Hard links and soft links

Learn the difference between hard links and soft links. This is an internal presentation that I gave at Continuum.

Transcript

1. Hard links and soft links Aaron Meurer
2. First understand a little bit about how filesystems work • Every file system is a little different, but the basic ideas are similar. I’ll be describing a very rough version of a (logical) Unix filesystem. • Filesystems are broken up into blocks • Blocks are the smallest unit of data that the filesystem deals with
3. Examples • On Unix, use df
4. • There are two basic types of blocks: • data blocks • inode blocks (metadata) • inode blocks represent files (or directories). • Every inode has a unique inode number • A file inode holds all the metadata for that file, as well as pointers on disk to all the data blocks for that file (one possibility, could also be a linked list)
5. • Directory inodes list all the inode numbers for the files in that directory • Block locations of all inodes are usually organized in an inode table so that they can be found by number
6. File inode example inode 41335 link count: 4 mtime … block list data data data
7. File inode example (cont.) • The inode contains a lot of metadata about the file (mtime, permissions, size) • But note that it does NOT contain the file name or directory location of the file
8. Directory inode example directory inode 41335 type: directory name: mydir pardir: 24312 mtime … inode table file name inode stuff.txt 23351 music.mp3 32154 presentation .pdf 41335 presentation 2.pdf 41335 … …
9. Directory inode example (cont.) • Notice that I have two files with the same inode (presentation.pdf and presentation2.pdf both have inode 41335) • Go back to the original file inode example
10. File inode example inode 41335 link count: 4 mtime … block list
11. Directory inode example (cont.) • The file has a link count of 4, meaning that there are four directory listings somewhere in the filesystem referencing it. • Whenever a file is deleted, it’s inode count is decremented. When the count reaches 0, the data is freed.
12. Hard links • A “hard link” is just a reference to an inode • Every file is a hard link! • When two files are “hard links to each other,” they are the exact same inode (i.e., data) on disk.
13. Hard links (cont.) inode number link count
14. Hard links (cont.) • If we read or write to the files, they will both be the same • Note that some text editors intentionally break hard links
15. Symlinks • A symbolic link (symlink) is a special kind of inode that references a path • Alternately a special entry in a directory listing • Paths can usually be relative or absolute • Symlinks are separate files from the files they point to • Most system calls read through symlinks by default (act as if they were called on the file the symlink points to)
16. A Python analogy Hard links are like this Symlinks are like this
17. A Python analogy (cont.) • Link counts are like reference counts • del a doesn’t delete a, it lowers its reference count. a is deleted when its reference count reaches 0. • Inode numbers are like id(a).