MySQL Inserting into Memory table triggers heavy disk activity

[Allan Poulsen]

I am confused. I am creating a Memory table in my Schema:

CREATE TABLE `memory_words` (
`word` varchar(128) NOT NULL ,
`COUNT` int NOT NULL,
PRIMARY KEY (`WORD`)
) ENGINE=Memory DEFAULT CHARSET=utf8MB4;

I am doing a lot of inserts into the table, for later statistics work. When monitoring the process, I see that my MySQL-server is starting a lot of writes to disk. Why? When using a Memory table, disk I/O should not be present?

The SQL for inserting (upserting) is:

INSERT INTO memory_words (word,COUNT) VALUES (_cleanstring,1) on duplicate key update COUNT=COUNT+1;

The MySQL Server is hosted on a Local Docker-Server and is mine alone, so I can rule out that the disk I/O is triggered by other than me. The server has enough resources to hold the Memory-Table.

Do I need to any config on table or server, to prevent disk I/O with Memory-tables?

Thanks in advance Allan