Richard,

I haven't looked at the source, but I do remember Nickolay explaining
NBackup to me/others.

> It seems to me from my understand of the source code, that when the
> alter database end backup occurs, the write-locking of the database
> header page prevents updates to the rest of the database? (although not
> reads?)

Yes, that is more-or-less correct. But ... the server will put changes into
a temporary file that will be merged with the database after NBackup
has finished (or something like that). So applications that use the database
can continue without problems.

NBackup also should be very fast, given it's nature of doing a page
level backup (almost the same as a file copy).

> This would mean that the fundamental issue that incremental backup is
> designed to address, databases getting bigger and bigger and more and
> more data changing would be defeated by this mechanism? The bigger and
> more active a site gets, the more downtime they will encounter when the
> ALTER DATABASE END BACKUP occurs because of merges that need to go back
> into the database?
>
> Clearly when the amount of data is down in the low megabytes range, this
> isn't a problem, but on busy multi-user sites, this seems to be a big
> problem.

Hope this helps.

Martijn Tonies
Database Workbench - tool for InterBase, Firebird, MySQL, NexusDB, Oracle &
MS SQL Server
Upscene Productions
http://www.upscene.com
My thoughts:
http://blog.upscene.com/martijn/
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