The z9-109 is designed to allow a single (processor) book, in a multibook server, to be concurrently removed from the server and reinstalled during an upgrade or repair action. To help minimize the impact on current workloads and applications, you should ensure that you have sufficient inactive physical resources on the remaining books to complete a book removal. For maximum availability, it is recommended that z9-109s be purchased with one additional book.
It supports up to 54 processors, vs 64 for the p595/i595, but other than the blue stripe vs red or orange stripe, the frame is the same. Still looking forward to the ecLipz (POWER6).