Trying it without the DB_PRIVATE flag in rev 153. We need to keep an eye on what's different.
On Windows at least, it creates six __db.001 - __db.006 files with sizes from 24K to 4MB. It doesn't delete them on exit, it just leaves them behind.
The docs say it uses memory mapped files. I assume they have the same file permissions as the database files, so the same user access restrictions apply.
Tests on Windows private LAN download of 78500 blocks:
with DB_PRIVATE 20 minutes 51 seconds
without DB_PRIVATE 20 minutes 51 seconds
I wasn't expecting them to come out exactly the same.