some question about fabric

shijian fu

Hi, our program group have used fabric for about 8 months. And there are some questions unsolved until now. 
here is the question list:
1. Would orderer node store all the block of a channel when there is only one channel and without using any private data? Or the orderer node just need to store the newest N block of a channel?
2. I have read some post in stack overflow which indicated that a peer 'A'  from one organization  will pull blocks from peers from other organizations when peer 'A' pull those blocks which are "missing"(not include private data) in orderer. 
In which condition would blocks missing in orderer?

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