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COMP 310:
Operating Systems
Lecture 23:
Restart File Systems
October 25, 2004
Christine Alvarado
Today’s goals
Understand basics of file management
Review properties of physical disks
The file system abstraction
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Slides for Restart File Systems - Operating Systems | COMP 310, Study notes of Operating Systems
Material Type: Notes; Class: Operating Systems; Subject: Computer Science; University: University of San Diego; Term: Fall 2004;
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COMP 310:
Operating Systems
Lecture 23:
Restart File Systems
October 25, 2004
Christine Alvarado
Today’s goals
Understand basics of file management
Review properties of physical disks
The file system abstraction
Midterm Results Scores:
Answers posted online Hours Spent on Projects 0 5 10 15 20 25 30 35 40 45 Proj 1 Proj 2
Disk Structure Disk Interaction Is Complicated Specifying a disk request requires a lot of detailed information
If you want to specify a place on a disk, what do
you have to know?
Modern disks are even worse…
Disk Performance
Disk request performance depends on many steps:
Goal of OS: Minimize time to perform these steps
How? Disk Scheduling Seeks are expensive! (i.e., milliseconds—how long was memory access?) How might we schedule disk seeks?
File Systems The file system is the ABSTRACTION the OS provides for the data on the disk File systems:
Organize files logically
Permit sharing of data between processes,
people and machines
Protect data from unwanted access
Just like VM did for PM!
Files A file is data with some properties A file can also have a type
Understood by file system
Understood by other parts of the OS
Executable, dll, source, text, etc…
Where is file’s type encoded?
Windows?
UNIX?
size Data
Basic File Operations File Access Methods Some file systems provide different access methods to files:
What method does Unix, NT provide?
Tree-Structured directories
root code bin classes
nachos java find reorder COMP
PSs
PS1 PS
threads userprog
synch.h sych.cc subdir
subdir
Acyclic Graph Structure
root code bin classes
nachos java find reorder COMP
code PSs
PS1 PS
threads userprog
synch.h sych.cc subdir
subdir
How are shared files implemented? Problems/Issues with Graph Structure
Protection File systems implement some kind of protection system
Who can access
How can they access
A protection system dictates whether an action (read/write/execute) performed by a process/thread/user on a file should be allowed
Where have you seen this recently?
Representing Protection Dr. Alvarado rx rx r OSStudent2 rwx rwx rw OSStudent1 rwx rwx rw /code /threads /synch.cc
ACLs FS must maintain a list of who has access to what
What happens when files are heavily shared?
How does UNIX solve this problem?
File System Layout
How do file systems use the disk to store files?
File systems define a block size (e.g. 4KB) Disk space allocated in terms of blocks A “Master Block” determines location of root directory A “Free Map” keeps track of which blocks are free or allocated Usually a bitmap (How?) Stored on disk—can be cached in memory Remaining disk blocks used to store files.
Unix Inodes and Path Search
Unix Inodes are NOT directories
Inodes describe where on disk the blocks for a file
are placed
Directories are files, so inodes also describe where they are placed
Directory entries map file names to inodes
To open /one user Master Block to find inode for / Open /, look for entry for one Read the inode for one into memory The inode says where first data block is on disk Read that block into memory to read file… Read Ahead Many File Systems implement “read ahead”
FS predicts that process will keep reading in file
FS requests next block before it is accessed
When is this an advantage?
Summary of File Systems Files
Operations, access methods
Directories
Operations using directories to do path searches
Sharing Protection File System Layouts