TDDI12:
Operating Systems, Real-time and
Concurrent programming
Agenda
[SGG7] Chapter 10 and 11
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File-System Interface and
Implementation
Explain the function of file systems
Describe interfaces to file systems
Discuss file system design and protection
Describe implementation of local and remote file systems
Discuss block allocation algorithms
File-System Interface
+ File Concept
+ Access Methods
+ Directory Structure
+ File-System Mounting
+ File Sharing
+ Protection
File-System Implementation
+ File-System Structure
+ File-System Implementation
+ Directory Implementation
+ Allocation Methods
+ Free-Space Management
+ Efficiency and Performance
+ Recovery
+ Log-Structured File Systems
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Copyright Notice: The lecture notes are mainly based on Silberschatz’s, Galvin’s and Gagne’s book (“Operating System
Concepts”, 7th ed., Wiley, 2005). No part of the lecture notes may be reproduced in any form, due to the copyrights
reserved by Addison-Wesley. These lecture notes should only be used for internal teaching purposes at the Linköping
University.
Andrzej Bednarski, IDA
Linköpings universitet, 2006
File Concept
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File Attributes
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None - sequence of words, bytes
Simple record structure
+ Lines
+ Fixed length
+ Variable length
Complex Structures
+ Formatted document
+ Relocatable load file
Can simulate last two with first method by inserting appropriate
control characters.
Who decides:
+ Operating system
+ Program
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File Operations
Name – only information kept in human-readable form.
Type – needed for systems that support different types.
Location – pointer to file location on device.
Size – current file size.
Protection – controls who can do reading, writing, executing.
Time, date, and user identification – data for protection, security, and
usage monitoring.
Information about files are kept in the directory structure, which
is maintained on the disk.
TDDI12, A. Bednarski, IDA, Linköpings universitet
Silberschatz, Galvin and Gagne ©2005
File Structure
Contiguous logical address space
Types:
+ Data
̶ numeric
̶ character
̶ binary
+ Program
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create
write
read
reposition within file – file seek
delete
truncate
open(Fi) – search the directory structure on disk for entry Fi, and
move the content of entry to memory.
close (Fi) – move the content of entry Fi in memory to directory
structure on disk.
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1
Open Files
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File Types – Name, Extension
Several pieces of data are needed to manage open files:
+ File pointer: pointer to last read/write location, per process
that has the file open
+ File-open count: counter of number of times a file is open – to
allow removal of data from open-file table when last processes
closes it
+ Disk location of the file: cache of data access information
+ Access rights: per-process access mode information
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Access Methods
TDDI12, A. Bednarski, IDA, Linköpings universitet
Directory
Direct Access
read n
write n
position to n
read next
write next
rewrite n
Files
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Information in a Device Directory
Name
Type
Address
Current length
Maximum length
Date last accessed (for archival)
Date last updated (for dump)
Owner ID (who pays)
Protection information (discuss later)
TDDI12, A. Bednarski, IDA, Linköpings universitet
F1
F2
F3
F4
Fn
n = relative block number
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A collection of nodes containing information about all files.
Sequential Access
read next
write next
reset
no read after last write
(rewrite)
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Silberschatz, Galvin and Gagne ©2005
Directory Structure
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9.8
9.11
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Operations Performed on Directory
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Both the directory structure and the files reside on disk.
Backups of these two structures are kept on tapes.
Search for a file
Create a file
Delete a file
List a directory
Rename a file
Traverse the file system
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Organize the Directory (Logically)
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Single-Level Directory
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Efficiency – locating a file quickly
Naming – convenient to users
+ Two users can have same name for different files
+ The same file can have several different names
Grouping – logical grouping of files by properties,
(e.g., all Java programs, all games, …)
A single directory for all users
Naming problem
Grouping problem
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Two-Level Directory
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Tree-Structured Directories
Separate directory for each user
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Path name
Can have the same file name for different user
Efficient searching
No grouping capability
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Tree-Structured Directories (Cont.)
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Efficient searching
Grouping Capability
Current directory (working directory)
+ cd /spell/mail/prog
+ type list
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Tree-Structured Directories (Cont.)
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Absolute or relative path name
Creating a new file is done in current directory.
Delete a file
rm <file-name>
Creating a new subdirectory is done in current directory.
mkdir <dir-name>
Example: if in current directory /spell/mail
mkdir count
mail
prog
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copy prt exp count
Deleting “mail” ⇒ deleting the entire subtree rooted by “mail”.
