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Lecture 12: File Management

Lecture 12: File Management. File Management. provides the file abstraction for data storage guarantees data validity (most of the time) performance: throughput and response time enforces protection. File-System Interface. File Concept Access Methods Directory Structure

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Lecture 12: File Management

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  1. Lecture 12: File Management

  2. File Management • provides the file abstraction for data storage • guarantees data validity (most of the time) • performance: throughput and response time • enforces protection

  3. File-System Interface • File Concept • Access Methods • Directory Structure • File-System Mounting • File Sharing • Protection

  4. File Concept • Contiguous logical address space • Persistent - stored on non-volatile memory (storage) • Identified by (external) name • pathname in UNIX

  5. File Structure • None - sequence of words, bytes • Simple record structure • Lines • Fixed length • Variable length • Complex Structures • Formatted document • Relocatable load file • Can simulate last two with first method by inserting appropriate control characters • Who decides: • Operating system • Program

  6. File Attributes • Name – only information kept in human-readable form • Identifier – unique tag (number) identifies file within file system • Type – needed for systems that support different types • Location – pointer to file location on device • Size – current file size • Protection – controls who can do reading, writing, executing • Time, date, and user identification – data for protection, security, and usage monitoring • Information about files are kept in the directory structure, which is maintained on the disk

  7. File Operations • Create • Write • Read • Reposition within file • Delete • Truncate • Open(Fi) –open read/write session for Fi • File pointer: pointer to last read/write location, • Close (Fi) – close read/write session

  8. Access Methods • Sequential Access read next write next reset no read after last write (rewrite) • Direct Access read n write n position to n read next write next rewrite n n = relative block number

  9. Directories • Set of files • Used to organize files • Translates external file names to internal file names (file labels, i-nodes, file control blocks) • Treated as a file but with special operations

  10. Operations Performed on Directory • Search for a file • Create a file • Delete a file • List a directory • Rename a file • Traverse the file system

  11. Organize the Directory (Logically) to Obtain • Efficiency – locating a file quickly • Naming – convenient to users • Two users can have same name for different files • The same file can have several different names • Grouping – logical grouping of files by properties, (e.g., all Java programs, all games, …)

  12. Single-Level Directory • A single directory for all users Naming problem Grouping problem

  13. Two-Level Directory • Separate directory for each user • Path name • Can have the same file name for different user • Efficient searching • No grouping capability

  14. Tree-Structured Directories

  15. Tree-Structured Directories (Cont) • Efficient searching • Grouping Capability • Current directory (working directory) • cd /spell/mail/prog • type list

  16. Tree-Structured Directories (Cont) • Absolute or relative path name • Creating a new file is done in current directory • Delete a file rm <file-name> • Creating a new subdirectory is done in current directory mkdir <dir-name> Example: if in current directory /mail mkdir count mail prog copy prt exp count Deleting “mail”  deleting the entire subtree rooted by “mail”

  17. Acyclic-Graph Directories • Have shared subdirectories and files

  18. Acyclic-Graph Directories (Cont.) • Two different names (aliasing) • If dict deletes list dangling pointer Solutions: • Backpointers, so we can delete all pointersVariable size records a problem • Backpointers using a daisy chain organization • Entry-hold-count solution • New directory entry type • Link – another name (pointer) to an existing file • Resolve the link – follow pointer to locate the file

  19. File System Mounting • A file system must be mounted before it can be accessed • A file system is mounted at a mount point (a directory)

  20. (a) Existing. (b) Unmounted Partition

  21. Mount Point

  22. File Sharing • Sharing of files on multi-user systems is desirable • Sharing may be done through a protection scheme • On distributed systems, files may be shared across a network • Network File System (NFS) is a common distributed file-sharing method

  23. File Sharing – Multiple Users • User IDs identify users, allowing permissions and protections to be per-user • Group IDs allow users to be in groups, permitting group access rights

  24. File Sharing – Remote File Systems • Uses networking to allow file system access between systems • Manually via programs like FTP • Automatically, seamlessly using distributed file systems • Semi automatically via theworld wide web • Client-server model allows clients to mount remote file systems from servers • Server can serve multiple clients • Client and user-on-client identification is insecure or complicated • NFS is standard UNIX client-server file sharing protocol • CIFS is standard Windows protocol • Standard operating system file calls are translated into remote calls

