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1 .. SPDX-License-Identifier: GPL-2.0
4 Inotify - A Powerful yet Simple File Change Notification System
9 Document started 15 Mar 2005 by Robert Love <rml@novell.com>
11 Document updated 4 Jan 2015 by Zhang Zhen <zhenzhang.zhang@huawei.com>
13 	- Deleted obsoleted interface, just refer to manpages for user interface.
22    Watches are associated with an open inotify device, not an open file.
23    This solves the primary problem with dnotify: keeping the file open pins
24    the file and thus, worse, pins the mount.  Dnotify is therefore infeasible
26    unmounted.  Watching a file should not require that it be open.
29    What is the design decision behind using an-fd-per-instance as opposed to
30    an fd-per-watch?
33    An fd-per-watch quickly consumes more file descriptors than are allowed,
35    select()-able.  Yes, root can bump the per-process fd limit and yes, users
37    A watch consumes less memory than an open file, separating the number
38    spaces is thus sensible.  The current design is what user-space developers
41    thousand times is silly.  If we can implement user-space's preferences
42    cleanly--and we can, the idr layer makes stuff like this trivial--then we
47    fd returns all watch events and also any potential out-of-band data.  If
50    - There would be no way to get event ordering.  Events on file foo and
51      file bar would pop poll() on both fd's, but there would be no way to tell
56    - We'd have to maintain n fd's and n internal queues with state,
60    - User-space developers prefer the current API.  The Beagle guys, for
64    - No way to get out of band data.
66    - 1024 is still too low.  ;-)
68    When you talk about designing a file change notification system that
74    need not be a one-fd-per-process mapping; it is one-fd-per-queue and a
81    The poor user-space interface is the second biggest problem with dnotify.
82    Signals are a terrible, terrible interface for file notification.  Or for
84    file descriptor-based one that allows basic file I/O and poll/select.
86    device file or a family of new system calls.  We decided to implement a