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Operating Systems
软件学院
高海昌
[email protected]
Operating Systems
Contents
 1.
Introduction
**
 2.
Processes and Threads
*******
 3.
Deadlocks
**
 4.
Memory Management
*****
 5.
Input/Output
***
 6.
File Systems
****
 8.
Multiple Processor Systems
*
 9.
Security
**
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Operating Systems
Chapter 6: File Systems
 6.1
Files
 6.2
Directories
 6.3
File system implementation
 6.4
Example file systems
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Operating Systems
Long-term Information Storage
Three essential requirements:
1.
Must be possible to store a large amount of information
2.
Information stored must survive the termination of the
process using it
3.
Multiple processes must be able to access the information
concurrently
The usual solution to all these problems is to store
information on disks and other external media in units
called files.
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Operating Systems
File Naming
 The extra rules for naming vary somewhat from system to
system.

bruce, device, multimedia

123, urgent!, fig.2-1

longlongagotherewasawarbetweentthebirdsandthebeasts (<255)
 Some file systems distinguish between upper and lower ease
letters (UNIX), whereas others do not (MS-DOS).
 Many OS support two-part file names, with the two parts
separated by a period, as in prog.c. The part following the
period is called the file extension.
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Operating Systems
File Naming
Typical file extensions.
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Operating Systems
File Structure
(a) byte sequence (UNIX and Windows)
(b) record sequence (80-character punched card)
(c) tree (commercial data processing large mainframe computers)
 Three kinds of files
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Operating Systems
File Types
 Sort by usage

system files

user files

library files
 Sort by data format

source files

object files

executable files
 Sort by access control attribution

executable-only files

read-only files

read/write files
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Operating Systems
File Types (2)
 Sort by file logical structure

structured files

unstructured files
 Sort by file physical structure

sequential files

linking files

index files
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Operating Systems
File Types (3)
 UNIX and Windows have regular files and directories.
UNIX also has character and block special files.

Regular files: contain user information.

Directories: system files for maintaining the structure of the file system.

Character special files: related to I/O.

