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| -rw-r--r-- | Documentation/filesystems/inotify.txt | 138 |
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diff --git a/Documentation/filesystems/inotify.txt b/Documentation/filesystems/inotify.txt new file mode 100644 index 00000000000..2c716041f57 --- /dev/null +++ b/Documentation/filesystems/inotify.txt @@ -0,0 +1,138 @@ + inotify + a powerful yet simple file change notification system + + + +Document started 15 Mar 2005 by Robert Love <rml@novell.com> + +(i) User Interface + +Inotify is controlled by a set of three sys calls + +First step in using inotify is to initialise an inotify instance + + int fd = inotify_init (); + +Change events are managed by "watches". A watch is an (object,mask) pair where +the object is a file or directory and the mask is a bit mask of one or more +inotify events that the application wishes to receive. See <linux/inotify.h> +for valid events. A watch is referenced by a watch descriptor, or wd. + +Watches are added via a path to the file. + +Watches on a directory will return events on any files inside of the directory. + +Adding a watch is simple, + + int wd = inotify_add_watch (fd, path, mask); + +You can add a large number of files via something like + + for each file to watch { + int wd = inotify_add_watch (fd, file, mask); + } + +You can update an existing watch in the same manner, by passing in a new mask. + +An existing watch is removed via the INOTIFY_IGNORE ioctl, for example + + inotify_rm_watch (fd, wd); + +Events are provided in the form of an inotify_event structure that is read(2) +from a inotify instance fd. The filename is of dynamic length and follows the +struct. It is of size len. The filename is padded with null bytes to ensure +proper alignment. This padding is reflected in len. + +You can slurp multiple events by passing a large buffer, for example + + size_t len = read (fd, buf, BUF_LEN); + +Will return as many events as are available and fit in BUF_LEN. + +each inotify instance fd is also select()- and poll()-able. + +You can find the size of the current event queue via the FIONREAD ioctl. + +All watches are destroyed and cleaned up on close. + + +(ii) Internal Kernel Implementation + +Each open inotify instance is associated with an inotify_device structure. + +Each watch is associated with an inotify_watch structure. Watches are chained +off of each associated device and each associated inode. + +See fs/inotify.c for the locking and lifetime rules. + + +(iii) Rationale + +Q: What is the design decision behind not tying the watch to the open fd of + the watched object? + +A: Watches are associated with an open inotify device, not an open file. + This solves the primary problem with dnotify: keeping the file open pins + the file and thus, worse, pins the mount. Dnotify is therefore infeasible + for use on a desktop system with removable media as the media cannot be + unmounted. + +Q: What is the design decision behind using an-fd-per-device as opposed to + an fd-per-watch? + +A: An fd-per-watch quickly consumes more file descriptors than are allowed, + more fd's than are feasible to manage, and more fd's than are optimally + select()-able. Yes, root can bump the per-process fd limit and yes, users + can use epoll, but requiring both is a silly and extraneous requirement. + A watch consumes less memory than an open file, separating the number + spaces is thus sensible. The current design is what user-space developers + want: Users initialize inotify, once, and add n watches, requiring but one fd + and no twiddling with fd limits. Initializing an inotify instance two + thousand times is silly. If we can implement user-space's preferences + cleanly--and we can, the idr layer makes stuff like this trivial--then we + should. + + There are other good arguments. With a single fd, there is a single + item to block on, which is mapped to a single queue of events. The single + fd returns all watch events and also any potential out-of-band data. If + every fd was a separate watch, + + - There would be no way to get event ordering. Events on file foo and + file bar would pop poll() on both fd's, but there would be no way to tell + which happened first. A single queue trivially gives you ordering. Such + ordering is crucial to existing applications such as Beagle. Imagine + "mv a b ; mv b a" events without ordering. + + - We'd have to maintain n fd's and n internal queues with state, + versus just one. It is a lot messier in the kernel. A single, linear + queue is the data structure that makes sense. + + - User-space developers prefer the current API. The Beagle guys, for + example, love it. Trust me, I asked. It is not a surprise: Who'd want + to manage and block on 1000 fd's via select? + + - You'd have to manage the fd's, as an example: Call close() when you + received a delete event. + + - No way to get out of band data. + + - 1024 is still too low. ;-) + + When you talk about designing a file change notification system that + scales to 1000s of directories, juggling 1000s of fd's just does not seem + the right interface. It is too heavy. + +Q: Why the system call approach? + +A: The poor user-space interface is the second biggest problem with dnotify. + Signals are a terrible, terrible interface for file notification. Or for + anything, for that matter. The ideal solution, from all perspectives, is a + file descriptor-based one that allows basic file I/O and poll/select. + Obtaining the fd and managing the watches could have been done either via a + device file or a family of new system calls. We decided to implement a + family of system calls because that is the preffered approach for new kernel + features and it means our user interface requirements. + + Additionally, it _is_ possible to more than one instance and + juggle more than one queue and thus more than one associated fd. + |