The users of my phpBB instance running on my server were greeted with

SQL-ERROR [mysqli]

The table „phpbb_login_attempts“ is full. [1114]

while I was greeted with greeted with

ERROR 3 (HY000): Error writing file './phpbb_4/phpbb_sessions.frm' (Errcode: 28 "No space left on device")

So - how do we free up some space?

Cleaning your tables

Before fiddeling around with your tables, it's always a good idea to do a backup

mysqldump -u root -p phpbb phpbb_sessions > phpbb_sessions_backup.sql

The by far biggest table in my database was the phpbb_4.phpbb_sessions table. Let's empty that

Easy way (prefer this)

TRUNCATE TABLE phpbb_sessions;

Hard way (only if easy does not work)

-- retrieve table structure 
show create table phpbb_4.phpbb_sessions

-- drop table
DROP TABLE phpbb_sessions;

-- recreate table
-- Run return value from first command

Cleaning your logfiles

Look for large log files

du -sh /var/log/*

Truncate them (in my case, adjust to return of du in your case)

> /var/log/auth.log
> /var/log/btmp
> /var/log/maillog
> /var/log/syslog