Block special files: used to model disks.
 Regular files are generally either ASCII files or binary files.
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Operating Systems
File Types (4)
(a) An executable file
(b) An archive 存档文件
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Operating Systems
File Types (exercise)
In an executable file, the instructions are usually put into
text segment.
the _______
header, text, data, relocation bits, symbol table
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Operating Systems
File Access
 Sequential access
 read
all bytes/records from the beginning
 cannot
jump around, could rewind or back up
 convenient when
medium was mag tape
 Random access
 bytes/records
 essential for
read in any order
data base systems
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Operating Systems
File Attributes
Possible file
attributes
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Operating Systems
File Operations
1.
Create
7.
Append
2.
Delete
8.
Seek
3.
Open
9.
Get attributes
4.
Close
10.
Set Attributes
5.
Read
11.
Rename
6.
Write
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Operating Systems
An Example Program Using File System Calls (1/2)
copyfile abc xyz
argc=3, argv[0]=“copyfile”, argv[1]=“abc”, argv[2]=“xyz”
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Operating Systems
An Example Program Using File System Calls (2/2)
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Operating Systems
Memory-Mapped Files
Access file as above is inconvenient.
Map files into the address space of a running process.
(a) Segmented process before mapping files into its address
space
(b) Process after mapping
existing file abc into one segment
creating new segment for xyz
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Operating Systems
Chapter 6: File Systems
 6.1
Files
 6.2
Directories
 6.3
File system implementation
 6.4
Example file systems
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Operating Systems
Single-Level Directory Systems
 A single level directory system
 contains
4 files
 owned by 3 different people, A, B, and C
 Problem: different users may accidently use the
same names for their files.
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Operating Systems
Two-level Directory Systems
Letters indicate owners of the directories and files
open (“x”)
open (“nancy/x”)
Problem: it is not satisfactory for users with a large number of
files.
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Operating Systems
Hierarchical Directory Systems
A hierarchical directory system (a tree of directories)
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Operating Systems
Path Names
– Absolute path name
/usr/ast/mailbox (UNIX)
\usr\ast\mailbox (Win)
– Relative path name
mailbox
– Working directory
( current directory)
/usr/ast
– . , ..
A UNIX directory tree
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Operating Systems
Directory Operations
1.
Create
2.
Delete
3.
Opendir
4.
Closedir
5.
Readdir
6.
Rename
7.
Link (hard link/symbol link)
8.
Unlink
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Operating Systems
Exercise
 In the file system shown in the following figure, rectangle
represents directory, circle represents file, “/” represents the
delimiter（分隔符）in the path name which represents the root
directory when placed on the head of path name. In the figure
（1） . If the present directory is ‘D1’, process A opens the ‘f1’
by the two ways listed blow:
① fd1=open(“ （2） /f1”, O_RDONLY)
② fd1=open(“/D1/W1/f1”, O_RDONLY)
 ① is more efficient than ②,
/
D1
the reason is （3） :
f1
W1
W2
f1
f2
D2
f2
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W3
f4
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Operating Systems
Exercise
 （1）A. f2 in subdirectory W2 is the same as the f2 in subdirectory D2
B. f2 in subdirectory W2 is different from the f2 in subdirectory D2
C. f2 in subdirectory W2 and f2 in subdirectory D2 may be same and may
be different
D. tree-shaped file system didn’t allow appear the file with same name
 （2）A. /D1/W1
B. D1/W1
C. W1
D. f1
 （3）A. ① can directly access the f1 in the root directory
B. ① can access the f1 started from searching the present directory
C. To get f1, ① needs only one time to access disk, but ② needs 2 times
D. To get f1, ① needs only one time to access disk, but ② needs 3 times
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Operating Systems
Lesson 2
Operating Systems
Chapter 6: File Systems
 6.1
Files
 6.2
Directories
 6.3
File system implementation
 6.4
Example file systems
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Operating Systems
File System layout 布局
 File system are stored on disk.
 Most disks can be divided up into one or more partitions,
with independent file systems on each partition.
 Sector 0 of the disk is called MBR (Master Boot Record) and
is used to boot the computer.
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Operating Systems
File System layout
A possible file system layout
Partition table: gives the starting and ending addresses of each partition.
Boot block: locate the active partition
Superblock: magic number, number of blocks, …
Free space mgmt: free blocks in the form of bitmap or a list of pointers.
i-nodes: an array of data structure, one per file.
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Operating Systems
Implementing Files
various methods are used in different systems
 Contiguous allocation
 Linked list allocation
 Linked list allocation using a table in memory
 i-nodes
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Operating Systems
Implementing Files
(a) Contiguous allocation of disk space for 7 files
(b) State of the disk after files D and E have been removed
Ad: simple to implement; read performance is excellent.
Dis: in time, the disk becomes fragmented.
Contiguous allocation is feasible in CD-ROM file system.
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Operating Systems
Implementing Files (2)
Storing a file as a linked list of disk blocks
Dis: random access is extremely slow.
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Operating Systems
Implementing Files (3)
Such a table in main memory is
called a FAT (File Allocation Table)
Linked list allocation using a file allocation table in RAM
Dis: entire table must be in memory all the time to make it work.
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Operating Systems
Implementing Files (4)
Associate with each file a
