Subject, Predicate, Predicate Nominative plural agreement

So I was writing a sentence and I came across an issue...
"The most common form of writing is logs."

I debated whether it was "writing are logs" or "writing is logs".
A friend on facebook told me that the verb has to agree with the predicate nominative "logs" as well as the subject.

He told me I could either do:
"The most common forms of writing are logs."
OR
"The most common form of writings are logs."

I'm not sure I agree with the second one, because if I remove all the fluff, then the sentence is:
"The form are logs."

I think the only acceptable forms of this sentence are:
"The most common forms of writing are logs."
OR
"The most common form of writing is a log."

Help me out, this is quite the tangle.