1. CSE 451: Operating Systems Spring 2006 Module 12
CSE 451: Operating Systems Spring Module 12.5 Secondary Storage Overview
John Zahorjan
Allen Center 534 1

2. Interface Layers Std. Runtime Library App. Code OS Disk
Device-type Dependent Commands
Procedure Calls
Whatever…
Syscalls
1/2/2019
© 2006 Gribble, Lazowska, Levy, Zahorjan

3. Exported Abstractions
App. Code
Std. Runtime Library OS
Disk
OS + a tiny bit of file type / structure
Directories, Directory Entries, Files,…
Array of Blocks
Whatever / /
etc
root
.xls
root
etc
1/2/2019
© 2006 Gribble, Lazowska, Levy, Zahorjan

4. Primary Roles of the OS (file system)
Hide hardware specific interface
Allocate disk blocks
Check permissions
Understand directory file structure
Maintain metadata
Performance
Flexibility OS
Disk
Why does the OS define directories? Why not leave that to the library/application layer?
(Why would you want to leave it to the app/library?) /
etc
root
1/2/2019
© 2006 Gribble, Lazowska, Levy, Zahorjan

5. © 2006 Gribble, Lazowska, Levy, Zahorjan
What Is A File?
At this level, what we think of as a file is an “inode”
An inode keeps track of where the data blocks are
It also keeps track of metadata (e.g., permissions, last modification time, etc.) OS
Disk /
root
etc
1/2/2019
© 2006 Gribble, Lazowska, Levy, Zahorjan

6. What Is A Directory? OS Disk Ok, so what is a “hard link”? / root etc /
passwd
root
etc
hosts
1/2/2019
© 2006 Gribble, Lazowska, Levy, Zahorjan

7. Obtaining Performance: CachingOS
Disk
read write
readblock writeblock
No…
Block in cache?
Why cache?
How does it affect applications?
1/2/2019
© 2006 Gribble, Lazowska, Levy, Zahorjan

8. Obtaining Performance: openOS
Disk
int fd = open(“/root/xxx”, O_RDONLY) [ read(fd,…) write(fd,…) ]
readblock writeblock
No…
Per-process open file table
inode / 1 2 3 …
root
etc
file pointer
1/2/2019
© 2006 Gribble, Lazowska, Levy, Zahorjan

9. Multiple File Systems readblock writeblock ext2 disk ext2 read write
fat
fat disk
readblock writeblock
Each filesystem:
Decides which blocks to allocate for what (inodes, directories, file data)
Decides what the format of a directory is
Not really disks, but partitions. We’ll get to that later…
1/2/2019
© 2006 Gribble, Lazowska, Levy, Zahorjan

10. Supporting Multiple File Systems: vfs
ext2 disk
read write
ext2
vfs
fat
fat disk
1/2/2019
© 2006 Gribble, Lazowska, Levy, Zahorjan

11. Extending Functionality
ext2 disk
read write
ext2
fat
vfs
/dev
fat disk
/proc
/dev: direct access to hardware blocks
/proc: “virtual files” (computed, not stored, results)
/proc is to the file system what cgi’s are to a web server (sort of)
1/2/2019
© 2006 Gribble, Lazowska, Levy, Zahorjan

12. Are Directories Fundamental?
What is the logical role of the directories?
What practical role(s) do they have?
Are there alternatives?
1/2/2019
© 2006 Gribble, Lazowska, Levy, Zahorjan