data structure called an inode (index-node), which
lists the attributes and
disk addresses of the file’s
blocks.
Ad: i-node need only be in
memory when the
corresponding file is open.
An example i-node
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Operating Systems
Implementing Directories
 The main function of the directory system is to map the
ASCII name of the file onto the information needed to locate
the data.
 Where the attributes should be stored?
 Store
them directly in the directory entry.
 For systems that use i-nodes, store the attributes in the inodes.
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Operating Systems
Implementing Directories (2)
(a) A simple directory, fixed size entries, disk addresses and
attributes in directory entry (Windows)
(b) Directory in which each entry just refers to an i-node (UNIX)
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Operating Systems
Implementing Directories (3)
 So far we have assumed that files have short, fixed-length
names.
 However, nearly all modern OS support longer, variable-
length file names. How can these be implemented?
 Simplest way: set a limit on file name length (255), and use
one of the designs above for each file name. (Waste)
 Second, give up the same size. Each dir entry contains a
fixed portion, followed by the actual file name, however long
it may be.
- dis: when a file is removed, a variable-sized gap is introduced into the dir
into which the next file to be entered may not fit.
 Third,
make the dir entries themselves all fixed length and
keep the file names together in a heap at the end of the dir.
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Operating Systems
Implementing Directories (4)
(a) In-line
(b) In a heap
 Two ways of handling long file names in directory
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Operating Systems
Shared Files
File system containing a shared file
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Operating Systems
Shared Files (2)
 If dir really do contain disk address, then a copy of the disk
address will have to be made in B’s dir when the file is
linked.
 If either B or C subsequently appends to the file, the new
blocks will be listed only in the dir of the user doing the
append.
 Problem can be solved in two ways:
 Disk
blocks are not listed in dir, but in a little data structure
associated with the file itself (i-node in UNIX).
 B links to C’s files by having the system create a new file, of
type LINK, and entering that file in B’s dir. The new file
contains just the path name of the file to which it is linked.
(symbolic linking)
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Operating Systems
Shared Files (3)
(a) Situation prior to linking
(b) After the link is created
(c)After the original owner removes the file
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Operating Systems
Shared Files (4)
 Drawbacks of first solution:
 If
C tries to remove a file, OS removes the file and clears the
i-node, B will have a dir entry pointing to an invalid i-node.
If the i-node is later reassigned to another file, B’s link will
point to the wrong file.
 The system can see from the count in the i-node that the file
is still in use, but there is no way for it to find all the dir
entries for the file, in order to erase them.
 The problem with symbolic links is the extra overhead
required.
 Drawbacks for both: programs that start at a given dir and find
all the files in that dir and its subdir will locate a linked file
multiple times.
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Operating Systems
Disk Space Management
 Two general strategies are possible for storing an n byte file:
n
consecutive bytes of disk space are allocated
 or the file is split up into a number of blocks.
 Nearly all file systems chop files up into fixed-size blocks
that need not be adjacent.
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Operating Systems
Block size
Block size
 The solid curve gives data rate of a disk. (data rate 数据速率 goes up with
block size)
 The dashed curve gives disk space efficiency. (With small blocks that are
powers of two and 2-KB files, no space is wasted in a block)
 All files are 2KB.
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Operating Systems
Keeping track of free blocks
(a) Storing the free list on a linked list
(b) A bit map
Bitmap requires less space, since it uses 1 bit per block, VS 32 bits
in the linked list model.
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Operating Systems
Lesson 3
Operating Systems
Disk Space Management (4)
When the block of pointers is almost
empty, a series of short-lived temporary
files can cause a lot of disk I/O.
(a) Almost-full block of pointers to free disk blocks in RAM
- three blocks of pointers on disk
(b) Result of freeing a 3-block file
(c) Alternative strategy for handling 3 free blocks
- shaded entries are pointers to free disk blocks
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Operating Systems
Disk Quotas 磁盘配额
Quotas for keeping track of each user’s disk use
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Operating Systems
File System Reliability
 Backups is very important
 It
is usually desirable to back up only specific dir and
everything in them rather than the entire file system.
 Incremental dumps 增量转储
 It may be desirable to compress the data before writing
them to tape, but a single bad spot on the backup tape can
foil the decompression algorithm and make an entire file
unreadable.
 It is difficult to perform a backup on an active file system.
Making rapid snapshots of the file system state by copying
critical data structures.
 Backup tapes should be kept off-site.
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Operating Systems
File System Reliability (2)
 Two strategies can be used for dumping a disk to tape:
 Physical
dump: start at block 0 of the disk, writes all the
disk blocks onto the output tape in order, and stops when it
has copied the last one.
Advantages: Easy, fast.
Disadvantages are the inability to skip selected dir, make
incremental dumps, and restore individual files upon
request.
 Logical dump: start at one or more specified dir and
recursively (递归地) dumps all files and dir found there
that
have changed since some given base data.
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Operating Systems
File System Reliability (3)
File that has
not changed
 A file system to be dumped