  25. File Sharing – Failure Modes • Remote file systems add new failure modes, due to network failure, server failure • Recovery from failure can involve state information about status of each remote request • Stateless protocols such as NFS include all information in each request, allowing easy recovery but less security

  26. File Sharing – Consistency Semantics • Consistency semantics specify how multiple users are to access a shared file simultaneously • Similar to Ch 7 process synchronization algorithms • Tend to be less complex due to disk I/O and network latency (for remote file systems • Andrew File System (AFS) implemented complex remote file sharing semantics • Unix file system (UFS) implements: • Writes to an open file visible immediately to other users of the same open file • Sharing file pointer to allow multiple users to read and write concurrently • AFS has session semantics • Writes only visible to sessions starting after the file is closed

  27. Protection • File owner/creator should be able to control: • what can be done • by whom • Types of access • Read • Write • Execute • Append • Delete • List

  28. Access Lists and Groups • Mode of access: read, write, execute • Three classes of users RWX a) owner access 7  1 1 1 RWX b) group access 6  1 1 0 RWX c) public access 1  0 0 1 • Ask manager to create a group (unique name), say G, and add some users to the group. • For a particular file (say game) or subdirectory, define an appropriate access. owner group public chmod 761 game Attach a group to a file chgrp G game

  29. Protection Mechanisms • files are OS objects: unique names and a finite set of operations that processes can perform on them • protection domain is a set of {object,rights}, where right is the permission to perform one of the operations • at every instant in time, each process runs in some protection domain • in Unix, a protection domain is {uid, gid} • protection domain in Unix is switched when running a program with SETUID/SETGID set or when the process enters the kernel mode by issuing a system call • how to store all the protection domains ?

  30. Protection Mechanisms (cont’d) • Access Control List (ACL): associate with each object a list of all the protection domains that may access the object and how • in Unix ACL is reduced to three protection domains: owner, group and others • Capability List (C-list): associate with each process a list of objects that may be accessed along with the operations • C-list implementation issues: where/how to store them (hardware, kernel, encrypted in user space) and how to revoke them

  31. A Sample UNIX Directory Listing

  32. Secondary Storage Management • Space must be allocated to files • Must keep track of the space available for allocation

  33. Preallocation • Need the maximum size for the file at the time of creation • Difficult to reliably estimate the maximum potential size of the file • Tend to overestimated file size to avoid running out of space

  34. Methods of File Allocation (1) • Contiguous allocation • Single set of blocks is allocated to a file at the time of creation • A single entry in the file allocation table • Starting block and length of the file • External fragmentation will occur

  35. Methods of File Allocation (2) • Chained allocation • Allocation on basis of individual block • Each block contains a pointer to the next block in the chain • A single entry in the file allocation table • Starting block and length of file • No external fragmentation • Best for sequential files • No accommodation for the principle of locality

  36. Methods of File Allocation (3) • Indexed allocation • File allocation table contains a separate one-level index for each file • The index has one entry for each portion allocated to the file • The file allocation table contains block number for the index

  37. File Allocation • contiguous: a contiguous set of blocks is allocated to a file at the time of file creation • good for sequential files • file size must be known at the time of file creation • external fragmentation • chained allocation: each block contains a pointer to the next one in the chain • consolidation to improve locality • indexed allocation: good both for sequential and direct access (UNIX)

  38. Free Space Management • bitmap: one bit for each block on the disk • good to find a contiguous group of free blocks • small enough to be kept in memory • chained free portions: {pointer to the next one, length} • index: treats free space as a file

  39. UNIX File System • Naming • External/Internal names translation using directories • Lookup • File blocks  Disk blocks • Protection • Free Space Management

  40. File Naming • External names (used by the application) • Pathname: /usr/users/file1 • Internal names (used by the OS kernel) • I-node: file number/index on disk File system on disk 0 1 superblock I-node area ( one I-node per file) File-block area

  41. Directories • Files that store translation tables (external names to internal names) usr usr users usr 23 users 41 file1 87 Root directory (always I-node 2) /usr/users/file1 corresponds to I-node 87

  42. File Content Lookup • address table used to translate logical file blocks into disk blocks • address table stored in the I-node 45 65 address table 85 0 1 2 File with i-node 87 File System disk 45 65 85

  43. File Protection • ACL with three protection domains (file owner, file owner group, others) • Access rights: read/write/execute • Stored in the i-node

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