1 .. SPDX-License-Identifier: GPL-2.0
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4 Inotify - A Powerful yet Simple File Change No
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7
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9 Document started 15 Mar 2005 by Robert Love <rm
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11 Document updated 4 Jan 2015 by Zhang Zhen <zhen
12
13 - Deleted obsoleted interface, just re
14
15 (i) Rationale
16
17 Q:
18 What is the design decision behind not tyin
19 the watched object?
20
21 A:
22 Watches are associated with an open inotify
23 This solves the primary problem with dnotif
24 the file and thus, worse, pins the mount.
25 for use on a desktop system with removable
26 unmounted. Watching a file should not requ
27
28 Q:
29 What is the design decision behind using an
30 an fd-per-watch?
31
32 A:
33 An fd-per-watch quickly consumes more file
34 more fd's than are feasible to manage, and
35 select()-able. Yes, root can bump the per-
36 can use epoll, but requiring both is a sill
37 A watch consumes less memory than an open f
38 spaces is thus sensible. The current desig
39 want: Users initialize inotify, once, and a
40 fd and no twiddling with fd limits. Initia
41 thousand times is silly. If we can impleme
42 cleanly--and we can, the idr layer makes st
43 should.
44
45 There are other good arguments. With a sin
46 item to block on, which is mapped to a sing
47 fd returns all watch events and also any po
48 every fd was a separate watch,
49
50 - There would be no way to get event orderi
51 file bar would pop poll() on both fd's, b
52 which happened first. A single queue tri
53 ordering is crucial to existing applicati
54 "mv a b ; mv b a" events without ordering
55
56 - We'd have to maintain n fd's and n intern
57 versus just one. It is a lot messier in
58 queue is the data structure that makes se
59
60 - User-space developers prefer the current
61 example, love it. Trust me, I asked. It
62 to manage and block on 1000 fd's via sele
63
64 - No way to get out of band data.
65
66 - 1024 is still too low. ;-)
67
68 When you talk about designing a file change
69 scales to 1000s of directories, juggling 10
70 the right interface. It is too heavy.
71
72 Additionally, it _is_ possible to more tha
73 juggle more than one queue and thus more th
74 need not be a one-fd-per-process mapping; i
75 process can easily want more than one queue
76
77 Q:
78 Why the system call approach?
79
80 A:
81 The poor user-space interface is the second
82 Signals are a terrible, terrible interface
83 anything, for that matter. The ideal solut
84 file descriptor-based one that allows basic
85 Obtaining the fd and managing the watches c
86 device file or a family of new system calls
87 family of system calls because that is the
88 interfaces. The only real difference was w
89 and ioctl(2) or a couple of new system call
90
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