"Auto Tier" will distribute data across all raid groups within the pool based on free capacity within the tier (disk type).  "Highest Tier" places as much data possible in the higher tiered disks within the pool.  If performance is a concern I would start on "Highest Tier" and move peformance sensitive LUN's (or VM's in your case) first; this will ensure they are placed in the high tier if space is available.  Auto Tier is the ideal setting for most moderate performance LUNs.  If you have any other questions feel free to ask; I'll share everything I can. I would also review this post as it talks about some things you need to remember should you start using different tiering settings for different VMware datastores.    - Jason
Hi,  A lot of EMC whitpapers explaining the mechanism regarding FAST have the following description on Inital Placement of data on a tiered LUN conifgured as 'Auto':   "...Initial placement with the pool set to auto-tier will result in the data being distributed across all storage tiers avaialble within the pool..."  Can anyone explain to me in a bit more detail how this will work in practice?  Specifically, I'm going to be storage vmotioning VMDK's into a LUN set on a pool backed by 2 tiers of disks (namely  FC and SATA) and I want to be able to maintain high IOPS on initial placement but allow the FAST algorithm to do it's wonderful stuff so that when the scheduled tiering happens over night the data is then relocated to a lower tier.  I could just put the LUN to highest available tier which will guarantee IOPS but I want to take advantage of sub LUN tiering.  Will it just be better for me to set 'Highest' until all VMDKs have been provisioned and then enable 'Auto' once all VMDKs have been storage vmotion'd across to get that tiering goodness?  Clarity on the above would be gratefully recieved right now [IMAGE]  Thanks,  Craig.
Initial Placement of Data on a FAST LUN with 'Auto-Tier' assigned