Why is the full page cache disabling block cache?

Question

I have a static part which is included on every page (navigation), but takes long to generate. That is why I have set a cache lifetime and a static cache key.

Works fine, until FPC is enabled. Then the block is still generated again for a newly called page and not served from the block cache.

There is this line in Enterprise_PageCache_Model_Observer:

Mage::app()
    ->getCacheInstance()
    ->banUse(Mage_Core_Block_Abstract::CACHE_GROUP); // disable blocks cache

What is the reason for this? It makes sense to me to still cache certain blocks, even if FPC is on - because otherwise generation of pages which are not fully served from the cache are slowed down.

1. What is the design decission behind that?
2. What is a good solution around that?
3. Would it be harmful if I just remove this block cache banning?

Answer 1

This bug has been fixed in Magento 1.14.2.0. (ref http://merch.docs.magento.com/ee/user_guide/magento/release-notes-ee-1.14.2.html under Known Issues)

Answer 2

To be able to disable FPC for dynamic blocks, for instance, in checkout or cart page. About 3rd question, in my opinion you should not do this which is might broken FPC process ( to be frank, to give right answer, we should check source code as well ).