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authorRobert Love <rml@novell.com>2005-07-12 17:06:03 -0400
committerLinus Torvalds <torvalds@g5.osdl.org>2005-07-12 23:38:38 -0400
commit0eeca28300df110bd6ed54b31193c83b87921443 (patch)
tree7db42d8a18d80eca538f5b7d25e0532b8fa38b85 /Documentation/filesystems/inotify.txt
parentbd4c625c061c2a38568d0add3478f59172455159 (diff)
[PATCH] inotify
inotify is intended to correct the deficiencies of dnotify, particularly its inability to scale and its terrible user interface: * dnotify requires the opening of one fd per each directory that you intend to watch. This quickly results in too many open files and pins removable media, preventing unmount. * dnotify is directory-based. You only learn about changes to directories. Sure, a change to a file in a directory affects the directory, but you are then forced to keep a cache of stat structures. * dnotify's interface to user-space is awful. Signals? inotify provides a more usable, simple, powerful solution to file change notification: * inotify's interface is a system call that returns a fd, not SIGIO. You get a single fd, which is select()-able. * inotify has an event that says "the filesystem that the item you were watching is on was unmounted." * inotify can watch directories or files. Inotify is currently used by Beagle (a desktop search infrastructure), Gamin (a FAM replacement), and other projects. See Documentation/filesystems/inotify.txt. Signed-off-by: Robert Love <rml@novell.com> Cc: John McCutchan <ttb@tentacle.dhs.org> Cc: Christoph Hellwig <hch@lst.de> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
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1 inotify
2 a powerful yet simple file change notification system
3
4
5
6Document started 15 Mar 2005 by Robert Love <rml@novell.com>
7
8(i) User Interface
9
10Inotify is controlled by a set of three sys calls
11
12First step in using inotify is to initialise an inotify instance
13
14 int fd = inotify_init ();
15
16Change events are managed by "watches". A watch is an (object,mask) pair where
17the object is a file or directory and the mask is a bit mask of one or more
18inotify events that the application wishes to receive. See <linux/inotify.h>
19for valid events. A watch is referenced by a watch descriptor, or wd.
20
21Watches are added via a path to the file.
22
23Watches on a directory will return events on any files inside of the directory.
24
25Adding a watch is simple,
26
27 int wd = inotify_add_watch (fd, path, mask);
28
29You can add a large number of files via something like
30
31 for each file to watch {
32 int wd = inotify_add_watch (fd, file, mask);
33 }
34
35You can update an existing watch in the same manner, by passing in a new mask.
36
37An existing watch is removed via the INOTIFY_IGNORE ioctl, for example
38
39 inotify_rm_watch (fd, wd);
40
41Events are provided in the form of an inotify_event structure that is read(2)
42from a inotify instance fd. The filename is of dynamic length and follows the
43struct. It is of size len. The filename is padded with null bytes to ensure
44proper alignment. This padding is reflected in len.
45
46You can slurp multiple events by passing a large buffer, for example
47
48 size_t len = read (fd, buf, BUF_LEN);
49
50Will return as many events as are available and fit in BUF_LEN.
51
52each inotify instance fd is also select()- and poll()-able.
53
54You can find the size of the current event queue via the FIONREAD ioctl.
55
56All watches are destroyed and cleaned up on close.
57
58
59(ii) Internal Kernel Implementation
60
61Each open inotify instance is associated with an inotify_device structure.
62
63Each watch is associated with an inotify_watch structure. Watches are chained
64off of each associated device and each associated inode.
65
66See fs/inotify.c for the locking and lifetime rules.
67
68
69(iii) Rationale
70
71Q: What is the design decision behind not tying the watch to the open fd of
72 the watched object?
73
74A: Watches are associated with an open inotify device, not an open file.
75 This solves the primary problem with dnotify: keeping the file open pins
76 the file and thus, worse, pins the mount. Dnotify is therefore infeasible
77 for use on a desktop system with removable media as the media cannot be
78 unmounted.
79
80Q: What is the design decision behind using an-fd-per-device as opposed to
81 an fd-per-watch?
82
83A: An fd-per-watch quickly consumes more file descriptors than are allowed,
84 more fd's than are feasible to manage, and more fd's than are optimally
85 select()-able. Yes, root can bump the per-process fd limit and yes, users
86 can use epoll, but requiring both is a silly and extraneous requirement.
87 A watch consumes less memory than an open file, separating the number
88 spaces is thus sensible. The current design is what user-space developers
89 want: Users initialize inotify, once, and add n watches, requiring but one fd
90 and no twiddling with fd limits. Initializing an inotify instance two
91 thousand times is silly. If we can implement user-space's preferences
92 cleanly--and we can, the idr layer makes stuff like this trivial--then we
93 should.
94
95 There are other good arguments. With a single fd, there is a single
96 item to block on, which is mapped to a single queue of events. The single
97 fd returns all watch events and also any potential out-of-band data. If
98 every fd was a separate watch,
99
100 - There would be no way to get event ordering. Events on file foo and
101 file bar would pop poll() on both fd's, but there would be no way to tell
102 which happened first. A single queue trivially gives you ordering. Such
103 ordering is crucial to existing applications such as Beagle. Imagine
104 "mv a b ; mv b a" events without ordering.
105
106 - We'd have to maintain n fd's and n internal queues with state,
107 versus just one. It is a lot messier in the kernel. A single, linear
108 queue is the data structure that makes sense.
109
110 - User-space developers prefer the current API. The Beagle guys, for
111 example, love it. Trust me, I asked. It is not a surprise: Who'd want
112 to manage and block on 1000 fd's via select?
113
114 - You'd have to manage the fd's, as an example: Call close() when you
115 received a delete event.
116
117 - No way to get out of band data.
118
119 - 1024 is still too low. ;-)
120
121 When you talk about designing a file change notification system that
122 scales to 1000s of directories, juggling 1000s of fd's just does not seem
123 the right interface. It is too heavy.
124
125Q: Why the system call approach?
126
127A: The poor user-space interface is the second biggest problem with dnotify.
128 Signals are a terrible, terrible interface for file notification. Or for
129 anything, for that matter. The ideal solution, from all perspectives, is a
130 file descriptor-based one that allows basic file I/O and poll/select.
131 Obtaining the fd and managing the watches could have been done either via a
132 device file or a family of new system calls. We decided to implement a
133 family of system calls because that is the preffered approach for new kernel
134 features and it means our user interface requirements.
135
136 Additionally, it _is_ possible to more than one instance and
137 juggle more than one queue and thus more than one associated fd.
138