This comment should explain why smaller size allows us to do so. Or it could explain why it is not possible when the size is greater.

Have you tested this code?

We always throw here:
if persistenceMethod = PersistenceMethod::UNKNOWN we throw because persistenceMethod != PersistenceMethod::BLOB
if persistenceMethod = PersistenceMethod::BLOB we throw because persistenceMethod != PersistenceMethod::UNKNOWN
in all the other cases we throw because persistenceMethod != PersistenceMethod::UNKNOWN

Please use sentences (capital letters and dots).

I'm not sure if I understand the idea behind this, but this doesn't seem to be correct. Let's suppose we want to persist two chunks, one bigger than LOG_DATA_SIZE_DATABASE_LIMIT and one smaller. After the first chunk, we set this->persistenceMethod = PersistenceMethod::BLOB. When processing the second, the first if condition is true, and we throw because this->persistenceMethod != PersistenceMethod::UNKNOWN. Am I missing something?

As a side note, it's better to avoid throwing when we can recover from invalid state. In this case, if we started writing to the db and then the size limit is exceeded, we can move the data to blob and write the rest there.

Do we need e.g. else here? I guess the intention wasn't to always throw.