Writes only hit FASTCache when the blocks have previously been promoted to FASTCache due to read activity.  It may also take some time for FASTCache to warm up (ie: a couple days) after which performance may improve.  FASTCache will alleviate backend IO to the disk, freeing those disks to handle IO that is not hitting FASTCache.  If you're primary performance problem is writes, and the data you are writing is not being promoted because you are not reading those blocks, then you may be better off using the EFDs with FASTVP, then setting the LUN tiering to "Highest Allocation".  New writes will hit highest tier as long as there is free space in that tier.  FASTVP keeps 10% of the tier space free for new writes after each reallocation has occurred.
Hello,  We recently implemented FAST and FAST Cache with some new SATA drives. Write performance of new data (for example storage vMotion of VM from old Raid Group to new disk pool) to SATA drives is very poor, and it seems like FAST Cache is doing nothing to improve write performance.  Can someone confirm how FAST Cache actually works for writes, I think I may have understood it wrong.  Are all writes committed from DRAM cache to FAST Cache and from there slowly to drives?  Or are writes committed to FAST Cache only when destination block has been previously promoted to FAST Cache based on read activity of that block?  Thanks!
FAST Cache with new writes?