79 lines
3.8 KiB
Text
79 lines
3.8 KiB
Text
inotify
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a powerful yet simple file change notification system
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Document started 15 Mar 2005 by Robert Love <rml@novell.com>
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Document updated 4 Jan 2015 by Zhang Zhen <zhenzhang.zhang@huawei.com>
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--Deleted obsoleted interface, just refer to manpages for user interface.
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(i) Rationale
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Q: What is the design decision behind not tying the watch to the open fd of
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the watched object?
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A: Watches are associated with an open inotify device, not an open file.
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This solves the primary problem with dnotify: keeping the file open pins
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the file and thus, worse, pins the mount. Dnotify is therefore infeasible
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for use on a desktop system with removable media as the media cannot be
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unmounted. Watching a file should not require that it be open.
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Q: What is the design decision behind using an-fd-per-instance as opposed to
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an fd-per-watch?
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A: An fd-per-watch quickly consumes more file descriptors than are allowed,
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more fd's than are feasible to manage, and more fd's than are optimally
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select()-able. Yes, root can bump the per-process fd limit and yes, users
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can use epoll, but requiring both is a silly and extraneous requirement.
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A watch consumes less memory than an open file, separating the number
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spaces is thus sensible. The current design is what user-space developers
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want: Users initialize inotify, once, and add n watches, requiring but one
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fd and no twiddling with fd limits. Initializing an inotify instance two
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thousand times is silly. If we can implement user-space's preferences
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cleanly--and we can, the idr layer makes stuff like this trivial--then we
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should.
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There are other good arguments. With a single fd, there is a single
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item to block on, which is mapped to a single queue of events. The single
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fd returns all watch events and also any potential out-of-band data. If
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every fd was a separate watch,
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- There would be no way to get event ordering. Events on file foo and
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file bar would pop poll() on both fd's, but there would be no way to tell
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which happened first. A single queue trivially gives you ordering. Such
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ordering is crucial to existing applications such as Beagle. Imagine
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"mv a b ; mv b a" events without ordering.
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- We'd have to maintain n fd's and n internal queues with state,
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versus just one. It is a lot messier in the kernel. A single, linear
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queue is the data structure that makes sense.
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- User-space developers prefer the current API. The Beagle guys, for
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example, love it. Trust me, I asked. It is not a surprise: Who'd want
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to manage and block on 1000 fd's via select?
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- No way to get out of band data.
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- 1024 is still too low. ;-)
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When you talk about designing a file change notification system that
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scales to 1000s of directories, juggling 1000s of fd's just does not seem
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the right interface. It is too heavy.
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Additionally, it _is_ possible to more than one instance and
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juggle more than one queue and thus more than one associated fd. There
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need not be a one-fd-per-process mapping; it is one-fd-per-queue and a
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process can easily want more than one queue.
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Q: Why the system call approach?
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A: The poor user-space interface is the second biggest problem with dnotify.
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Signals are a terrible, terrible interface for file notification. Or for
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anything, for that matter. The ideal solution, from all perspectives, is a
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file descriptor-based one that allows basic file I/O and poll/select.
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Obtaining the fd and managing the watches could have been done either via a
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device file or a family of new system calls. We decided to implement a
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family of system calls because that is the preferred approach for new kernel
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interfaces. The only real difference was whether we wanted to use open(2)
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and ioctl(2) or a couple of new system calls. System calls beat ioctls.
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