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Files in Unix

Files in Unix. From the System's View. File Handling Data Structures. Process Table File Table V-Node Table inodes. Process Table. Process A. File Descriptor Flags. File Table Entry. Process B. Pointer to File Table Entry. Process C. Process D. File Table. File 1.

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Files in Unix

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  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. 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

  10. 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

  11. File Systems Expanded B S i-list data data directory data directory data data data inode inode inode inode # File Name inode # File Name inode # File Name

  12. 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

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

  14. 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 0 1 2 3 4 5 6 7 8 9 10 indirect 11 Double indirect 12 triple indirect

  15. Direction and Indirection inode Disk Blocks 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

  16. 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

  17. 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

  18. System V File System (s5fs) B S Inode list Data blocks Boot area superblock • 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

  19. 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)

  20. 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 • Inodenumber 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

  21. 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

  22. 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

  23. 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

  24. 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

  25. 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

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

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