From the Operating Systems course (CMPSCI 377) at UMass Amherst, Fall 2007.

1. Operating Systems
CMPSCI 377
File Systems
Emery Berger
University of Massachusetts Amherst
UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science

2. Files
Associate

names with
data
Usually

stored on
persistent
media
(disks)
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3. File Names
Hierarchical

directory
structure
Absolute,

relative
to current
Windows

names =
location +
dir
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4. Directories
Directory – just special file

Contains metadata, filenames, inodes

inode = unique number for file

May contain other directory, etc.

Beware: fixed max filename length

File systems use directory structure to

improve locality
E.g., ext2 – all files in same directory

clustered in same region of disk
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5. Poor Man’s Database
Because files & directories are easy to use,

they get used as de facto databases
e.g., Internet Explorer web cache

~ 1000 files in each hash subdirectory

C:Documents and SettingsEmeryLocal SettingsTemporary Internet FilesContent.IE5>
ls -ltra
total 1873
-rwx------+ 1 Emery None 67 Jan 10 17:31 desktop.ini
drwx------+ 2 Emery None 0 Jan 17 22:42 0NDWKTYT
drwx------+ 7 Emery None 0 Feb 19 19:53 .
drwx------+ 7 Emery None 0 Apr 20 14:45 ..
drwx------+ 2 Emery None 0 May 1 21:41 8HZD6WS6
drwx------+ 2 Emery None 0 May 1 21:54 I4F15DOK
drwx------+ 2 Emery None 0 May 1 22:03 XM0N4Q4W
-rwx------+ 1 Emery None 1916928 May 3 12:21 index.dat
drwx------+ 2 Emery None 0 May 3 12:21 S0RKZRFZ
C:Documents and SettingsEmeryLocal SettingsTemporary Internet FilesContent.IE5>
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6. File Systems Abstraction
File system manages files

Traditionally: file system maps files to disk

But: files convenient abstraction

use same, easy interface (read, write)
Block devices (/dev/scsi0)

Disk drives – transfer in blocks

Character devices (/dev/tty)

Console, printer

Proc filesystem (/proc/mem)

FIFO (named pipes)

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7. Device files
Unix devices live in /dev,

act like ordinary files
elnux14> echo quot;fooquot; > /dev/tty
foo
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8. /proc filesystem
Normal file access to kernel internals

elnux14> ls -l /proc/30917/
total 0
dr-xr-xr-x 2 emery fac 0 May 3 13:18 attr
-r-------- 1 emery fac 0 May 3 13:18 auxv
-r--r--r-- 1 emery fac 0 May 3 13:01 cmdline
lrwxrwxrwx 1 emery fac 0 May 3 13:18 cwd -> /nfs/elsrv4/users5/fac/em
-r-------- 1 emery fac 0 May 3 13:18 environ
lrwxrwxrwx 1 emery fac 0 May 3 13:18 exe -> /bin/tcsh
dr-x------ 2 emery fac 0 May 3 12:06 fd
-rw-r--r-- 1 emery fac 0 May 3 13:18 loginuid
-r-------- 1 emery fac 0 May 3 13:18 maps
-rw------- 1 emery fac 0 May 3 13:18 mem
-r--r--r-- 1 emery fac 0 May 3 13:18 mounts
lrwxrwxrwx 1 emery fac 0 May 3 13:18 root -> /
-r--r--r-- 1 emery fac 0 May 3 13:01 stat
-r--r--r-- 1 emery fac 0 May 3 13:18 statm
-r--r--r-- 1 emery fac 0 May 3 13:01 status
dr-xr-xr-x 3 emery fac 0 May 3 13:18 task
-r--r--r-- 1 emery fac 0 May 3 13:10 wchan
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9. Named Pipes (FIFO)
Special file: acts like unnamed pipe

E.g., cat file | wc -l

elnux14> mkfifo thePipe
elnux14> ls -ld thePipe
prw-r----- 1 emery fac 0 May 3 14:00 thePipe
elnux14> cat simplesocket.h > thePipe &
[1] 32242
elnux14> wc -l < thePipe
155
[1] Done cat simplesocket.h
> thePipe
elnux14>
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10. Named Pipes (FIFO)
Special file: acts like unnamed pipe

E.g., cat file | wc -l

elnux14> mkfifo thePipe
elnux14> ls -ld thePipe
prw-r----- 1 emery fac 0 May 3 14:00 thePipe
elnux14> cat simplesocket.h > thePipe &
[1] 32242
elnux14> wc -l < thePipe
155
[1] Done cat simplesocket.h
> thePipe
elnux14>
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11. Named Pipes (FIFO)
Special file: acts like unnamed pipe

E.g., cat file | wc -l

elnux14> mkfifo thePipe
elnux14> ls -ld thePipe
prw-r----- 1 emery fac 0 May 3 14:00 thePipe
elnux14> cat simplesocket.h > thePipe &
[1] 32242
elnux14> wc -l < thePipe
155
[1] Done cat simplesocket.h
> thePipe
elnux14>
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12. Named Pipes (FIFO)
Special file: acts like unnamed pipe

E.g., cat file | wc –l

elnux14> mkfifo thePipe
elnux14> ls -ld thePipe
prw-r----- 1 emery fac 0 May 3 14:00 thePipe
elnux14> cat simplesocket.h > thePipe &
[1] 32242
elnux14> wc -l < thePipe
155
[1] Done cat simplesocket.h
> thePipe
elnux14>
Useful when cannot do redirection

Especially for compression

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13. Named Pipes (FIFO)
Exercise:

Program named “joe” outputs file “joe.out”

Huge (~ 3 GB)

Compress it automagically using gzip -c &

named FIFO to “joe.out.gz”
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14. Named Pipes (FIFO)
Exercise:

Program named “joe” outputs file “joe.out”

Huge (~ 3 GB)

Compress it automagically using gzip -c &

named FIFO to “joe.out.gz”
elnux14> mkfifo joe.out
elnux14> gzip –c < joe.out > joe.out.gz &
[1]
elnux14> joe
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15. File Metadata
Files have a lot of associated “metadata”;

ex.: Unix (from stat)
Date created, last modified, last accessed

Size (bytes)

User & group ID of file’s owner

File type (not content type)

Directory

Regular file

Block / character device (disk drive, screen)

FIFO

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16. Untyped Files
Unix, Windows – file contents untyped

Stream of bytes

Type implied by convention (extensions)

.ppt, .pdf, …

Mac: file types stored in metadata

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17. File Types
Unix, Windows – file contents untyped

Stream of bytes

Type implied by convention (extensions)

.ppt, .pdf, …

Mac: file types stored in metadata

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18. Access Control
Unix: each file has associated bits that

control access (& other stuff)
Read

Write

Execute

Can specify for three “users”

User (file owner)

Group (set of users)

Other (everyone else)

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19. Access Control - chmod
Can read bits via ls, set bits via chmod

elnux14> ls -l ack.scm
-rw-r----- 1 emery fac 197 Feb 25 15:19 ack.scm
elnux14> chmod -r ack.scm
elnux14> ls -l ack.scm
--w------- 1 emery fac 197 Feb 25 15:19 ack.scm
elnux14> cat ack.scm
cat: ack.scm: Permission denied
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20. The End
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