David,

I think that you need to be careful not to impose your idea of when to
take snapshots and how long to keep them onto the design. For example
why take snapshots every 15 minutes? Why not every 10 or every hour?

  crontab is anyways changeable by the admin, I think we can have that
  flexibility.

Why treat monthly snapshots as special when it does not fit into most
working weeks? would weekly be more logical? What about 2 weekly (When
I worked at Nokia, internal releases where done on Tuesday of each even
numbered week, so we would have wanted the snapshot taken on that day
to be retained longer than snapshots taken on other days, or Tuesdays
in odd numbered weeks.)

  agreed. weekly is more important. (I had that in mind but missed it
  when writing, sorry for that).

I think a more flexible design would be to allow the user to specify
(via a config file for each subvolume) a label for each type of snapshot
and how long to keep snapshots depending on when they are taken. This
can be done using syntax similar to crontab:

  simple and nice idea. thanks for explaining, will try to get this
  in the initial release.

Cheers, Anand