If a value of the session is updated, the data field of piwik_session gets fully rewritten (it's a text blob) instead of simply updating the one value.
This text blob is already a data structure - but not a data structure mysql understands. This breaks mysql database replication binlog, because this change is logged there (to apply it to an other server) and causes higher database traffic as needed.
do you find this issue is really bad, or just a little "nice to have", in other words, did you notice a real database traffic increase due to this issue?
It is really bad, because there are cases, like #8307, where 16KB(and rising) are written multiple times in a second.
I understand the convenience to store data this way, because it is similar to a filebased sessionstore. ;-)
Until somebody finds a way to put arbitrary large data in this field. I think #8307 is just one known way to trigger this bug. And as the replication binlog just saves differences, a rewrite causes more traffic even if a change is minimal inside this field-datastructure.