Operating Systems - File Systems

UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science

Operating SystemsCMPSCI 377File Systems

Emery Berger

University of Massachusetts Amherst


Files

Associate names with data

Usuallystored on persistent media (disks)

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File Names

Hierarchicaldirectory structure

Absolute, relativeto current

Windows names = location + dir

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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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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

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C:\Documents and Settings\Emery\Local Settings\Temporary Internet Files\Content.IE5>ls -ltratotal 1873-rwx------+ 1 Emery None 67 Jan 10 17:31 desktop.inidrwx------+ 2 Emery None 0 Jan 17 22:42 0NDWKTYTdrwx------+ 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 8HZD6WS6drwx------+ 2 Emery None 0 May 1 21:54 I4F15DOKdrwx------+ 2 Emery None 0 May 1 22:03 XM0N4Q4W-rwx------+ 1 Emery None 1916928 May 3 12:21 index.datdrwx------+ 2 Emery None 0 May 3 12:21 S0RKZRFZ

C:\Documents and Settings\Emery\Local Settings\Temporary Internet Files\Content.IE5>


File Systems Abstraction

File system manages files

Traditionally: file system maps files to disk

But: files convenient abstractionuse 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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Device files

Unix devices live in /dev,act like ordinary files

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elnux14> echo "foo" > /dev/ttyfoo


/proc filesystem

Normal file access to kernel internals

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elnux14> ls -l /proc/30917/total 0dr-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 cmdlinelrwxrwxrwx 1 emery fac 0 May 3 13:18 cwd -> /nfs/elsrv4/users5/fac/emery-r-------- 1 emery fac 0 May 3 13:18 environlrwxrwxrwx 1 emery fac 0 May 3 13:18 exe -> /bin/tcshdr-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 mountslrwxrwxrwx 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 statusdr-xr-xr-x 3 emery fac 0 May 3 13:18 task-r--r--r-- 1 emery fac 0 May 3 13:10 wchan


Named Pipes (FIFO)

Special file: acts like unnamed pipe

E.g., cat file | wc -l

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elnux14> mkfifo thePipeelnux14> ls -ld thePipeprw-r----- 1 emery fac 0 May 3 14:00 thePipeelnux14> cat simplesocket.h > thePipe &[1] 32242elnux14> wc -l < thePipe155[1] Done cat simplesocket.h> thePipeelnux14>


Named Pipes (FIFO)



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Named Pipes (FIFO)



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Named Pipes (FIFO)


E.g., cat file | wc –l

Useful when cannot do redirection

Especially for compression12



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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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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elnux14> mkfifo joe.outelnux14> gzip –c < joe.out > joe.out.gz &[1]elnux14> joe


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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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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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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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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Access Control - chmod

Can read bits via ls, set bits via chmod

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elnux14> ls -l ack.scm-rw-r----- 1 emery fac 197 Feb 25 15:19 ack.scmelnux14> chmod -r ack.scmelnux14> ls -l ack.scm--w------- 1 emery fac 197 Feb 25 15:19 ack.scmelnux14> cat ack.scmcat: ack.scm: Permission denied

UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 20

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Operating Systems - File Systems

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