Used_serial.db size

Hi i am wonderning why my ‘used_serial.db’ file is 850MB.

My node hosts 3.4TB of data and has been running since 11th month on a USB drive.

According to what i red on the forum, it looks like something wrong to be that big.

As a result “usedserialsdb error: database is locked” error is the norm on my node and it does not process any transaction anymore.

What is ‘expiration’ column in this DB meant for ?

select count (*)from used_serial_ where expiration < ‘2020-05-08 00:00:00.000000000+00:00’
returns 4295667 rows

Mine are all under 70MB but I did recently run a vacuum job against it: "Action": "GET_AUDIT", "error": "usedserialsdb error: database is locked",