squares are directories, circles are files
shaded items, modified since last dump
each directory & file labeled by i-node number
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Operating Systems
File System Reliability (4)
Bit maps used by the logical dumping algorithm
 Step
1: examine all the entries. For each modified file, its inode is marked in the bitmap. Each dir is also marked.
 Step 2: unmark any dir that have no modified files or dir in
them or under them.
 Step 3: dump all the dir that are marked for dumping.
 Step 4: the files marked are also dumped.
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Operating Systems
File System Reliability (5)
 Tricky issues 棘手之处:
 Since
the free block list is not a file, it is not dumped and
hence it must be reconstructed from scratch after all the
dumps have been restored.
 If a file is linked to two or more dir, it is important that the
file is restored only one time.
 UNIX files may contain holes. The blocks in between are not
part of the file and should not be dumped and not be restored.
 Special files, named files, and the like should never be
dumped.
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Operating Systems
File System Consistency
If the system crashes before all the modified blocks have been written
out, the file system can be left in an inconsistent state.
 File system states
(a) Consistent
(c) duplicate block in free list
(b) missing block
(d) duplicate data block
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Operating Systems
File System Performance
 Access to disk is much slower than access to memory.
 Many file system designed with various optimizations to
improve performance:
 Caching
 Block read ahead
 Reducing disk arm motion
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Operating Systems
Caching
The block cache data structures
 When
a block has to be loaded into a full cache, some block
has to be removed. (like paging, use LRU)
 By writing critical blocks quickly, we greatly reduce the
probability that a crash will wreck the file system.
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Operating Systems
Caching
 It is undesirable to keep data blocks in the cache too long
before writing them out.
 Systems take two approaches:
 UNIX,
have a system call, sync, which forces all the
modified blocks out onto the disk immediately.
Sleeping 30 sec between calls, thus no more than 30 sec
work is lost due to a crash.
 MS-DOS, write every modified block to disk as soon as it
has been written. Write-through caches 通写高速缓存.
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Operating Systems
Block read ahead
 Try to get blocks into the cache before they are needed to
increase the hit rate.
 Read ahead strategy only works for files that are being read
sequentially.
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Operating Systems
Reducing disk arm motion
 By putting blocks that are likely to be accessed in sequence
close to each other. Easy to choose in bitmap record.
 Even with a free list, some block clustering can be done.
 For i-node system, reading even a short file requires two disk
accesses. One for the i-node, and one for the block.
 i-node are usually near the beginning of the disk.
 Put the i-nodes in the middle of the disk.
 Divide the disk into cylinder groups, each with its own inodes, blocks, and free list.
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Operating Systems
Reducing disk arm motion
 I-nodes placed at the start of the disk
 Disk divided into cylinder groups

