Download File Systems

File Systems
Tanenbaum Chapter 4
Silberschatz Chapters 10, 11, 12
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1
File Systems
Essential requirements for long-term
information storage:
•
•
•
It must be possible to store a very large amount
of information.
The information must survive the termination of
the process using it.
Multiple processes must be able to access the
information concurrently.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639
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File Structure
• None:
– File can be a sequence of words or bytes
• Simple record structure:
– Lines
– Fixed Length
– Variable Length
• Complex Structure:
– Formatted documents
– Relocatable load files
• Who decides?
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File Systems
Think of a disk as a linear sequence of fixed-size
blocks and supporting reading and writing of
blocks. Questions that quickly arise:
•
•
•
How do you find information?
How do you keep one user from reading another’s data?
How do you know which blocks are free?
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Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639
4
File Naming
Figure 4-1. Some typical file extensions.
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Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639
5
File Access Methods
• Sequential Access
– Based on a magnetic tape model
– read next, write next
– reset
• Direct Access
–
–
–
–
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Based on fixed length logical records
read n, write n
position to n
relative or absolute block numbers
6
File Structure
Figure 4-2. Three kinds of files. (a) Byte sequence.
(b) Record sequence. (c) Tree.
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Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639
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File Types
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Figure 4-3. (a) An executable file. (b) An archive.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639
8
File Attributes
Figure 4-4a. Some possible file attributes.
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Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639
9
File Operations
The most common system calls relating to files:
•
•
•
•
•
•
Create
Delete
Open
Close
Read
Write
•
•
•
•
•
Append
Seek
Get Attributes
Set Attributes
Rename
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Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639
10
Example Program Using File System
Calls (1)
...
Figure
4-5. A simple program to copy a file.
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Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639
11
Example Program Using File System
Calls (2)
Figure 4-5. A simple program to copy a file.
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Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639
12
Directory Structure
• Collection of nodes containing
information on all files
F2
F1
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F4
F5
F3
13
Information in a Device Directory
•
•
•
•
•
•
•
•
•
File name:
File Type:
Address:
Current Length
Maximum Length
Date Last accessed (for archiving)
Date Last updated (for dumping)
Owner ID
Protection information
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Directory Operations
•
•
•
•
•
•
Search for a file
Create a file
Delete a file
List a directory
Rename a file
Traverse the file system
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Objectives for a Directory System
• Make it efficient
– It should be easy to locate a file quickly
• Make file (and directory) naming convenient
– Allow 2 users to have the same name for different
files
– Allow the same file to have more than 1 name
• Allow logical grouping of files
– All word processing files together
– All c++ files together
– etc.
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Alternative Directory Structures
• Single-Level Directory
cat
bo
a
test
data mail cont hex
word calc
• Issues:
– Naming
– Grouping
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Alternative Directory Structures
• Two-Level Directory
User1
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User2
User3
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Tree-Structured Directory
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Path Names
Figure 4-8. A UNIX directory tree.
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Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639
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File System Structure
• A data structure on a disk that holds files
• Disk Storage attributes:
– Allows direct access to any given block of information
– Supports re-writing in place (copy block, modify,
rewrite)
• Organized in Layers:
–
–
–
–
–
–
Application Programs
Logical file system
file-organization module
basic file system
I/O control
devices
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File System Organization
• Devices
– Peripheral equipment- disk drives, tapes, etc.
• I/O Control
– device drivers and interrupt handlers
– Communicates directly with peripheral devices
– Starts and closes out I/O request
• Basic File System
– Requires knowledge of physical device (track,
cylinder, head, etc.)
– Manages data transfer at block level (buffering,
placement)
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File System Organization
• File-organization Module
– Maps logical block structure of file to physical
block structure of disk
– Includes free space manager
• Logical file system
– Manages file directory information
– Provides security, protection
• Applications
– Maintains basic data about the file
– Allows users to access file information
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File System Structure
• Access to File
– file control block (generic)
– file descriptor (UNIX)
– file handle (Windows)
• File System Requirements
–
–
–
–
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boot mechanism
file system information
file information
file data
24
Allocation Methods
Contiguous Allocation
• Each file occupies a set of contiguous blocks on the
disk.
