>> i use statements like this to lock the rows I need:
>> SELECT mycolumns FROM thetable WHERE thekey=? FOR LOCK
>>
>> do such locks involve WRITE disk activity (if I do not actually
>> modify the raw)?

Yes, it will create new record version (unless the transaction
is rolled back and cleans after itself).

>
> Yes, but only if the page is in contention, which in turn depends on
> whether you're using SuperServer or Classic.

???

Ivan