securityjm,

There is another thread with some more information around how the "when-source-modified" functions which may help answer some questions around that. Along with some things to think about with RPO and RTO.

Re: what is minimum Isilon's minimum RTO, and can snapshot be supported?

Which also has a link to the white paper:

https://www.emc.com/collateral/hardware/white-papers/h8224-replication-isilon-synciq-wp.pdf

Being in support and working with snapshots, on a continuous bases, I have seen large backups of pending snapshots deletes due to "when-source-modified" (400k+ pending deletes)

This is incredibly dependent on the type of work flow. As Burhan Halilov said, if this is done on a very busy directory, say home directories for thousands of users, the cluster may have a hard time (pending hardware configuration) keeping up with the 100k+ snapshots (files being modified) a day.

So something to keep in mind is the type of work flow in the directory that when-source-modified is going to be utilized on. Being mindful of the amount of changes on files within the directory.