• Number of blocks needed identified at file creation
– May be increased using file extensions
• Advantages:
– Simple to implement
– Good for random access of data
• Disadvantages
– Files cannot grow
– Wastes space
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Contiguous Allocation
FileA
0
1
2
3
4
5
6
7
8
FileB
9
10
11
12
13
14
15
16
17
18
FileC
19
20
21
22
23
FileE
24
25
26
27
FileD
28
29
30
31
33
34
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File Allocation Table
File Name
FileA
FileB
FileC
FileD
FileE
Start Block Length
2
9
18
30
26
3
5
8
2
3
26
Allocation Methods
Linked Allocation
• Each file consists of a linked list of disk
blocks.
data
ptr
data
ptr
data
ptr
data Null
• Advantages:
– Simple to use (only need a starting address)
– Good use of free space
• Disadvantages:
– Random Access is difficult
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Linked Allocation
0
FileB
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
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File Allocation Table
File Name
...
FileB
...
Start Block End
...
1
...
...
28
...
28
Linked Allocation
0
FileB
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
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File Allocation Table
File Name
...
FileB
...
Start Block End
...
1
...
...
28
...
29
Allocation Methods
Indexed Allocation
• Collect all block pointers into an index block.
Index Table
• Advantages:
– Random Access is easy
– No external fragmentation
• Disadvantages
– Overhead of index block
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Indexed Allocation
File Allocation Table
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
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File Name
Index Block
Jeep
24
1
3
8
14
28
31
Indexed Allocation
File Allocation Table
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
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File Name
Index Block
Jeep
24
1
8
3
14
28
32
UNIX File System
File Types
•
•
•
•
•
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Regular File
Directory
Block-oriented Device
Character-oriented Device
Symbolic Link
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UNIX File Ownership
read
write
execute
owner
1
1
0
group
1
0
0
others
1
0
0
(for directories: read, write, search)
-rwx-----drwxr-xr-x
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1 rsmith
2 rsmith
is
is
785 Feb 11 1994 send_exe
512 Apr 16 13:49 sysmon_server
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File Access Model
• open -- read -- write -- close
fdesc = open (*filename, r / w / a / +);
n = read (fdesc, buff, 24);
n = write (fdesc, buff, 24);
lseek (fdesc, 100L, L_SET);
(L_SET, L_CUR, L_END)
close (fdesc);
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Concurrent File Access
• From separate processes
– OPEN generates a new file descriptor.
– Allows independent access to the same file.
– No inherent blocking in access
• From parent / child processes
– fork duplicates all active file descriptors.
– all processes access a common “current” pointer
• File locking
– flock, lockf
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UNIX File Sharing
- unrelated processes process table entry
fd 0
fd flags
ptr
File Table
file status flags
current file offset
v-node ptr
V-node Table
v-node info
I-node info
file size
process table entry
fd 0
fd flags
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ptr
file status flags
current file offset
v-node ptr
37
UNIX File Sharing
- related processes process table entry
fd 0
fd flags
ptr
File Table
file status flags
current file offset
v-node ptr
V-node Table
v-node info
I-node info
file size
process table entry
fd 0
fd flags
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ptr
file status flags
current file offset
v-node ptr
38
UNIX i-node
mode
owners(2)
timestamps(3)
size block
count
data
data
data
direct blocks
single indir
double indir
triple indir
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UNIX i-node
mode
owners(2)
timestamps(3)
size block
count
data
data
data
direct blocks
single indir
double indir
:
:
data
data
triple indir
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UNIX i-node
mode
owners(2)
timestamps(3)
size block
count
data
data
data
direct blocks
single indir
double indir
triple indir
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:
:
data
data
:
:
:
:
:
:
data
data
data
data
41
I-node File Sizes
1 KB block
2KB block
4KB block
Direct
12 k bytes
24 k bytes
48 k bytes
Single
indirect
Double
indirect
Triple
indirect
256 k bytes 1 M bytes
4 M bytes
64 M bytes 512 M
bytes
16 G bytes 256 G
bytes
2 G bytes
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2 T bytes
42
Disk Space Management Block Size (1)
Figure 4-20. Percentage of files smaller than a given size (in bytes).
