I had some more thoughts as relate to EC2 deployments and SSD. SSDs can have vastly more than the 4000k IOPs that you can obtain for EBS with provisioned IOPs. Therefore, we probably want to setup on hi1.4xlarge instances with an EBS volume for durable storage of the service directory, transaction logs, and snapshots. The data directory would be on the SSD disk. This would allow very large database instances with high IOPs using the instance SSDs and durable storage using EBS. A nice combination.
I suggest that the deployment concept for the HA replication cluster might look like: Use an instance type with local SSD. For example, either of the following might work, but the Storage Optimized is likely to be the better choice for bigdata:
Memory optimized cr1.8xlarge 64-bit 32 88 244 2 x 120 SSD - 10 Gigabit4
Storage optimized hi1.4xlarge 64-bit 16 35 60.5 2 x 1,024 SSD2 - 10 Gigabit4 <== best choice.
Then put the snapshots, transaction logs, and the process log files on EBS so that they are durable. We use sequential reads and writes for the snapshots and transaction logs so EBS should be Ok and provisioned IOPs could be used if necessary to ensure good sequential write performance.
Since the SSDs are instance disks, the data on the SSDs will not survive a restart. Therefore, if you restart an instance node, you would create a new journal from the most recent snapshot. On startup, the HAJournalServer would automatically roll forward the transaction logs since that snapshot. This could all be automated on the startup of an SSD-backed instance node.