12: Filesystems: The User View

Transcription

1 12: Filesystems: The User View Mark Handley Goals for Long-term Information Storage 1. Store large amounts of data. 2. Information stored must survive the termination of the process using it and reboot or crash of OS. 3. Multiple processes must be able to access the information concurrently. 4. Aid the user in retrieving their data. 5. Protect each user s data from other users. 1

2 Outline of Upcoming Lectures User s view of filesystems. File naming, directories, file operations. System view of filesystems. How filesystems are built internally. Specific examples of filesystems. File Naming Users want to name their files so they can find them again. What are the restrictions to file naming? The system wants to store metadata about files. What metadata? Where does it store it? Who uses it? 2

3 File Name Extensions File Naming MS-DOS/Windows US US ASCII charset case-insentitive name/extension extension used used by by operating system Unix/Linux US US ASCII charset case case sensitive character names inc inc extension extension optional Win Win 98/2K/XP, MacOS X HFS+ Unicode case case insensitive long long names + extension extension used used by by operating system MacOS classic unicode charset case case insensitive character names separate resource fork fork for for extensive metadata 3

4 File Structure Three kinds of files: (a) Simple sequence of bytes (b) Sequence of file records (c) Tree with indexed records File Access Sequential access Read all bytes/records from the beginning Cannot jump around, could rewind or back up Convenient when medium was mag tape Random access Bytes/records read in any order Essential for data base systems Read can be move file marker (seek), then read or read and then move file marker 4

5 File Structure Unix, Windows Sequential files of bytes. Sequential read access. Random access using seek() system call. Essentially the same interface to disk and tape. Nothing fancy, but lets you implement almost any structure on top yourself. File Structure Vax VMS/OpenVMS Files can be sequential, relative, or indexed. Access can be sequential or random Records can be fixed size, variable size, stream of bytes, or variable with fixed control. sequential access mode random by relative record number random by key value random by record file address file organisation sequential relative indexed yes yes yes yes yes no no no yes yes yes yes 5

6 File Types Windows: Unix: Regular Files + Directories everything is a file - very elegant uniform interface. Regular Files Directories Character Special Files Eg: terminals, printers, network, sound devices cat britney.wav > /dev/audio Block Special Files Eg: disks Symbolic Link File Attributes: Typical Metadata 6

7 Example File Attributes: BSD FFS File Attributes File Permissions (9 bits): Read/Write/Execute for User/Group/Other SetUID: set User ID on execute bit SetGID: set Group ID on execute bit Sticky bit - only allow a user to delete their own file. Hard link reference count Time of last access, modification, or file status change Flags: archived, opaque, nodump, sappnd, uappnd system/user append-only schg, uchg system/user immutable sunlnk, uunlnk system/user undeletable File Operations (Unix, etc) 1. Create 2. Delete 3. Open 4. Close 5. Read 6. Write 7. Append 8. Seek 9. Get attributes 10.Set Attributes 11.Rename 7

8 Example: Copying a File #include <fcntl.h> #include <stdlib.h> #include <unistd.h> #define BUF_SIZE #define OUTPUT_MODE int int main(int argc, argc, char char *argv[]) {{ int int in_fd, in_fd, out_fd; int int rd_size = 1, 1, wr_size; char char buf[buf_size]; if if (argc (argc!=!= 3) 3) exit(1); in_fd in_fd = open(argv[1], O_RDONLY); if if (in_fd (in_fd < 0) 0) exit(2); }} out_fd out_fd = creat(argv[2], OUTPUT_MODE); if if (out_fd < 0) 0) exit(3); while while (rd_size > 0) 0) {{ rd_size = read(in_fd, buf, buf, BUF_SIZE); if if (rd_size < 0) 0) exit(4); wr_size = write(out_fd, buf, buf, rd_size); if if (wr_size <= <= 0) 0) exit(5); }} close(in_fd); close(out_fd); Memory-Mapped Files (a) Segmented process before mapping files into its address space (b) Process after: mapping existing file abc into one segment creating a new segment for new file xyz 8

9 Example: Copying a Memory-mapped File #include <fcntl.h> #include <stdlib.h> #include <unistd.h> #include <sys/stat.h> #include <sys/mman.h> #define OUTPUT_MODE int int main(int argc, argc, char char *argv[]) {{ int int in_fd, in_fd, out_fd, size; size; struct struct stat stat statbuf; char char *buf; *buf; if if (argc (argc!=!= 3) 3) exit(1); in_fd in_fd = open(argv[1], O_RDONLY); if if (in_fd (in_fd < 0) 0) exit(2); }} fstat(in_fd, &statbuf); size size = statbuf.st_size; out_fd out_fd = creat(argv[2],output_mode); if if (out_fd < 0) 0) exit(3); buf buf = mmap(0, size, size, PROT_READ, MAP_SHARED, in_fd, in_fd, 0); 0); write(out_fd, buf, buf, size); size); munmap(buf, size); size); close(in_fd); close(out_fd); Directories Single-Level Directory Systems A single level directory system Contains 4 files Owned by 3 different people, A, B, and C 9

10 Two-level Directory Systems Letters indicate owners of the directories and files Hierarchical Directory Systems 10

11 A Unix Directory Tree File Paths and Naming Windows: \home\mjh\mail Unix: /home/mjh/mail MULTICS: >home>mjh>mail MacOS Classic: :home:mjh:mail Absolute Names: /home/mjh/mail Relative Names: Mail when in dir: /home/mjh../mail when in dir: /home/mjh/foobar 11

12 Directory Operations 1. Create 2. Delete 3. Opendir 4. Closedir 5. Readdir 6. Rename 7. Link 8. Unlink 12