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File Management
2
Operating System Components
Processor(s) Main Memory Devices
Process &ResourceManager
MemoryManager
DeviceManager
FileManager
Computer Hardware
Operating System
3
• Persistent storage• Shared device
Why Programmers Need Files
HTMLEditor
HTMLEditor
<head>…</head><body>…</body>
WebBrowser
WebBrowser
• Structured information• Can be read by any applic
• Accessibility• Protocol
<head>…</head><body>…</body>
<head>…</head><body>…</body>
foo.html
FileManager
FileManager
FileManager
FileManager
4
Fig 13-2: The External View of the File Manager
Hardware
ApplicationProgram
ApplicationProgram
Fil
e M
gr
Dev
ice
Mgr
Mem
ory
Mgr
Pro
cess
Mgr
UNIXF
ile
Mgr
Dev
ice
Mgr
Mem
ory
Mgr
Pro
cess
Mgr
Windows
open()read()
close()
write()
lseek()
CreateFile()ReadFile()CloseHandle()
SetFilePointer()
WriteFile()mount()
5
Introduction
• What is a file?
• Where is a file located physically?
• What are the steps to access a file?
6
File Management
• File is a named, ordered collection of information
• The file manager administers the collection by:– Storing the information on a device– Mapping the block storage to a logical view– Allocating/deallocating storage– Providing file directories
• What abstraction should be presented to programmer?
7
File system context
8
Levels in a file system
9
Levels of data abstraction
10
Logical structures in a file
11
Information Structure
Records
Applications
Structured Record Files
Record-Stream Translation
Stream-Block Translation
Byte Stream Files
Storage device
12
Byte Stream File Interface
• Implements the block-stream interface• Info on file held in file descriptor
– described later
• Typical operations on file– fileID = open(fileName)– close(fileID)– read(fileID, buffer, length)– write(fileID, buffer, length)– seek(fileID, filePosition)
13
Low Level Files
Stream-Block Translation
b0 b1 b2 bi......
fid = open(“fileName”,…);…read(fid, buf, buflen);…close(fid);
int open(…) {…}int close(…) {…}int read(…) {…}int write(…) {…}int seek(…) {…}
Storage device response to commands
14
File meta-data
• File contain data plus information about the data, that is, meta-data– Meta-data is kept in a file descriptor
15
File Descriptor Information
• External name• Current state• Sharable• Owner• User• Locks• Protection settings• Length• Time of creation• Time of last modification• Time of last access• Reference count• Storage device details
16
File Descriptor in Unix
• File descriptor in UNIX is called an inode (index node), containing the following entries
17
Structured Files
• A file is a stream of bytes
• Usually want to access in a structured manner– May have no structure imposed (UNIX)
• Must be provided by application
– May have a structure imposed (VMS)• Need to maintain additional information
– Type of file
– Access methods
– Other information
18
Block Record Translation
Records
Record-Block Translation
19
Record-Oriented Sequential Files
• A structured sequential file is a named sequence of logical records, indexed by nonnegative integers
• Records may be of fixed size, or variable size– This is determined by file manager
Logical Record
fileID = open(fileName)close(fileID)getRecord(fileID, record)putRecord(fileID, record)seek(fileID, position)
20
Record-Oriented Sequential Files
...H byte header k byte logical record
Logical Record
• Header contains record descriptor information(occupies H bytes)
• Logical record takes up k bytes – fixed size
Next Header
21
Record-Oriented Sequential Files
...H byte header k byte logical record
...
