[olpc-software] Package manager stuff

[Alan Cox]

On Tue, Mar 14, 2006 at 10:24:50PM +0000, Mike Hearn wrote:
> I'd expect the change to be reflected up, so if I'm watching a directory
> that is a composite of A and B, and a file is added in A then the watch
> says "new file!". And if a file with the same name is then added to B,
> nothing happens, because the file in A overrides it and so nothing has
> changed view-wise.

Then you will need to add code for it. In the case of subtrees it gets quite
foul because a subtree can be entirely masked then re-appear just due to
changes in the underlying trees. Right now nobody has code doing all this
propogation (and with networking you also need to teach the user space 
network file monitors about it - eg fam)

> there wasn't". So it would work the same as a naive polling based
> implementation would. Why is that affected by network vs something else?

Its affected directly because NFS and friends don't have a notifier interface
at all so you have to go via user space (eg fam). It also means the underlying
fs of the union can change without the change coming via the unionfs from
another node. That gets 'exciting' when you are trying to open a file as
the name paths change and really requires "intent" data (as done with
lustre) to get permissions right. In the notification case it also means that
you have to be very careful how you deal with subtrees because you can end
up with the things that have changed being hidden by something else.

In the traditional case you can ask to hear about all changes, in the union
case with all the other paths its extremely hard to work out what you must 
be told about, and that list changes dynamically.