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Acyclic-Graph Directories
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Acyclic-Graph Directories (Cont.)
Have shared subdirectories and files
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Two different names (aliasing)
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If dict deletes list ⇒ dangling pointer
Solutions:
+ Backpointers, so we can delete all pointers
+ Backpointers using a daisy chain organization
+ Entry-hold-count solution
New directory entry type
+ Link – another name (pointer) to an existing file
+ Resolve the link – follow pointer to locate the file
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General Graph Directory
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File System Mounting
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How do we guarantee no cycles?
+ Allow only links to file not subdirectories
+ Garbage collection
+ Every time a new link is added use a cycle detection
algorithm to determine whether it is OK
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File Sharing
A file system must be mounted before it can be used
Mounting
+ Provide device name to OS
+ Provide mount point
+ OS verifies validity of file system (device directory)
+ OS updates its directory structure
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General Graph Directory (Cont.)
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Sharing of files on multi-user systems is desirable
+ (Control version software: CVS, SVN, CADESE, …)
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Sharing may be done through a protection scheme
+ User IDs identify users, allowing permissions and
protections to be per-user
+ Group IDs allow users to be in groups,
permitting group access rights
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On distributed systems, files may be shared across a network
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File Sharing – Remote File Systems
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Uses networking to allow file system access between systems
+ Manually via programs like FTP
+ Automatically, seamlessly using distributed file systems
+ Semi automatically via the world wide web
Client-server model allows clients to mount remote file systems
from servers
+ Server can serve multiple clients
+ Client and user-on-client identification is insecure or
complicated
+ NFS is standard UNIX client-server file sharing protocol
+ CIFS is standard Windows protocol
+ Standard operating system file calls are translated into remote
calls
Distributed Information Systems (distributed naming services) such as
LDAP, DNS, NIS, Active Directory implement unified access to
information needed for remote computing
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Protection
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File Sharing – Consistency Semantics
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Consistency semantics specify how multiple users are to access a
shared file simultaneously
+ Similar to process synchronization algorithms
+ Unix file system (UFS) implements:
̶ Writes to an open file visible immediately to other users of
the same open file
̶ Sharing file pointer to allow multiple users to read and
write concurrently
+ AFS has session semantics
̶ Writes only visible to sessions starting after the file is
closed
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Access Lists and Groups
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File owner/creator should be able to control:
+ what can be done
+ by whom
Types of access
+ Read
+ Write
+ Execute
+ Append
+ Delete
+ List
Mode of access: read, write, execute
Three classes of users
RWX
111
RWX
b) groups access
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⇒
110
RWX
c) public access
1
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001
Ask manager to create a group (unique name), say G,
and add some users to the group.
For a particular file (say game) or subdirectory, define
an appropriate access:
a) owner access
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⇒
owner
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File-System Implementation
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chmod 761
public
game
Silberschatz, Galvin and Gagne ©2005
File-System Structure
File-System Structure
File-System Implementation
Directory Implementation
Allocation Methods
Free-Space Management
Efficiency and Performance
Recovery
Log-Structured File Systems
TDDI12, A. Bednarski, IDA, Linköpings universitet
Attach a group to a file
$ chgrp G game
group
• File structure
+ Logical storage unit
+ Collection of related information
• File system resides on secondary storage (disks)
• File system organized into layers
• File control block
storage structure consisting of information about a file
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In-Memory File System Structures
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Allocation Methods
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Directory Implementation
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Linear list of file names with pointer to the data blocks.
+ simple to program
+ time-consuming to execute
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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
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Contiguous Allocation
An allocation method refers to
how disk blocks are allocated for files:
• Each file occupies a set of contiguous blocks on the disk.
• Simple – only starting location (block #) and length
(number of blocks) are required.
• Random access.
• Wasteful of space (dynamic storage-allocation problem).
• Files cannot grow.
• Mapping from logical to physical.
+ Contiguous allocation
+ Linked allocation
+ Indexed allocation
Q
LA/512
R
+ Block to be accessed = Q + starting address
+ Displacement into block = R
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Contiguous Allocation of Disk Space
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Linked Allocation
• Each file is a linked list of disk blocks: blocks may be scattered
anywhere on the disk.
Block
=
Pointer
Data
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Linked Allocation (Cont.)
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Indexed Allocation
Simple – need only starting address
Free-space management system – no waste of space
No random access
File-allocation table (FAT)
Mapping
disk-space allocation used by
Q
MS-DOS and OS/2.
