Operating System:
Chap10 File System Interface
National Tsing-Hua University
2016, Fall Semester
Chapter10 FS Interface Operating System Concepts – NTHU LSA Lab 2
Overview
File Concept
Access Methods
Directory Structure
File System Mounting
File Sharing
Protection
File Concept
File : a logical storage unit created by OS
v.s. physical storage unit in disk ( sector, track )
File attributes
Identifier: non-human-readable name
Name
Type
Location
Size
sector track
platter
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File Operations
File operations include
Creating a file
Writing a file
Reading a file
Repositioning within a file (i.e. file seek)
Deleting a file
Truncating a file
Process: open-file table
OS: system-wide table
Open-File Tables
Per-process table
Tracking all files opened by this process
Current file pointer for each opened file
Access rights and accounting information
System-wide table
Each entry in the per-process table points to this table
Process-independent information such as disk location, access dates, file size
Open count
files
P1 PCB Open-file table a.txt
b.txt
System open-file table
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Open File Attributes
Open-file attributes (metadata)
File pointer (per-process)
File open count (system table)
Disk location (system table)
Access rights (per-process)
File types
.exe, .com, .obj, .cc, .mov, etc
Hint for OS to operate file in a reasonable way
Access Method
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Access Methods
Sequential access
Read/write next (block)
Reset: repositioning the file pointer to the beginning
Skip/rewind n records
Access Methods
Direct (relative) access
Access an element at an arbitrary position in a sequence
File operations include the block # as parameter
Often use random access to refer the access pattern from direct access
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Index Access Methods
Index: contains pointers to blocks of a file
To find a record in a file:
search the index file find the pointer
use the pointer to directly access the record
With a large file index could become too large
Review Slides ( I )
File vs. Sector, Track
Open-file (in-memory) attributes
Per-process, system-wide?
File-access methods?
Sequential access
Direct access
Index access
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Directory Structure
Partition, Volume & Directory
A partition (formatted or raw)
raw partition (no file system): UNIX swap space, database
Formatted partition with file system is called volume
a partition can be a portion of a disk or group of multiple disks (distributed file system)
Some storage devices (e.g.: floppy disk) does not and cannot have partition
Directories are used by file system to store the
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File-System Organization
Directory vs. File
Directory: A collection of nodes containing information about all files
Both the directory structure and the files reside on disk
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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
Single-Level Directory
All files in one directory
Filename has to be unique
Poor efficiency in locating a file as number of files
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Two-Level Directory
a separate dir for each user
path = user name + file name
single-level dir problems still exists per user
Tree-Structured Directory
Absolute path : starting from the root
Relative path : starting from a directory
current
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Acyclic-Graph Directory
Use links to share files or directories
UNIX-like: symbolic link (ln -s /spell/count /dict/count)
A file can have multiple absolute paths
When does a file actually get deleted?
deleting the link but not the file
deleting the file but leaves the link dangling pointer
deleting the file when reference counters is 0
General-Graph Directory
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General-Graph Directory
May contain cycles
Reference count does not work any more
E.g. self-referencing file
How can we deal with cycles?
Garbage collection
First pass traverses the entire graph and marks accessible files or directories
Second pass collect and free everything that is un-marked
Poor performance on millions of files …
Use cycle-detection algorithm when a link is created
/
home bin
g1 g2
t1
Review Slides ( II )
Directory structure: pros & cons
One-level directory
Two-level directory
Tree-structured directory
Acyclic-graph directory
General-graph directory
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File-System Mounting &
File Sharing
File System Mounting
A file system must be mounted before it can be accessed
Mount point : the root path that a FS will be mounted to
Mount timing:
boot time
automatically at run-time
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File System Mounting Example
/
local usr home
jane john
games
/home/john/games Partition A
Partition C Partition B
Root Info local
m usr
P1
Root Info students m home
P2
Root Info P3
\
usr local
…...
…
…...
…
students home
…...
…
P3
m usr m home
P2
m home
…...
… students home
…...
…
Mount Table
usr P2
P3 home
Linux command: mount -t type device dir mount –t ext2 /dev/sda0 /mnt mnt
m mnt
P4 mnt
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File Sharing on Multiple Users
Each user: (userID, groupID)
ID is associated with every ops/process/thread the user issues
Each file has 3 sets of attributes
owner, group, others
Owner attributes describe the privileges for the owner of the file
same for group/others attributes
group/others attributes are set by owner or root
group1
group2
group3
Owner Other users
Group user
Access-Control List
We can create an access-control list (ACL) for each user
check requested file access against ACL
problem: unlimited # of users
3 classes of users 3 ACL ( RWX ) for each file
owner (e.g. 7 = RWX = 111)
group (e.g. 6 = RWX = 110)
public (others) (e.g. 4 = RWX = 100) chmod 664 intro.ps
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File Protection
File owner/creator should be able to control
what can be done
by whom
Access control list (ACL)
Files should be kept from
physical damage (reliability): i.e. RAID
improper access (protection): i.e. password
Review Slides ( III )
File system mounting point, timing?
Access-control list? How does it function?
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Reading Material & HW
Chap 10
Problems
10.1: Consider a file system where a file can be deleted and its disk space reclaimed while links to that file still exist.
What problems may occur if a new file is created in the same storage area or with the same absolute path name?
How can these problems be avoided?
10.4: Provide examples of applications that typically access files according to “sequential” and “random”.
10.6: If the operating system knew that a certain application was going to access file data in a sequential manner, how could it exploit this information to improve performance?
Consistency Semantics
When files are shared, ops from different users to the same file must be synchronized
UNIX semantics
write is visible to all other users opening the same file
Open-file option: share the same file pointer
Session semantics (AFS file system)
write is not visible to all other users
once a file is closed, changes are visible for sessions
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File Sharing on Remote File Systems
Uses networking to allow file system access between systems
Manually via programs like FTP
Semi automatically via the world wide web
Automatically, seamlessly using distributed
file systems
Client-server model
Allows clients to mount remote file systems from servers
Sever : the machine that owns the files and serves multiple clients
Client: the machine that accesses remote files
Standard OS file calls are translated into remote calls
Client and user-on-client identification is insecure or complicated
Example:
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Distributed Information Systems
Distributed naming services
Provide unified access to the info for remote computing
Clusters/
Datacenters Frontend nodes Client side
Server side DIS:
DNS (Domain Name System) NIS (Network Info Service)
Failure Modes
Failures:
HW: disk, network cable, switch, server, etc.
SW: corruption or inconsistency of file, directory structure, etc.
We need to recover:
Data: files, directory contents
Metadata: data and system management info.
Stateful vs. Stateless communication protocol:
Stateless: treats each request as an independent transaction
