hi Stack, as you rightly pointed out, there are two issues at play here.
Case1. where to store the namenode transaction log?
Case2. how to make the active namenode transfer control to a standby namenode and how long does this failover process take?
In my view, these two issues are not tied up with one another.One can use NFS, DRBD or even newer things like zookeeper/bookkeeper for storing the transaction logs in Case1.
For Case2, we still have to work out the details. Sanjay's proposal seems to say that we should attempt to address Case2 first. A design for Case2 should work irrespective of the type of storage we chose in Case1. In fact, getting the right design for Case2 is the more challenging part of implementing a fast failover.