Today I got this message from the Dude: db failure: database or disk is full: stopped
Database size is ~940 MiB, it's located on the ramdisk, that has ~450 MiB free of 2 GiB.
Wtf? Looks like another one artifical limit.
pragma for internal SQLite3 database... too few pages?
or maybe if it doesn't have double the capacity for temporary files it considers the disk full?
/dude vacuum-db ?
I don't know about internal SQLite3 limits.
But regarding temporary files, they take only about 11 MiB:
Also, there was a period, when I had only ~150 MiB of free RAM and the Dude worked without problems, so it doesn't seem to be some disk limit.
Vacuuming behaves strange. I've started it, and it looks like it has freezed. At the beginning it has created temporary db files, and there was some noticable 'dude' and 'flash' activity in the Profile tool, but after a minute or so, the command has returned with 'vacuuming' status, but there is no any activity anymore - no CPU usage, no any date/time changes in database files, nothing...
UPD: after almost 24h still no any status change...
My bad. I didn't notice, that ramdisk uses only 1/2 of RAM by default, if max size is not specified 
So, it was really not enough of space. Specified larger max size, and now it works. I think, the topic can be closed.