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.