there is a limit of how many external LUNs you can present to a DX port, per engine.  When i was setting it up it was 2048 per engine. I needed about ~200TB of logical capacity on VMAX so i created about 400 x 500G LUNs on XIO,  presented 200 to each VMAX engine.  Get your hands on document "Design and Implementation Best Practices for EMC FAST.X", excellent read.
I wasn't sure where to ask, but this has been bugging me for some time and I'd like to get other admins/engineers opinions or experience (apologies if this isn't in the right community). I've been reading white papers on this, articles, and even discussions and FAST.X (previously FTS/Federated Tiered Storage?) seems like a great solution. We have a 250F and 200K in our environment and they both have sufficient cache to support it (1.5TB), however when I talk to our EMC group or even consultants they write off FAST.X/FTS as old technology, unnecessary now with flash- I feel strongly that this isn't the case but the only knowledge I have is theoretical and, well quick info sessions from EMC world. Seems though this is where VMAX really brings value, being able to extend features to otherwise dumb arrays. I haven't been able to find too much stuff after 2015 in regards to it.  We have some environments we manage where I'd like to Tier, we have XtremIO and VNX 8000's I'd like to Federate behind VMAX, and possibly start stretching clusters between our two data centers with SRDF. We also have Vplex, however I haven't been too happy with the feature set and lack of caching (somehow migrating to this from our SVC added latency). It's certainly more complex than this but that's the short.   I'd like to hear peoples experiences in using this technology, and maybe some pros and cons- or perhaps why it just is or isn't worth doing. 
FAST.X - What's it really like?