Steve French wrote: > We really need to revisit the dnotify/inotify code ... the SMB spec > lists what the protocol allows, but AFAIK no one has had a chance to > prototype this. It is among the highest priority features still > needed to implement in cifs. > > As we have talked about before, dnotify (or inotify) is even more > useful for network file systems than local file systems (since the > alternative, polling, is too expensive to do over the network). CIFS > and SMB2 protocols, unlike NFS, has a mechanism to handle dnotify, but > mapping it to the VFS has not been investigated sufficiently I think NFSv4 can do it with delegations, although the exact semantics an app could rely on would be different. There are several different kinds of notify that apps could use for different purposes: 1. Queued (an event is posted after a change, will eventually reach the app). 2. Coherent (the app can ask "any notifications for me" and if the answer is no, it can be sure that an attribute/data read issued prior to asking would have yielded what the app already cached; a sort of app cache validation). 3. Lease (the app is notified and must respond prior to a change being allowed to proceed). These form a hierarchy: if the OS/filesystem provides Lease (3), the app can use it in place of Coherent (2) and Queued (1). If the OS/filesystem provides Lease (3) or Coherent (2), the app can use either in place of Queued (1). All of these have different, useful applications. GUIs like Nautilus are happy with Queued (1). For content indexers, it depends how you want them to behave, and how up to date they should seem. For something which computes things from file contents or attributes, such as (possible beneficiaries) any scripting language, Make, Git, JIT system, or web templating system, it needs Coherent (2) notifications; Queued (1) is not reliable for caching. Lease (3) is potentially useful for distributed databases. For CIFS/SMB it looks like all three could be implemented, in different ways. I'm not sure if NFSv4 can do Queued, but I think with delegations it can do Coherent and Lease. For local filesystems, I think Linux provides Coherent but I haven't looked closely. If not, it provides Queued. Al Viro said (long ago) apps should not rely upon timely dnotify/inotify events, and therefore should not use them for consistent app caching. However, timely delivery isn't required, what's required is that reading the inotify descriptor (or reading a flag set by delivery of the dnotify signal / inotify SIGIO?) will definitely return an event if the corresponding file change has become observable by other means. It is about ordering guarantees. If there is a revamp of the fsnotify code for networking especially, please at least have a little think about the different event delivery models and what apps can expect, when which semantics to support. The different models are each useful, and may involve different parts of the networked filesystem protocol. -- Jamie -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
