File System Mounting

1
File System Mounting

• A file system must be mounted before it can be
  accessed.
• A unmounted file system is mounted at a mount
  point within the directory structure.
• UNIX
• All files are arranged in one big tree rooted at
  /.
• Automatic mounting at boot time according to a
  system configuration file containing a list of
  devices and mount points.
• Mounts can be done manually using the mount
  command, which attaches the file system on some
  device to the directory tree.
• Windows
• Devices and partitions assigned a drive letter.
• OS automatically discover all devices and mount
  all located file systems at boot time.

2
File System Mounting Example
Partition mounted over /users
(a) An existing file system. (b) An unmounted
partition.
3
File Sharing on Multi-User Systems

• Add owner and group attributes to files
• Owner has full control over the file
• Group defines a subset of users who may share
  access to the file
• Owner defines which operations can be executed by
  group members and other users
• Implementation
• Each user have a user name and a user ID
• Each group have a group name and a group ID
• A user can be in multiple groups
• Owner ID and group ID of a given file are stored
  with other file attributes
• When a user requests an operation on a file, the
  user ID and groups ID(s) will be compared to the
  owner ID and group ID of the file to determine
  whether the operation is allowed.

4
File Sharing on Distributed Systems

• In distributed systems, files may be shared
  across a network via
• Manual transfer (FTP)
• Distributed file system (DFS)
• World Wide Web (WWW)
• DFS remote directories visible from local
  machine
• A host can mount a file system from a remote host
• The host containing the files is called server
• The host wanting access to the files is called
  client
• Once a remote file system is mounted, the client
  can access the files as if they are local.

5
Consistency Semantics

• Consistency semantics specifies the semantics of
  multiple users accessing a shared file
  simultaneously
• File session the series of accesses between the
  open and close operations
• UNIX semantics
• Writes to an open file by a user are visible
  immediately to other users that have this file
  open at the same time.
• One mode of sharing allows users to share the
  current-file-position pointer of a file. If one
  user advances the pointer, all users are
  affected.
• Session semantics
• Writes to an open file by a user are not visible
  immediately to other users that have the same
  file open simultaneously.
• Once a file is closed, the changes made to it are
  visible only in sessions starting later.
• Used in Andrew File System (an influential DFS)

6
Protection

• Goal keep files safe from improper access
• File owner should be able to control
• What can be done
• By whom
• Types of access that may be controlled
• Read
• Write
• Execute
• Append
• Delete
• List (file name and attributes)

7
Access-Control List (ACL)

• Make access dependent on the identity of the user
• Each file/directory is associated with an
  access-control list (contained in directory
  entry) specifying user name and allowed access
• Drawbacks of ACL
• Long length
• Variable size directory entry

8
Protection in UNIX

• Three classifications of users for each file
• Owner the user who created the file
• Can be changed with chown command by superuser
• Group a set of users who need similar access to
  the file
• Created by superuser
• Can be changed with chgrp command by superuser
• Universe all other users
• Can specify read, write, execute permissions for
  owner, group, and universe
• RWX
• owner access 6 ? 1 1 0 group access 4
  ? 1 0 0
• universe access 0 ? 0 0 0
• chmod command is used to change file access
  permissions
• E.g. chmod 640 paper.doc

9
Protection in UNIX

• Protection bits for directories
• r a user can list the content of the directory
  if set
• w a user can create or delete files in the
  directory if set
• x a user can change into the directory if set
• A sample directory listing
• drwxr----- 1 ruan cs352 4096 May 10
  1002 HWSolution
• -rw-r----- 1 ruan cs352 4259 Sep 12
  1505 sample-code.c
• -rw-r--r-- 1 ruan cs352 4300 Oct 3
  1732 homepage.html