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Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639
43
Disk Drive Layout
(simplified)
Disk
Drive
Partition
Block
Group
partition
Block group
partition
Block group
Block group
directory blocks and data blocks
I-list
I-node
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partition
I-node
I-node
I-node
44
Sample filesystem:
Creating the directory “testdir”
directory
block
i-list
i-node
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i-node
1267
i-node
2549
i-node
45
Sample filesystem:
Creating the directory “testdir”
directory
block
i-list
i-node
i-node
1267
i-node
2549
i-node
1267
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•
46
Sample filesystem:
Creating the directory “testdir”
directory
block
i-list
i-node
i-node
1267
i-node
2549
i-node
1267
2549
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•
testdir
47
Sample filesystem:
Creating the directory “testdir”
directory
block
i-list
i-node
i-node
1267
i-node
2549
i-node
1267
2549
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•
testdir
48
Sample filesystem:
Creating the directory “testdir”
directory
block
i-list
i-node
i-node
1267
i-node
2549
directory
block
i-node
2549
1267
•
• •
1267
2549
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•
testdir
49
Sample filesystem after creating
the directory “testdir”
directory
block
i-list
i-node
i-node
1267
i-node
2549
directory
block
i-node
2549
1267
•
• •
1267
2549
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•
testdir
50
Viewing directory structure
• ls –ai /
–2
–:
– 1730
.
home
• ls –ai /home
–
–
–
–
1730
2
:
199
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.
..
rcotter
• ls –ai /home/rcotter
–
–
–
–
199
.
1730 . .
:
241224 cs431
• ls –ai ~/cs431
–
–
–
–
241224 .
199
..
:
245436 exec.cpp
51
Shared Files (1)
Figure 4-16. File system containing a shared file.
ln –s shared_file new_link
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Shared Files (2)
Figure 4-17. (a) Situation prior to linking. (b) After the link is
created. (c) After the original owner removes the file.
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Linux File System Structure
• Linux uses a Virtual File System (VFS)
– Defines a file object
– Provides an interface to manipulate that object
• Designed around OO principles
– File system object
– File object
– Inode object (index node)
• Primary File System - ext2fs
– Supports (or maps) several other systems
(MSDOS, NFS (network drives), VFAT (W95),
HPFS (OS/2), etc.
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Virtual Filesystem
• A kernel software layer that handles all system
calls related to a standard UNIX filesystem.
• Supports:
– Disk-based filesystems
• IDE Hard drives (UNIX, LINUX, SMB, etc.)
• SCSI Hard drives
• floppy drives
– Network filesystems
• remotely connected filesystems
– Special filesystems
• /proc
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VF Example
inf = open (“/floppy/test”,
O_RDONLY, 0);
outf = open (“/tmp/test”,
O_WRONLY|O_CREATE|O_TRUNC,
0600);
do {
cnt = read(inf, buf, 4096);
write (outf, buf, cnt);
} while (cnt);
close (outf);
close (inf);
cp
VFS
ext2
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MS-DOS
56
Virtual file system Model
• Superblock object
– metadata about a mounted filesystem
• inode object
– general information about a specific file
• file object
– information about process and open file interaction
• dentry object
– directory entry information
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Virtual Filesystem and
Processes
disk file
superblock
Process 1
inode object
file object
dentry
object
Process 2
file object
Process 3
file object
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dentry
object
58
Ext2fs History
• Minux
• extfs
• ext2fs - 1994
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Ext2fs Blocks
• 1k default
• 2k, 4k possible
• choice based on file sizes
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Ext2fs Disk Data Structures
Boot
Block
Super
Block
Block Group 0
Group
Data block Inode Inode
Descriptors Bitmap Bitmap Table
Block Group N
Data Blocks
• Each Block Group contains:
–
–
–
–
–
–
A copy of the Super Block
A copy of the group of block group descriptors
A data block bitmap (for this group)
An inode bitmap (for this group)
A group of inodes (for this group)
Data blocks
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Disk Data Structures
• SuperBlock
– Contains metadata about the block group
– # free blocks,
– # free inodes,
– block size,
– times (last mount, last write),
– check status,
– blocks to pre-allocate,
– alignments, etc. (~38 items)
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Disk Data Structures
• Block Groups
– Block group limited to 8 * block size by data
block bitmap.
