152 lines
6.0 KiB
Plaintext
152 lines
6.0 KiB
Plaintext
inotify
|
|
a powerful yet simple file change notification system
|
|
|
|
|
|
|
|
Document started 15 Mar 2005 by Robert Love <rml@novell.com>
|
|
|
|
|
|
(i) User Interface
|
|
|
|
Inotify is controlled by a set of three system calls and normal file I/O on a
|
|
returned file descriptor.
|
|
|
|
First step in using inotify is to initialise an inotify instance:
|
|
|
|
int fd = inotify_init ();
|
|
|
|
Each instance is associated with a unique, ordered queue.
|
|
|
|
Change events are managed by "watches". A watch is an (object,mask) pair where
|
|
the object is a file or directory and the mask is a bit mask of one or more
|
|
inotify events that the application wishes to receive. See <linux/inotify.h>
|
|
for valid events. A watch is referenced by a watch descriptor, or wd.
|
|
|
|
Watches are added via a path to the file.
|
|
|
|
Watches on a directory will return events on any files inside of the directory.
|
|
|
|
Adding a watch is simple:
|
|
|
|
int wd = inotify_add_watch (fd, path, mask);
|
|
|
|
Where "fd" is the return value from inotify_init(), path is the path to the
|
|
object to watch, and mask is the watch mask (see <linux/inotify.h>).
|
|
|
|
You can update an existing watch in the same manner, by passing in a new mask.
|
|
|
|
An existing watch is removed via
|
|
|
|
int ret = inotify_rm_watch (fd, wd);
|
|
|
|
Events are provided in the form of an inotify_event structure that is read(2)
|
|
from a given inotify instance. The filename is of dynamic length and follows
|
|
the struct. It is of size len. The filename is padded with null bytes to
|
|
ensure proper alignment. This padding is reflected in len.
|
|
|
|
You can slurp multiple events by passing a large buffer, for example
|
|
|
|
size_t len = read (fd, buf, BUF_LEN);
|
|
|
|
Where "buf" is a pointer to an array of "inotify_event" structures at least
|
|
BUF_LEN bytes in size. The above example will return as many events as are
|
|
available and fit in BUF_LEN.
|
|
|
|
Each inotify instance fd is also select()- and poll()-able.
|
|
|
|
You can find the size of the current event queue via the standard FIONREAD
|
|
ioctl on the fd returned by inotify_init().
|
|
|
|
All watches are destroyed and cleaned up on close.
|
|
|
|
|
|
(ii)
|
|
|
|
Prototypes:
|
|
|
|
int inotify_init (void);
|
|
int inotify_add_watch (int fd, const char *path, __u32 mask);
|
|
int inotify_rm_watch (int fd, __u32 mask);
|
|
|
|
|
|
(iii) Internal Kernel Implementation
|
|
|
|
Each inotify instance is associated with an inotify_device structure.
|
|
|
|
Each watch is associated with an inotify_watch structure. Watches are chained
|
|
off of each associated device and each associated inode.
|
|
|
|
See fs/inotify.c for the locking and lifetime rules.
|
|
|
|
|
|
(iv) 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 preffered 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.
|
|
|