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Files in Unix

From the System's View

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File Handling Data Structures

Process Table File Table V-Node Table inodes

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Process Table

Process A

Process B

Process C

Process D

File DescriptorFlags

Pointer toFile Table Entry

FileTableEntry

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File Table

File Status FlagsRead, Write, Etc.

Current File Offset

Pointer to V-node Table Entry

File 1

File 2

File 3

File 4

File Table forOpen Files

V-node Table

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V-Node Table Entry

FileTableEntry

File Type

Pointers to Functionsfor this File

Inode Information

V-node TableEntry

Disk Drive

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File Types

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

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Special File Protection Bits

Group Locking

Set group/User ID on execution

Sticky Bit (saved-text-bit)

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Disk Drives and File Systems

Partition Partition Partition

File System

inode inode inode inode inode

i-list directory blocks and data blocks

BootBlock

SuperBlock

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Layout: Boot area: Bootstrap code Superblock: Attributes and metadata of file system Inode list: one inode/file 64 bytes, fix the size of file

system Data area: files, directories and indirect blocks which

hold pointers to other file data blocks Directories:

File containing list of files and subdirectories Fixed record of 16 bytes

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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

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File Systems Expanded

B S i-list data data data directorydirectory datadata data

inode inode inode

inode # File Name

inode # File Name

inode # File Name

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2 bytes ( 216= 65535 files) inode number, 14 bytes file name

0 inode number means file no longer exist Root directory and parent have inode number equal to 2

73 .

38 ..

9 File1

0 Deleted file

110 Subdirectory1

65 File2

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Inodes ( Index nodes): Each file has one unique inode Inode contains metadata of the file on-disk inode and in-core inode

Field Size Description

di_mode 2 File type, permissions

di_uid 2 Owner UID

di_gid 2 Owner GID

di_size 4 Size in bytes

di_addr 39 Array of block addresses

: : :

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di_addr: File is not stored in contiguous blocks, prevents

fragmentation An array of block address is required, Stored in inode,

prevent extra read Size of array depends on the size of file

012345

76

89

101112

indirectDouble indirect

triple indirect

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Direction and Indirection

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

DiskBlocks

inode

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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

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RFS (Remote File System)

An AT&T 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

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System V File System (s5fs)

Single logical disk or partition, one FS per partition Each FS has own root, sub directories, Files, data and

metadata Disk Block = 512 * n, granularity of disk allocation for a file Translated by disk drivers in to track sectors and cylinders

B S Inode list Data blocks

Boot area superblock

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Superblock Metadata about File system One Superblock per File system Kernel reads Superblock when mounting the File

system Superblock contains following information

Size in blocks of the file system Size in blocks of the inode list Number of free blocks and inodes Free block list (Partial) Free inode list (Full)

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Kernel Organization

In-Core Inodes Represented by struct inode All fields of on-disk inode and following extra fields

vnode: contains the vnode of the file Device ID of the partition containing the file Inode number of the file Flags for synchronization and cache management Pointers to keep the inode on a free list Pointers to keep the inode on a hash queue Block number of last block read

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Inode Lookup Lookuppn(), a file system independent function performs

pathname parsing When searching s5fs directory it translates to a call to

s5lookup() s5lookup first checks directory name lookup cache On miss it reads the directory one block at a time If directory contains a valid filename entry, s5lookup()

obtains inode number of file iget() is called to locate inode iget() searches the appropriate hash table to get the

inode

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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

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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

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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

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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

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