LA/511
R
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+ Block to be accessed is the Qth block
in the linked chain of blocks
representing the file.
+ Displacement into block = R + 1
TDDI12, A. Bednarski, IDA, Linköpings universitet
Brings all pointers together into the index block.
Logical view.
index table
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Example of Indexed Allocation
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Indexed Allocation (Cont.)
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Need index table
Random access
Dynamic access without external fragmentation, but have
overhead of index block.
Mapping from logical to physical in a file of maximum size
of 256K words and block size of 512 words. We need only
1 block for index table.
Q
LA/512
R
Q = displacement into index table
R = displacement into block
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Indexed Allocation – Mapping (Cont.)
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Mapping from logical to physical in a file of unbounded length
(block size of 512 words).
Linked scheme – link blocks of index table (no limit on size).
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Indexed Allocation – Mapping (Cont.)
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Two-level index (maximum file size is 5123)
Q1
LA / (512 x 512)
R1
Q1
LA / (512 x 511)
R1
Q1 = block of index table
R1 is used as follows:
Q1 = displacement into outer-index
R1 is used as follows:
Q2
R1 / 512
R2
Q2 = displacement into block of index table
R2 displacement into block of file:
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Q2
R1 / 512
R2
Q2 = displacement into block of index table
R2 displacement into block of file:
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Combined Scheme: UNIX (4K bytes per block)
Free-Space Management
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Bit vector (n blocks)
0 1 2
n-1
bit[i] =
678
…
0 ⇒ block[i] free
1 ⇒ block[i] occupied
Block number calculation
(number of bits per word) *
(number of 0-value words) +
offset of first 1 bit
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Free-Space Management (Cont.)
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Free-Space Management (Cont.)
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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
Grouping
Counting
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TDDI12, A. Bednarski, IDA, Linköpings universitet
Silberschatz, Galvin and Gagne ©2005
Efficiency and Performance
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 ] = 1 in memory and bit[i ] = 0 on disk
+ Solution:
̶ Set bit[i ] = 1 in disk
̶ Allocate block[i ]
̶ Set bit[i ] = 1 in memory
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Page Cache
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• Efficiency dependent on:
+ Disk allocation and directory algorithms
+ Types of data kept in file’s directory entry
• Performance
+ Disk cache
separate section of main memory for frequently used blocks
+ Free-behind and read-ahead
techniques to optimize sequential access
+ Improve PC performance by dedicating section of memory as
virtual disk, or RAM disk.
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A page cache caches pages rather than disk blocks using virtual
memory techniques
Memory-mapped I/O uses a page cache
Routine I/O through the file system uses the buffer (disk) cache
A unified buffer cache uses the same page cache to cache both
memory-mapped pages and ordinary file system I/O
I/O using a unified buffer cache
I/O without a unified buffer cache
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Redundant Arrays of Independent Disks
(RAID)
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RAID – multiple disk drives provides
reliability via redundancy.
RAID is arranged into six different levels.
Several improvements in
disk-use techniques involve the use
of multiple disks working cooperatively.
Disk striping uses a group of disks
as one storage unit.
RAID schemes improve performance
and improve reliability of
the storage system by
storing redundant data.
+ Mirroring or shadowing
keeps duplicate of each disk.
+ Block interleaved parity
uses much less redundancy.
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Log Structured File Systems
Log structured (or journaling) file systems record each update to
the file system as a transaction
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All transactions are written to a log
+ A transaction is committed once it is written to the log
+ However, the file system may not yet be updated
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Transactions in the log are asynchronously written
to the file system
+ When the file system is modified,
the transaction is removed from the log
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If the file system crashes, all remaining transactions in the log must
still be performed
9.51
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Consistency checking – compares data in directory structure with
data blocks on disk, and tries to fix inconsistencies
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Use system programs to back up data from disk to another
storage device (floppy disk, magnetic tape, magnetic disk, optical)
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Recover lost file or disk by restoring data from backup
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Recommended Reading and Exercises
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TDDI12, A. Bednarski, IDA, Linköpings universitet
Recovery
Silberschatz, Galvin and Gagne ©2005
Exercise (Memory Management)
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Reading:
+ [SGG7] Chapter 10 and 11
+ Chapter 11 and 12 (sixth edition)
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Exercises:
+ All
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Moving-head Disk Mechanism
For each of the following decimal addresses,
compute the virtual page number and the offset
for a 4KB (K Byte) and 8KB page:
20000, 32768, and 60000.
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