diff options
Diffstat (limited to 'Documentation/filesystems')
| -rw-r--r-- | Documentation/filesystems/inotify.txt | 151 | ||||
| -rw-r--r-- | Documentation/filesystems/ntfs.txt | 29 |
2 files changed, 178 insertions, 2 deletions
diff --git a/Documentation/filesystems/inotify.txt b/Documentation/filesystems/inotify.txt new file mode 100644 index 000000000000..6d501903f68e --- /dev/null +++ b/Documentation/filesystems/inotify.txt @@ -0,0 +1,151 @@ + 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. + diff --git a/Documentation/filesystems/ntfs.txt b/Documentation/filesystems/ntfs.txt index f89b440fad1d..eef4aca0c753 100644 --- a/Documentation/filesystems/ntfs.txt +++ b/Documentation/filesystems/ntfs.txt @@ -21,7 +21,7 @@ Overview ======== Linux-NTFS comes with a number of user-space programs known as ntfsprogs. -These include mkntfs, a full-featured ntfs file system format utility, +These include mkntfs, a full-featured ntfs filesystem format utility, ntfsundelete used for recovering files that were unintentionally deleted from an NTFS volume and ntfsresize which is used to resize an NTFS partition. See the web site for more information. @@ -149,7 +149,14 @@ case_sensitive=<BOOL> If case_sensitive is specified, treat all file names as name, if it exists. If case_sensitive, you will need to provide the correct case of the short file name. -errors=opt What to do when critical file system errors are found. +disable_sparse=<BOOL> If disable_sparse is specified, creation of sparse + regions, i.e. holes, inside files is disabled for the + volume (for the duration of this mount only). By + default, creation of sparse regions is enabled, which + is consistent with the behaviour of traditional Unix + filesystems. + +errors=opt What to do when critical filesystem errors are found. Following values can be used for "opt": continue: DEFAULT, try to clean-up as much as possible, e.g. marking a corrupt inode as @@ -432,6 +439,24 @@ ChangeLog Note, a technical ChangeLog aimed at kernel hackers is in fs/ntfs/ChangeLog. +2.1.23: + - Stamp the user space journal, aka transaction log, aka $UsnJrnl, if + it is present and active thus telling Windows and applications using + the transaction log that changes can have happened on the volume + which are not recorded in $UsnJrnl. + - Detect the case when Windows has been hibernated (suspended to disk) + and if this is the case do not allow (re)mounting read-write to + prevent data corruption when you boot back into the suspended + Windows session. + - Implement extension of resident files using the normal file write + code paths, i.e. most very small files can be extended to be a little + bit bigger but not by much. + - Add new mount option "disable_sparse". (See list of mount options + above for details.) + - Improve handling of ntfs volumes with errors and strange boot sectors + in particular. + - Fix various bugs including a nasty deadlock that appeared in recent + kernels (around 2.6.11-2.6.12 timeframe). 2.1.22: - Improve handling of ntfs volumes with errors. - Fix various bugs and race conditions. |