FragmentPhysical Storage Blocks
Logical Record
22
Electronic Mail Examplestruct message {/* The mail message */ address to; address from; line subject; address cc; string body;};struct message *getRecord(void) { struct message *msg; msg = allocate(sizeof(message)); msg->to = getAddress(...); msg->from = getAddress(...); msg->cc = getAddress(...); msg->subject = getLine(); msg->body = getString(); return(msg);}
putRecord(struct message *msg) { putAddress(msg->to); putAddress(msg->from); putAddress(msg->cc); putLine(msg->subject); putString(msg->body);}
23
Record-Oriented Sequential Files
• Fixed size records can be a problem– Applications requiring large record sizes would
require that the programmer break the records into smaller pieces
– Applications only requiring small record sizes would waste space
• A solution is for the file system to be enhanced to include a function to define the record size for a file – encoded in header
24
Indexed Sequential File
• Suppose we want to directly access records
• Add an index to the file
fileID = open(fileName)close(fileID)getRecord(fileID, index)index = putRecord(fileID, record)deleteRecord(fileID, index)
25
Indexed Sequential File (cont)
Account #012345123456294376...529366...965987
Index
ik
j
index = i
index = k
index = j
Application structure
26
More Abstract Files
• Inverted files– System index for each datum in the file– Records accessed based on appearance in table
rather than their logical location• Company accounts may be accessed by customer
name, but customer may have several accounts• Set up external index table by name with pointers to
the main table
• Multimedia storage– Records contain radically different types– Access methods must be general
27
Database Management Systems
• A database is a very highly structured set of information– Stored across different files – Optimized to minimize access time
• DBMSs implementation– Some DBMSs use the normal files provided by
the OS for generic use– Some use their own storage device block
28
File systems
• File system– A data structure on a disk that holds files
• actually a file system is in a disk partition
• a technical term different from a “file system” as the part of the OS that implements files
• File systems in different OSs have different internal structures
29
A file system layout
30
Implementing Low Level Files
• Process needs to be able to read from and write to storage devices
• Simplest system is byte stream file system– (will consider record-oriented systems later)
• Storage device may be accessed 2 ways– Sequentially – like a tape drive– Randomly – like a magnetic disk
31
Low-level File System Architecture
b0 b1 b2 b3 bn-1 … …
Block 0
...
Sequential Device Randomly Accessed Device
32
Low Level Files Management
• Secondary storage device contains:– Volume directory (sometimes a root directory
for a file system)– External file descriptor for each file– The file contents
• Manages blocks– Assigns blocks to files (descriptor keeps track)– Keeps track of available blocks
• Maps to/from byte stream
33
File Manager Data Structures
External File Descriptor
Open FileDescriptor
Copy info from external to the open file descriptor
1
Process-FileSession
Keep the state of the process-file session
2
Return a reference to the data structure
3
34
An open Operation
• Locate the on-device (external) file descriptor
• Extract info needed to read/write file• Authenticate that process can access the file • Create an internal file descriptor in primary
memory• Create an entry in a “per process” open file
status table• Allocate resources, e.g., buffers, to support
file usage
35
A close Operation
• Completes all pending operations
• Release I/O buffers
• Release locks process holds on file
• Update external file descriptor
• Deallocate file status table entry
36
Opening a UNIX File
fid = open(“fileA”, flags);…read(fid, buffer, len);
0 stdin1 stdout2 stderr3 ...
Open File Table
File structure
inode
Internal File Descriptor
On-Device File Descriptor
37
Block Management
• The job of selecting & assigning storage blocks to the file
• For a fixed sized file of k blocks– File of length m requires N = m/k blocks
– Byte bi is stored in block i/k
• The logical file is divided into logical blocks
• Each logical block is mapped to a physical disk block
38
Locating file data
• The file descriptor contains data on how to perform this mapping– there are many methods for performing this
mapping
• Three basic strategies:– Contiguous allocation– Linked lists– Indexed allocation
39
Dividing a file into blocks
40
Disk Organization
Blk0Blk0 Blk1
Blk1 Blkk-1Blkk-1
BlkkBlkk Blkk+1
Blkk+1 Blk2k-1Blk2k-1
Track 0, Cylinder 0
Track 0, Cylinder 1
BlkBlk BlkBlk BlkBlk Track 1, Cylinder 0
BlkBlk BlkBlk BlkBlk Track N-1, Cylinder 0
BlkBlk BlkBlk BlkBlk Track N-1, Cylinder M-1
…
…
…
…
…
…
…
…
Boot Sector Volume Directory
41
Contiguous Allocation
• Maps the N blocks into N contiguous blocks on the secondary storage device– Simple to implement– Random access
• Does not provide for dynamic file sizes– If you want to extend a file, hope there is an empty
block following, or recopy the entire file to a larger group of unallocated contiguous blocks
Head position 237…First block 785Number of blocks 25
File descriptor
42
A contiguous file
43
Keeping a file in pieces
• We need a block pointer for each logical block, an array of block pointers– block mapping indexes into this array– Each file is a linked list of disk blocks
• But where do we keep this array?– usually it is not kept as contiguous array– the array of disk pointers is like a second related
file (that is 1/1024 as big)
44
Block pointers in the file descriptor
45
Block pointers in contiguous disk blocks
46
Linked Lists
• Each block contains a header with– Number of bytes in the block– Pointer to next block
• Blocks need not be contiguous
• Files can expand and contract
• Seeks can be slowFirst block…
Head: 417...
