mysqld and akonadiserver keeps on runing and writing things on my hard drive

Context :
opensuse 15.0
kontact  5.7.3
kmail/akonadi 17.12.3
libmysqlcppconn7 1.1.9

For more than a week now, i have this strange behavior :
i see on the htop, runing in background window, that some processes are eating 
the cpu time constantly :
akonadiserver and some other mysqld (more than 10....)
And it doesn't stop with a reboot of the os.
And worse : i see on the gkrellm that the main disk is constantly written .
That's o problem, because it is going to wear the ssd very fast. And i don't 
want that.

Could you give me an advise to stop such bad behaviour, please ?

-- 
christophe