Filesystem - Operating System Design and Implementation - Lecture Slides, Slides of Operating Systems

Download Filesystem - Operating System Design and Implementation - Lecture Slides and more Slides Operating Systems in PDF only on Docsity! 15-410, F’13 1 1   File System (Interface) Nov. 13, 2013 Dave Eckhardt & Todd Mowry Contributions from n  Rahul Iyer L31_Filesystem 15-410 “...RADIX-50??...” 15-410, F’13 2 2   Synchronization Today n  Chapter 10, File system interface n  Ok to skip: remote/distributed (10.5.2!!) Also read Chapter 13 n  Might help demystify readline() some 15-410, F’13 5 5   “Extended” file attributes BSD Unix n  archived n  nodump n  append-only (by user/by operating system) n  immutable (by user/by operating system) MacOS n  icon color Plan 9 n  Identity of most recent mutator 15-410, F’13 6 6   Operations on Files Create – locate space (maybe), enter into directory Write, Read – often via position pointer/“cursor” Seek – adjust position pointer for next access Delete – remove from directory, release space (maybe) Truncate n  Trim some data from end of file (common case: all data) Append – write at end of file (implicit synchronization) Rename n  Change name of file inside a directory n  Move a file between two directories (maybe) 15-410, F’13 7 7   I/O to a File – Take 1 Users will read/write files n  Not being able to defies the point in having them So, how do you read from and write to one? n  read(“README.dox”, input_buffer, num_bytes); n  What's the problem with this? 15-410, F’13 10 10   Open files (Unix Model) “In-core” file state – avoid going to disk repeatedly n  Mirror of on-disk structure n  File number, size, permissions, modification time, ... n  Housekeeping info n  Back pointer to enclosing file system n  Pointer to disk device hosting the file n  Who holds locks on ranges of file n  How to access file (vector of methods) n  Pointer to file's type-specific data Shared when file is opened multiple times 15-410, F’13 11 11   Open files (Unix Model) “Open file” state – result of one open() call n  Results are retained for use by multiple I/O calls n  Pointer to underlying “open file” n  Credentials of process (when it opened the file) n  Access mode (read vs. write, auto-append, ...) n  Cursor position Shared by multiple processes n  “copied” by fork() n  inherited across exec() 15-410, F’13 12 12   Example int fd1, fd2, fd3; off_t pos2, pos3; char buf[10]; fd1 = open(“foo.c”, O_RDONLY, 0); fd2 = dup(fd1); fd3 = open(“foo.c”, O_RDONLY, 0); read(fd1, &buf, sizeof (buf)); pos2 = lseek(fd2, 0L, SEEK_CUR); /* ⇒? */ pos3 = lseek(fd3, 0L, SEEK_CUR); /* ⇒? */ 15-410, F’13 15 15   File types (or not) Goal n  Avoid printing a binary executable file n  Find program which “understands” a file selected by user Derive “type” from file names n  *.exe are executable, *.c are C Tag file with type information (extended attributes) n  MacOS: 4-byte type, 4-byte creator Unix: Both/neither n  Leave it (mostly) up to users (maybe: GUI, libraries, etc.) 15-410, F’13 16 16   File Structure What's in a file? n  Stream of bytes? n  What character set? US-ASCII? Latin-1? Unicode? n  Stream of records? n  Array of records? Tree of records? Record structure? n  End of “line” n  CR, LF, CR+LF n  Fixed-length? Varying? Bounded? 15-410, F’13 17 17   File Structure - Unix Program loader needs to know about executables n  “Magic numbers” in first two bytes n  obsolete A.OUT types - OMAGIC, NMAGIC, ZMAGIC n  ELF n  #! - script Otherwise, array of bytes n  User/application remembers meaning (hopefully!) For a good time... n  Try the “file” command n  Read /usr/share/misc/magic or /usr/share/file/magic n  Marvel at the dedication of the masses – 16,000 lines! 15-410, F’13 20 20   Access Methods – Indexed File contains records Records contain keys Index maps keys ⇒ records n  Sort data portion by key n  Binary search in multi-level list Fancy extensions n  Multiple keys, multiple indices n  Are we having a database yet? n  Missing: relations, triggers, consistency, transactions, ... n  Unix equivalent: dbm/ndbm/gdbm/bdb/... 15-410, F’13 21 21   Directory Operations Lookup(“index.html”) Create(“index.html”) Delete(“index.html”) Rename(“index.html”, “index.html~”); Iterate over directory contents Scan file system n  Unix “find” command n  Backup program “Log changes to directory tree ____” 15-410, F’13 22 22   Directory Types Single-level n  Flat global namespace – only one test.c n  Ok for floppy disks (maybe) Two-level n  Every user has a directory n  One test.c per user n  [1003,221]PROFILE.CMD vs. [1207,438]PROFILE.CMD n  Typical of early timesharing Are we having fun yet? 15-410, F’13 25 25   Tree Directories Directories are special files n  Created with special system calls – mkdir() n  Format understood & maintained by OS Current directory (“.”) n  “Where I am now” (e.g., /usr/zzz) n  Start of relative pathname n  ./stuff/foo.c or stuff/foo.c ⇒ /usr/zzz/stuff/foo.c n  ../joe/foo.c ⇒ /usr/joe/foo.c n  Directory reference in, e.g., p->p_fd->fd_cdir 15-410, F’13 26 26   DAG Directories Share files and directories between users Not mine, not yours: ours Destroy when everybody deletes Unix “hard link” n  Remove an open file? n  Stays “alive” until last close n  Files, not directories n  (“.. problem”) usr mji / paper.ms owens 15-410, F’13 27 27   Soft links Hard links “too hard”? n  Need a level of indirection in file system? n  Want “one true name” for a file? n  Need to cross to a different file system of a different type? Alternative: soft link / symbolic link / “short cut” n  Tiny file, special type n  Contains name of another file n  OS dereferences link when you open() it n  Link can point to a file anywhere n  A file in a different type of file system n  A remote file 15-410, F’13 30 30   Mounting Multiple disks on machine Multiple partitions on disk File system within a partition n  Or, within a volume / logical volume / ... How to name files in “another” file system? n  Wrong way n  C:\temp vs. D:\temp n  [1003,221]PROFILE.CMD vs. [1207,438]PROFILE.CMD 15-410, F’13 31 31   Mounting mji owens / dae jdi / usr0 usr1 / 15-410, F’13 32 32   Multiple Users Users want to share files What's a user? n  Strings can be cumbersome n  Integers are nicer for OS to compare n  Unix: User ID / “uid” n  Windows: Security ID / “SID” What's a group? n  A set of users n  Typically has its own gid / SID 15-410, F’13 35 35   Protection – typical File specifies owner, group n  Permissions for owner, permissions for group members n  Read, write, ... n  Permissions for “other” / “world” n  Read, write, ... Unix n  r, w, x = 4, 2, 1 n  r w x r – x - — x = 0751 (octal) n  V7 Unix: 3 16-bit words specified all permission info n  permission bits, user #, group # n  As of 2013-03-25, Andrew supports ~33,963 users... 16 bits is a little tight. 15-410, F’13 36 36   Summary File n  Abstraction of disk/tape storage n  Records, not sectors n  Type information n  Naming n  Complexity due to linking n  Ownership, permissions n  Semantics of multiple open()s Extra details in 20.7, 20.8 15-410, F’13 37 37   The “.. Problem” Foo Bar $ ln .. bar