Length
Byte 0
Byte 4095...
Length
Byte 0
Byte 4095...
Length
Byte 0
Byte 4095...
Block 0 Block 1 Block N-1
NULL
47
Linked Lists – cont.
48
Indexed Allocation
• Extract headers and put them in an index
• Simplify seeks
• May link indices together (for large files)
Index block…
Head: 417...
Byte 0
Byte 4095...
Byte 0
Byte 4095...
Byte 0
Byte 4095...
Block 0
Block 1
Block N-1
Length
Length
Length
49
Block pointers in an index block
50
Block pointers in an index block – cont.
51
Chained index blocks
52
Two-level index blocks
53
Two-level index blocks – cont.
primary index
secondary index table data blocks
54
File system layout variations
• New UNIX file systems use cylinder groups (mini-file systems) to achieve better locality of file data
• MS/DOS uses a FAT (file allocation table) file system– so does the Macintosh OS (although the MacOS
layout is different)
UNIX FilesDatamode
owner…Direct block 0Direct block 1…Direct block 11Single indirectDouble indirectTriple indirect
inode
Data
Data
Index
Data
DataIndex
Index
Index
Index
Index
IndexIndex
Index
Data
Data
Data
Data
56
DOS FAT Files
DiskBlock
File Descriptor
DiskBlock
DiskBlock
…43
107254
Logical Linked List
57
DOS FAT Files
DiskBlock
File Descriptor
DiskBlock
DiskBlock
…
File Access Table (FAT)
DiskBlock
DiskBlock
DiskBlock
…
43
43
107
107
10743
254
254
254
File Descriptor
58
Unallocated Blocks
• How should unallocated blocks be managed?
• Need a data structure to keep track of them– Block status map (or disk bitmap)
• Small enough to be held in primary memory
– Linked list (or free list)• Very large
• Hard to manage spatial locality (need to scan list to find blocks ‘close to’ each other)
59
Free-Space Management
• Bit vector (n blocks)
…
0 1 2 n-1
bit[i] = 1 block[i] free
0 block[i] occupied
• First free block number
(number of bits per word) *(number of 0-value words) +offset of first 1 bit
60
Free-Space Management - cont.
• Bit map requires extra space. Example:
block size = 212 bytes
disk size = 230 bytes (1 gigabyte)
n = 230/212 = 218 bits (or 32K bytes)
• Easy to get contiguous files
• Linked list (free list)– Cannot get contiguous space easily– No waste of space
61
Free list organization
62
Free-Space Management – cont.
• Need to protect:– Pointer to free list– Bit map
• Must be kept on disk• Copy in memory and disk may differ.• Cannot allow for block[i] to have a situation where
bit[i] = 0 in memory and bit[i] = 1 on disk.
– Solution:• Set bit[i] = 0 in disk.• Allocate block[i]• Set bit[i] = 0 in memory
63
Marshalling the Byte Stream
• Must read at least one buffer ahead on input
• Must write at least one buffer behind on output
• Seek flushing the current buffer and finding the correct one to load into memory
• Inserting/deleting bytes in the interior of the stream
64
Buffering
• Storage devices use Block I/O• Files place an explicit order on the bytes• Therefore, it is possible to predict what will be
read after bytei
• When file is opened, manager reads as many blocks ahead as feasible
• After a block is logically written, it is queued for writing behind, whenever the disk is available
• Buffer pool – usually variably sized, depending on virtual memory needs– Interaction with the device manager and memory
manager
65
Supporting Other Storage Abstractions
• Low-level file systems avoid encoding record-level functionality– If applications use very large or very small
records, a generic file manager may be efficient– Some operating systems provide a higher-layer
file system to support applications with large or small files
– Database management systems and multimedia documents are examples
66
Other Storage Abstractions
• Modern, open operating systems tend towards low-level file systems
• Proprietary operating systems designed for specific applications implement higher layer files systems
• Structured Sequential Records– Contain collections of logical records– Need to read from or write to entire records
67
Other Storage Abstractions
• Indexed sequential files– File manager keeps table for each open file and
maps index to block containing the record– Consumes space– Read/write operations more complex– Buffering is not of much value (records accessed
in arbitrary order)
• Multimedia– Requires large files and high bandwidth
• Use larger block sizes• Try to use contiguous block allocation
68
Directories
• A directory is a set of logically associated files and other directories of files– Directories are the mechanism we use to organize
files
• The file manager provides a set of commands to manage directories– Traverse a directory– Enumerate a list of all files and nested directories
69
Directories
• Directory commands– enumerate– copy– rename– delete– traverse– etc.
