I'm using kontact/kmail 4.6beta4.

There is a directory 'file_db_data' inside '~/.local/share/akonadi/' that is growing very big (more than 1GB).
The content of this directory is mostly e-mails but there are also a few contact files. But all these files are copies(?) of files that are found elsewhere on the same harddisk (e-mails in '~/.local/share/.local-mail.directory/' and contacts in '~/.local/share/contacts/').
I can not say that these files are exact copies, because after inspecting some e-mails using the command 'diff', some headers have newline-characters and quotes (") added, all the other characters and the body of the e-mail are equal.

My question is, is it save to delete this 'file_db_data' directory to save diskspace.

As far as I know, it is safe to delete this file as long as Akonadi is stopped. Note however that Akonadi will likely start rebuilding it as soon as it is started up.

Note for others having the same problem:
Be careful deleting this 'file_db_data' directory. In the current beta version of kmail/kontact this directory should not be deleted. I don't know if this will change in the future.

The content of the 'file_db_data' directory will not be recreated after deletion. Only new e-mails downloaded from a POP3-server are put inside a newly created 'file_db_data' directory.
In kmail the e-mails that used to be inside the old 'file_db_data' directory are missing the body (only the header is readable).
I can rebuild the akonadi database to make all e-mails complete again, but when I do that the whole database will be recreated, that means all flags (read/unread/important) and tags will be erased/reset.

Luckily I've only renamed (didn't delete) the 'file_db_data' directory and I've also made a mysql dump of the Akonadi database... so it was easy to undo the changes.

But still I find it strange why kmail or akonadi is storing 2 copies of every downloaded e-mail from a POP3-server. One copy in '~/.local/share/akonadi/file_db_data/' and an other in '~/.local/share/.local-mail.directory/'.
I have relatives sending me e-mails with attachments of holiday and/or baby pictures. Those e-mails can easily be bigger than 10MB, so a 2nd copy stored on the harddisk is a waste of diskspace.

Indeed, might be worth to file a bug report, Akonadi shouldn't do any caching for locally stored email.

I've tested this with the new 4.6beta5 (4.5.95) version of kdepim and it has the same problem.
Bug report posted here:
https://bugs.kde.org/show_bug.cgi?id=271231

bcooksley wrote:Indeed, might be worth to file a bug report, Akonadi shouldn't do any caching for locally stored email.

Are you sure?

With my non-akonadi-developer hat on (I don't have the other one at all ) it should index local mail as well. Otherwise, when searching, it would need to search through mbox or maildir which would be painfully slow (like in kmail1 or even more)

It should of course index for search, but not cache the data in it's database (which is what I meant)