understanding a 2.4 db directory?

[Carl Brewer]

G'day,

I have an old 2.4.17 installation, and am planning to bump it up to 3.0 
soon-ish. It's on an older NetBSD system running from a pkgsrc install.

As part of this, I am trying to understand the old directory structure, 
and in particular, the contents of the old conf directory, which in 2.4 
has a couple of directories called db, db.backup1 and db.backup2

db is pretty big :

du -sh db*
11G     db
94K     db.backup1
94K     db.backup2


db is full if log.00000xxx files, and a "skipstamp" file

The log.0000... files are all Berkeley DB, which is why I am concerned 
about an in-place upgrade to 3.0.

bash-4.3# file log.0000001340
log.0000001340: Berkeley DB (Log, version 16, native byte-order)

bash-4.3# file skipstamp
skipstamp: data

Can anyone tell me what they are?  As well as being Berkeley DB, can 
they be cleaned up/deleted? 11GB of "I don't know what this is?" isn't 
doing me any favours.  The doco here :
http://www.cyrusimap.org/imap/reference/admin/locations/configuration-state.html
doesn't seem to help much for this version of imapd.

Pointers to relevant doco much appreciated!
Thank you,
Carl