– 1k block limited to 8,192 blocks or 8 mbytes
per block group. 4k block = 128 mbytes
• Group descriptor - 24 bytes per group
– block numbers for bitmaps, start of tables, etc.
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Disk Data Structures
• Inode Table
– Inodes - 128 bytes
– File type and access rights
– owner
– length
– timestamps (last access, last change, etc.)
– hard links counter
– pointers to data blocks,
– etc.
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Disk Data Structures
• Items cached in memory on filesystem
mount
– Superblock - always cached
– Group descriptor - always cached
– Block bitmap - fixed limit - most recent only
– Inode bitmap - fixed limit - most recent only
– Inode - dynamic
– Data block - dynamic
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File Blocks Grouped
•
•
•
•
Physically and logically
Inodes distributed throughout disk
Data blocks allocated close to inode
Pre-allocation of free blocks to minimize
frag.
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Summary Features
•
•
•
•
Fast Symbolic Links
pre-allocation of data blocks
file-updating strategy
immutable files or append only files
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Journal File Systems
• Designed to speed file system recovery from
system crashes.
• Traditional systems use an fs recovery tool (e.g.
fsck) to verify system integrity
• As file system grows, recovery time grows.
• Journal File Systems track changes in meta-data
to allow rapid recovery from crashes
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Journal File System
• Uses a version of a transaction log to track
changes to file system directory records.
• If a system crash occurs, the transaction log can
be reviewed back to a check point to verify any
pending work (either undo or redo).
• For large systems, recovery time goes from
hours or days to seconds.
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Journal File System - Examples
• XFS
–
–
–
–
–
–
–
Commercial port by SGI (IRIX OS)
Based on 64 bit system (ported to 32 bit)
Supports large systems 2TB -> 9 million TB
Uses B+trees for improved performance
Journals file system meta-data
Supports Quotas, ACLs.
Supports filesystem extents (contiguous blocks)
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Journal File System - Examples
• JFS
–
–
–
–
–
–
–
Commercial Port by IBM. Used in OS/2
64 bit system ported to 32 bits.
Journal support of meta-data.
System Size 2TB -> 32PB
Built to scale on SMP architectures
Uses B+trees for improved performance
Supports use of extents.
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Journal File System - Examples
• ReiserFS
–
–
–
–
–
–
Designed originally for Linux (32 bit system)
Supports file systems to 2TB -> 16TB
Journal support of meta-data
Btree structure supports large file counts
Supports block packing for small files
No fixed inode allocation - more flexible.
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Journal File System - Examples
• Ext3fs
– Simple extension of ext2fs that adds journaling
– All virtual file system operations are journaled.
– Other limitations of ext2fs remain.
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Journal File System - Examples
• ext4fs
– Next generation of file system development, incorporating
improvements and changes to ext3fs.
– Journalling filesystem
• Improvements is journal checksumming
– Larger file systems – 1 MTB (exabyte)
• Larger files – 16 TB
– Delayed block allocation, multi-block allocator
• Allows files to be written as contiguous sequences of blocks:
improves read performance of streaming data files.
• Poses the risk of lost data if system crashes before data is written.
– Use of extents
• Large (<= 128 MB) contiguous physical blocks.
