> Better thing:
> - if main file can't be opened, try open in read-only mode.
> - if open succeed and the file is locked, continue

Yes, I was thinking about this option too...

> For example:
> - replication

You mean "hot standby"? Issue is that one need another config param
saying how many transactions can be lost. In other words, commit on the
main db should be blocked until difference in tx count in main and
standby DBs is less or equal to this number. Otherwise feature is not
controllable.

> - flash back queries

Why do I need NBAK here? I always thought that this can be achieved by
providing a "flash back depth" parameter to garbage collector to prevent
garbage-collecting those records and providing the "flash back"
parameter with the query.

> - database "branches" - database with various independent delta files -
> very useful for group of people developing/testing on one copy of big
> production database without interfering with others

I like it :) We have to take care, since people might begin to ask for a
"MERGE BRANCH <name> INTO HEAD" functionality - might be not very
trivial to implement.

Roman