--- /dev/null
+ inotify
+ a powerful yet simple file change notification system
+
+
+
+Document started 15 Mar 2005 by Robert Love <rml@novell.com>
+Document updated 4 Jan 2015 by Zhang Zhen <zhenzhang.zhang@huawei.com>
+ --Deleted obsoleted interface, just refer to manpages for user interface.
+
+(i) 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. Watching a file should not require that it be open.
+
+Q: What is the design decision behind using an-fd-per-instance 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?
+
+ - 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.
+
+ Additionally, it _is_ possible to more than one instance and
+ juggle more than one queue and thus more than one associated fd. There
+ need not be a one-fd-per-process mapping; it is one-fd-per-queue and a
+ process can easily want more than one queue.
+
+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 preferred approach for new kernel
+ interfaces. The only real difference was whether we wanted to use open(2)
+ and ioctl(2) or a couple of new system calls. System calls beat ioctls.
+
Code Review / kvmfornfv.git / blobdiff