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Free Space Management
• Bit Vector management
0
1
1
0
.....
1
• One bit for each block
– 0 = free; 1 = occupied
• Use bit manipulation commands to find free
block
• Bit vector requires space
– block size = 4096 = 2 12
– disk size = 1 gigabyte = 2 30
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– bits = 2 (30-12) = 2 18 = 32k bytes
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Free Space Management
• Bit vector (advantages):
– Easy to find contiguous blocks
• Bit vector (disadvantages):
– Wastes space (bits allocated to unavailable
blocks)
• Issues:
– Must keep bit vector on disk (reliability)
– Must keep bit vector in memory (speed)
– cannot allow memory != disk (memory = 1, disk =
0)
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Free Space Management
• Linked List management
– Use linked list to identify free space
• Advantages:
– no wasted space
• Disadvantages:
– harder to identify contiguous space.
• Issues:
– Must protect pointer to free list
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Free Space Management
• Grouping of blocks
Boot
block
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File
File
system
descriptors
descriptor
78
Reducing Disk Arm Motion
Figure 4-29. (a) I-nodes placed at the start of the disk.
(b) Disk divided into cylinder groups, each with its own blocks
and i-nodes.
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Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639
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Keeping Track of Free Blocks (1)
Figure
4-22. (a) Storing the free list on a linked list. (b) A bitmap.
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Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639
Disk Quotas
Figure 4-24. Quotas are kept track of on a per-user basis
in a quota table.
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81
File System Backups (1)
Backups to tape are generally made to handle
one of two potential problems:
•
•
Recover from disaster.
Recover from stupidity.
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File System Backups (2)
Figure 4-25. A file system to be dumped. Squares are directories,
circles are files. Shaded items have been modified since last
dump. Each directory and file is labeled by its i-node number.
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File System Backups (3)
Figure 4-26. Bitmaps used by the logical dumping algorithm.
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File System Consistency
Figure 4-27. File system states. (a) Consistent. (b) Missing block.
(c) Duplicate block in free list. (d) Duplicate data block.
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Caching (1)
Figure 4-28. The buffer cache data structures.
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Caching (2)
•
Some blocks, such as i-node blocks, are rarely
referenced two times within a short interval.
•
Consider a modified LRU scheme, taking two
factors into account:
•Is the block likely to be needed again soon?
•Is the block essential to the consistency of the file system?
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Example File Systems
• CD-ROM
– ISO 9660
– Rock Ridge
– Joliet
• MS-DOS
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The ISO 9660 File System
Figure 4-30. The ISO 9660 directory entry.
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Rock Ridge Extensions
Rock Ridge extension fields:
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PX - POSIX attributes.
PN - Major and minor device numbers.
SL - Symbolic link.
NM - Alternative name.
CL - Child location.
PL - Parent location.
RE - Relocation.
TF - Time stamps.
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Joliet Extensions
Joliet extension fields:
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Long file names.
Unicode character set.
Directory nesting deeper than eight levels.
Directory names with extensions
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The MS-DOS File System (1)
Figure 4-31. The MS-DOS directory entry.
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The MS-DOS File System (2)
Figure 4-32. Maximum partition size for different block sizes. The empty
boxes represent forbidden combinations.
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Summary
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File Structures
Directory Structures
File System Implementation
File System Management
Example File Systems
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Questions
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Two primary access methods are used to manage
files. Discuss these two methods, including which types of
physical devices each is normally associated with.
What is the difference between a tree structured file
directory and an acyclic graph directory? Please give an
example Operating System used today that uses each.
Discuss 3 ways in which files might be allocated to file
stores (disks). What are the advantages or disadvantages
of each?
How might a file system manage its free space so that it
can easily assign space for a new file when needed? Give
at least 2 approaches.
What can an OS designer to improve the efficiency of a file
system? What factors should be considered? How do those
affect the efficiency of the system?
Discuss at least 2 mechanisms that might be used to
ensure the recovery of information stored in a file system.95
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