group mapping in Samba3

[tridge at samba.org]

Hi Simo,

Sorry for not being able to finish our conversation on IRC earlier
today. I had to run off to a meeting.

In regard to your patch be026a6fd89b44ba7e6bdf5bef049959b242c61e, I
wonder if you have thought about the scaling and clustering issues?

Your commit comment indicates that you made this change as you wanted
to make the default be cluster friendly. Unfortunately it also means
that we're back to the original problem that led to the group mapping
change 4 years ago, which is that the group mapping code now doesn't
scale. It uses traverses to do lookups, because it has no indexing. I
haven't run any profiling, so perhaps this is no longer an issue with
the way idmap and related subsystems work in s3?

The next logical step beyond your current patch would be to
re-introduce the multi-key tdb system that Volker worked on 4 years
ago, but if you do, then please be aware that this is not quite as
easy as it seems if your aim is for clustering. You could easily
introduce deadlock scenarios because multi-key systems on top of tdb
do not have predictable lock ordering if you use hash chain
locks. Possibly clustered tdb transactions solve this?

Cheers, Tridge