Kernel Korner - Intro to inotify
One of the biggest issues with dnotify (aside from the signals and basically everything else) is that a dnotify watch on a directory requires that said directory remain open. Consequently, watching a directory on, say, a USB keychain drive prevents the drive from unmounting. inotify solves this problem by not requiring that any file be open.
inotify takes this one step further, though, and sends out the IN_UNMOUNT event when the filesystem on which a file resides is unmounted. It also automatically destroys the watch and cleanup.
Move events are complicated because inotify may be watching the directory that the file is moved to or from, but not the other. Because of this, it is not always possible to alert the user of the source and destination of a file involved in a move. inotify is able to alert the application to both only if the application is watching both directories.
In that case, inotify emits an IN_MOVED_FROM from the watch descriptor of the source directory, and it emits an IN_MOVED_TO from the watch descriptor of the destination directory. If watching only one or the other, only the one event will be sent.
To tie together two disparate moved to/from events, inotify sets the cookie field in the inotify_event structure to a unique nonzero value. Two events with matching cookies are thus related, one showing the source and one showing the destination of the move.
The size of the pending event queue can be obtained via FIONREAD:
unsigned int queue_len; int ret; ret = ioctl (fd, FIONREAD, &queue_len); if (ret < 0) perror ("ioctl"); else printf ("%u bytes pending in queue\n", queue_len);
This is useful to implement throttling: reading from the queue only when the number of events has grown sufficiently large.
inotify is configurable via procfs and sysctl.
/proc/sys/filesystem/inotify/max_queued_events is the maximum number of events that can be queued at once. If the queue reaches this size, new events are dropped, but the IN_Q_OVERFLOW event is always sent. With a significantly large queue, overflows are rare even if watching many objects. The default value is 16,384 events per queue.
/proc/sys/filesystem/inotify/max_user_instances is the maximum number of inotify instances that a given user can instantiate. The default value is 128 instances, per user.
/proc/sys/filesystem/inotify/max_user_watches is the maximum number of watches per instance. The default value is 8,192 watches, per instance.
These knobs exist because kernel memory is a precious resource. Although any user can read these files, only the system administrator can write to them.
inotify is a simple yet powerful file change notification system with an intuitive user interface, excellent performance, support for many different events and numerous features. inotify is currently in use in various projects, including Beagle, an advanced desktop indexing system, and Gamin, a FAM replacement.
What application will use inotify next?
Resources for this article: /article/8534.
Robert Love is a senior kernel hacker in Novell's Ximian Desktop group and the author of Linux Kernel Development (SAMS 2005), now in its second edition. He holds degrees in CS and Mathematics from the University of Florida. Robert lives in Cambridge, Massachusetts.
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