There are a few different ways in which you can connect data stored on an iSCSI storage array to VMware, to give access to the guest virtual machines that run on the hypervisor.
VMDK In guest iSCSI RDM
Zerto is a replication solution that integrates with VMware, shadowing the writes that VMware sees made by guest VMs and based on your configuration, replicating these to a remote target - with an RPO typically within seconds (depending on bandwidth etc).
To do this however, the storage has to be VMDK or RDM, which rules out using in guest iSCSI initiated connections.
Talbert Houle, a contributor to Nimble’s community site, has submitted research on the performance characteristics on Nimble Storage between in guest iSCSI and VMDK access methods (link below).
This illustrates that to obtain the maximum possible performance whilst retaining Zerto compatibility, the VMs should reside within VMDKs, with the guest data drives stored on VMDKs that are located on a different nimble datastore than the VM’s OS/page file/system files.
Additionally, if you can ensure the data type stored on the data store (e.g. only SQL data, or only SQL logs), you can set the relevant Nimble performance profile on that data datastore.
Guest iSCSI vs VMDK Performance comparison on Nimble Storage: https://connect.nimblestorage.com/message/3576#3576
One last point, store the Zerto VRA VM’s on a datastore that is set to not be cached by Nimble, to avoid cache pollution (or on the internal datastore of the hypervisor host, if one is available).