each with its own blocks and i-nodes
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Operating Systems
Log-Structured File Systems
LFS, 日志结构文件系统
 With CPUs faster, memory larger
 disk caches can also be larger
 increasing number of read requests can directly come from
file system cache, with no disk access needed
 thus, most disk accesses will be writes
 LFS Strategy structures entire disk as a log
 have
all writes initially buffered in memory
 periodically write these to the end of the disk log
 when file opened, using map to locate i-node, then find
blocks
 LFS has a cleaner thread that spends its time scanning the
log circularly to compact it.
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Operating Systems
Chapter 6: File Systems
 6.1
Files
 6.2
Directories
 6.3
File system implementation
 6.4
Example file systems
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Operating Systems
CD-ROM File Systems
 Simple, have no provision for keeping track of free blocks.
 There is a single continuous spiral containing the bits in a
linear sequence.
The ISO 9660 directory entry
 Restriction: The max depth of dir nesting is 8
 Rock Ridge extensions (UNIX)
 Joliet extensions (Windows)
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Operating Systems
The CP/M File System
Control Program for Microcomputers, CP/M 微机控制程
序. A surprisingly powerful disk-based OS.
 It is a historically very
important system and was the
direct ancestor of MS-DOS.
 The system can ran quite well in
16KB of RAM. (The size of OS
in CP/M 2.2 is 3584 bytes!)
 Embedded systems are going to
be extremely widespread.
Memory layout of CP/M
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Operating Systems
The CP/M File System (2)
1KB each block, maximum
file size is 16KB.
The CP/M directory entry format (only 1 dir in CP/M)
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Operating Systems
The MS-DOS File System
Run only on Intel platforms and only in the PC’s real mode. Does
not support multiprogramming.
 MS-DOS was patterned closely on the CP/M file system
including the use of 8+3 character file names.
 From MS-DOS 2.0, has a hierarchical file system in which dir
could be nested.
 Unlike CP/M, there is no concept of different users in MS-DOS.
 Unlike CP/M, MS-DOS store the exact file size.
 MS-DOS does not store a file’s disk address in its dir entry. It
keeps track of file blocks via a file allocation table (FAT) in
main memory.
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Operating Systems
The MS-DOS File System
32-bit number is used, file size
is 4GB largest in theory.
The MS-DOS directory entry
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Operating Systems
The MS-DOS File System (2)
 Maximum partition for different block sizes (The
empty boxes represent forbidden combinations)
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Operating Systems
The Windows 98 File System
Win98 use long file names and FAT-32.
 Files created using Win98 can be accessible from Win3.x. It was
backward compatible with the old MS-DOS 8+3 naming system.
 When a file is created whose name does not obey the MS-DOS
naming system. Win98 invents an MS-DOS name for it
according to a certain algorithm.
Bytes
The extended MS-DOS directory entry used in Windows 98
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Operating Systems
The Windows 98 File System (2)
 Every file has an MS-DOS file name.
 If a file also has a long name, the name is stored in one or more
dir entries directly preceding the MS-DOS file name. Each longname entry holds up to 13 characters.
Bytes
Checksum
An entry for (part of) a long file name in Windows 98
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Operating Systems
The Windows 98 File System (3)
An example of how a long name is stored in Windows 98
“The quick brown fox jumps over the lazy dog”
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Operating Systems
The UNIX V7 File System
 Multiuser file system.
 The file system is in the form of a tree starting at the root dir,
with the addition of links, forming a directed acyclic graph.
A UNIX V7 directory entry
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Operating Systems
The UNIX V7 File System (2)
A UNIX i-node
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Operating Systems
The UNIX V7 File System (3)
The steps in looking up /usr/ast/mbox
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Operating Systems
Linux文件系统
 EXT2 (Linux Second Extend File System, ext2fs).
 superblock：记录文件系统的整体信息，包括inode/block
的总量、使用量、剩余量， 以及文件系统的格式与相关
信息等；
 inode：记录文件的属性，一个文件占用一个inode，同时
记录此文件的数据所在的 block 号码；
 block：实际记录文件的内容，若文件太大时，会占用多
个 block 。
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Operating Systems
Linux文件系统
 EXT2 所支持的 block 大小有 1K, 2K 及 4K 三种
文件最大限制
最大容量
1KB
2KB
4KB
16GB
256GB
2TB
2TB
8TB
16TB
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Operating Systems
Linux文件系统
 可以使用 dumpe2fs 这个命令来查询EXT2 文件系统
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Operating Systems
Linux文件系统
 在 Linux 下的 ext2 文件系统创建一个目录时， ext2 会分配
一个 inode 与至少一块 block 给该目录。
 可以使用 ls -i 这个选项来查看 root家目录内的文件所占用的
inode 号码
 使用ll 查看
Gao Haichang , Software School, Xidian University
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Operating Systems
Linux文件系统
 EXT3是一个日志文件系统.
 性能（速度）不如它的竞争对手，例如JFS2和XFS，但它具
有重要的优势，那就是它允许在适当的时候从流行的ext2文
件系统升级，而无需备份和恢复数据；除此之外，它还具
有比XFS更低的的CPU使用率.
 EXT3的增强特性：
 日志
 位目录跨越多个块提供基于树的目录索引
 在线系统增长
文件最大限制
最大容量
1KB
2KB
4KB
8KB*
16GB
256GB
2TB
16TB
2TB
8TB
16TB
32TB
Gao Haichang , Software School, Xidian University
80
Operating Systems
Linux文件系统
 EXT3跟EXT2一样，还是使用15个inode来查找数据块，前
12个为直接数据块，直接指向存储数据的数据块，接下来
分别为一级间接块，二级间接块，三级间接块，如图
Gao Haichang , Software School, Xidian University
81
Operating Systems
Linux文件系统
 EXT4扩展日志文件系统(Linux Kernel 2.6.28后支持)，特点：
 兼容EXT3
 更大的文件系统和更大的文件。
Ext4 分别支持 1EB
（1,048,576TB）的文件系统，以及 16TB 的文件
 无限数量的子目录
 引入了现代文件系统中流行的 extents 概念，每个 extent
为一组连续的数据块
 多块分配，支持一次调用分配多个数据块
 延迟分配。现代文件系统都尽可能地延迟分配，直到文
件在 cache 中写完才开始分配数据块并写入磁盘
 快速 fsck
 日志校验
 无日志模式
…
Gao Haichang , Software School, Xidian University
82
Operating Systems
Linux文件系统
 XFS日志文件系统(CentOS7缺省)，特点：
 支持超大文件和文件系统，特别擅长处理大文件（大数
据时代？）
 分配组
 条带化分配
 支持extents
 可变块尺寸
 延迟分配
 稀疏文件
 原生备份/恢复工具
…
Gao Haichang , Software School, Xidian University
83
Operating Systems
Linux文件系统
 Linux支持的文件系统：
 传统文件系统：ext2
/ minix / MS-DOS / FAT (用 vfat 模块)
/ iso9660 (光盘)
 日志式文件系统： ext3 / ReiserFS / Windows' NTFS /
IBM's JFS / SGI's XFS
 网络文件系统： NFS / SMBFS
 支持的文件系统查看命令
 查看已经加载到内存中支持的文件系统
Gao Haichang , Software School, Xidian University
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Operating Systems
Linux虚拟文件系统VFS
 整个 Linux 系统通过 Virtual Filesystem Switch 的核心功能去
读取 filesystem。整个 Linux 认识的 filesystem 都是 VFS 在
进行管理，使用者并不需要知道每个 partition 的 filesystem
是什么
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