70
Directory Structures
• How should files be organized within directory?– Flat name space
• All files appear in a single directory
– Hierarchical name space• Directory contains files and subdirectories
• Each file/directory appears as an entry in exactly one other directory -- a tree
• Popular variant: All directories form a tree, but a file can have multiple parents.
71
Directory Structures
72
Directory Structures – cont.
73
A directory tree
74
Directory Implementation
• Device Directory– A device can contain a collection of files– Easier to manage if there is a root for every file on
the device -- the device root directory
• File Directory– Typical implementations have directories
implemented as a file with a special format– Entries in a file directory are handles for other
files (which can be files or subdirectories)
75
Directory Implementation – cont.
• Sorted linear list of file names with pointers to the data blocks– simple to program– time-consuming to execute
• Hash Table – linear list with hash data structure– decreases directory search time– collisions – situations where two file names hash
to the same location– fixed size
76
Directory Implementation – cont.
• Physical disk may be divided into two or more logical disks– Bitmap table doesn’t need to be as large– Easier to archive– Can handle several operating systems
• Requires partitioning at device driver level
77
Mounting file systems
• Each file system has a root directory
• We can combine file systems by mounting– that is, link a directory in one file system to the
root directory of another file system
• This allows us to build a single tree out of several file systems
• This can also be done across a network, mounting file systems on other machines
78
Mounting a file system
79
UNIX mount Command
/
bin usr etc foo
bill nutt
abc
bar
blah
cde xyz
80
UNIX mount Command
/
bin usr etc foo
bill nutt
abc
bar
blah
cde xyz
/
bin usr etc foo
bill nutt
abc
bar
blah
cde xyz
mount bar at foo
More on Files
82
File names
• Directory – Maps component names into objects (files or directories)
• Path name– A sequence of component names specifying a path of
directories• absolute path: starts at the root directory
• relative path: starts at the working directory
• File name extension: suffix of a component names that indicate the type of the file
• Alias: alternate path names for a file
83
File name space topologies
84
Some common file extensions
• file.c -- a C program file• file.txt -- a text file• file.s -- an assembly language file• file.obj -- an object file (in MS/DOS)• file.o -- an object file (in UNIX)• file.exe -- an executable file (in MS/DOS)• file.wk1 -- spreadsheet worksheet• file.tex -- tex or latex (a text formatter) file• file.mif -- Framemaker interchange file• file.scm -- Scheme program file• file.tmp -- a temporary file
85
Path name examples
• /home/faculty/egle/os/book/ch02 – UNIX
• /home/student/jdoe/os/proj2 – UNIX
• book/ch02 –UNIX relative path name
• E:\egle\class\os\book\ch02 – MS/DOS
• users:u1:egle:os:book:ch02 – Macintosh
• disk$faculty:[egle.os.book]ch02 – VMS
• [.book]ch02 – VMS relative path name
86
Open Files
• When a file is opened, the file manager keeps additional dynamic information – File position
87
File Operations
• openFile = open(file name)
• openFile = create(file name)
• fileMetaData = status(file name)
• okay = access(file name, access type)
• okay = change mode(file name, new mode)– changes protection information
• okay = change owner(file name,new owner)
88
Open file operations
• bytesRead = read(open file)• bytesWritten = write(open file)• newFilePos = seek(open file, how much,
how) -- move file position• close(open file)• openFile = duplicate(open file)• fileLock(open file)• fileControl(open file)• twoOpenFiles = pipe()
89
Directory operations in UNIX
• link(file name, alias name)
• unlink(file name): delete file
• rename(old name, new name)
• makeDirectory(directory name)
• removeDirectory(directory name)
90
Two major parts of a file system
The file manager needs to map a filename to a collectionof physical blocks on the storage devices
91
File system data structures
92
Flow of control for an open
93
Flow of control for a read
94
Connecting files and devices
95
Special files
• Special files are not ordinary files– e.g. directories, devices, pipes, message queues,
remote files, mounted directories, etc.
• They are marked by flags in the file descriptor
• The read and write operations are directed to code for that type of special file– a case statement determines how to handle a read
or write operation
96
Fork data structure changes
97
System call data structure changes
98
Duplicate data structure changes
99
Pipe data structure changes
100
Avoiding data copies
101
Path name lookup algorithm
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