Files in Unix

1
Files in Unix

• From the System's View

2
File Handling Data Structures

• Process Table
• File Table
• V-Node Table
• inodes

3
Process Table
Process A
File Descriptor Flags
File Table Entry
Process B
Pointer to File Table Entry
Process C
Process D
4
File Table
File 1
File Status Flags Read, Write, Etc.
File 2
Current File Offset
File 3
Pointer to V-node Table Entry
File 4
File Table for Open Files
V-node Table
5
V-Node Table Entry
File Type
File Table Entry
Pointers to Functions for this File
Inode Information
Disk Drive
V-node Table Entry
6
File Types

• Regular files
• Directory files
• Character special files
• Block special files
• FIFOs
• Sockets
• Symbolic Links

7
Special File Protection Bits

• Group Locking
• Set group/User ID on execution
• Sticky Bit (saved-text-bit)

8
Disk Drives and File Systems
Partition
Partition
Partition
File System
i-list
directory blocks and data blocks
Boot Block
Super Block
inode
inode
inode
inode
inode
9
File Systems Expanded
B
S
i-list
data
data
data
directory
directory
data
data
data
inode inode inode
inode File Name inode File Name inode
File Name
10
Direction and Indirection
Disk Blocks
inode
Direct 0 Direct 1 Direct 2 Direct 3 Direct 4
Direct 5 Direct 6 Direct 7 Direct 8 Direct 9
Sgl Ind Dbl Ind Triple Ind
11
inodes

• inodes contain a lot of information about a file
• mode and type of file
• number of links to the file
• owner's UID
• owners GID
• number of bytes in file
• times (last accessed, modified, inode changed)
• physical disk addresses (direct and indirect
  blocks)
• number of blocks
• access information

12
In-core inodes

• When a file is being used, it's inode is copied
  to memory (core) and additional fields are added
• access status
• node is locked
• process is waiting for node to unlock
• in-core inode has been modified
• file has changed so inode has also changed
• Number of processes using inode and file
• Device ID and disk address where inode is located
• Current file position

13
RFS (Remote File System)

• An ATT invention
• Independent of any particular network hardware or
  software
• Designed for transparent sharing of disk storage
  and peripherals
• RFS mounts a remote directory structure under an
  existing structure in the user's filesystem
• Remote files are then accessed as though they are
  part of the local file system
• RFS also supports special files and named pipes

14

• The server keeps track of how many remote users
  have the file open at a given time
• Server also maintain security by distinguishing
  between local and remote opens
• Remote access can be restricted to the privileges
  of selected local accounts

15
NFS (Network File System)

• Developed by Sun Microsystems
• Design goal was portability to other OS's and
  architectures
• Uses RPCs (Remote Procedure Calls) for protocol
  definition and remote system access
• Uses the External Data Representation (XDR) to
  define data in a machine independent manner
• Furthers machine independence by defining a
  Virtual File System (VFS) that defines operations
  that can be performed on a file

16

• NFS fileservers are stateless and do not track
  the state of files or provide any standard Unix
  file locking capabilities
• Conflicts among users of the same file can arise
• Sun has provided a locking scheme by maintaining
  client requested locks in memory only
• Both client and server keep locks in memory for
  consistency
• However, because of the tight coupling between
  client and server, loss of a server can cause
  data consistency problems

17

• NFS does not allow shared root filesystems
• In concept, shared directories like /dev, /bin,
  and /etc should reside on a common file server
• This simplifies the design but the failure of the
  fileserver then impacts all attached nodes
• The user or sysadmin must know what directories
  should be mounted at each network node
• The number of required mounts can take quite a
  while to perform on system reboot
• Since files are shared in place (on the server)
  there are no revision conflicts

18

• The stateless nature of NFS (except for the
  locking kludge) ensures consistency of the system
  after recovery from a crash