ASM re-balance or storage level replications are all proven solutions for data migration, however, a good planning and full backup before migration is a must for any unforeseen situation.
what's VNAX ? Did you mean VNX ?
You have a few options, my DBAs like to use ASM rebalance. If that's not available you can use Open Replicator to copy at storage array level, SANCopy pull, PowerPath migration enabler (host based migration if using PowerPath).
that's one option to migrate the data. Present new LUNs from VNX, add to ASM disk group, rebalance, remove old disks.
Is this your question just for migration methodologies? Why would you considering to migrate from high-end VMAX to VNX? For cost, no performance concerns?
Could you please share main points where we can get performance impact through VNX?
Is there more IO latency in VNX?
There are significant differences between High-end VMAX and mid-range VNX. Generally speaking, due to the fundamental different architecture design, VMAX with multiple engines and directors will provide much better performance (IOPS, throughput etc.), scalability and high availability over VNX.
From performance point of view, It is hard to say whether VMAX or VNX fit to your case as it all depends on what the specific applications, IO patterns are for the right storage sizing (base on both performance and capacity). FAST Cache can also be used to significantly improve VNX performance. So if VNX can meet your current and future requirements (performance, scalability and high availability), it will also be a good choice.
Would FAST cache have to be enabled or this is enabled by default?Does VNX support fiber cable ? and I believe this cache would be part of storage controller right?
FAST Cache uses Flash drives to add an extra layer of cache between DRAM cache and Rotating disk drives. It is optional function need to be enabled.
For more detail knowledge and best practices, please refer to: EMC Performance for Oracle - EMC VNX, EFD, FAST Cache, VMware vSphere