file description

returned in open system call.

• Q3: True _ False _ Typically, directories are stored as

files

• Q4: True _ False _ With FAT, pointers are maintained in the data blocks

• Q5: True _ False _ Unix file system is more efficient than FAT for random access

Question: File Systems

• Q1: True _ False _x inumber is the id of a block

• Q2: True _ False _ x inumber is a file description

returned in open system call.

• Q3: True _x False _ Typically, directories are stored as

files

• Q4: True _ False _x With FAT, pointers are maintained in the data blocks

• Q5: True _x False _ Unix file system is more efficient than FAT for random access

Question: File Systems

Summary

• File access sequential random

• File-System Structure Layered file system Multi-level directory

• Allocation Methods of Disk Space Linked allocation Contiguous allocation Block-oriented indexing and maximum file size

– One-level vs. multi-level– Unix inode, inumber

7/2/2015 1 finish proj 3a now! no deadline extension for the rest of quarter project 0 resubmission...

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