In addition to what Dynamox has posted, there can be impact when expanding pools. After the drives are added, a rebalance of the pool will be triggered. This is the same type of process that runs when Fast VP does a tier/data relocation nightly or on schedule.  While the impact may not be big, if a lot of data is relocated, it me be prudent to do it out of peak.  I've blogged about the process if you want the full details. EMC VNX/MCx Storage Pool Expansion – NavisecCLI or Unisphere | Pragmatic IO
We have a VNX5200 in our environment. we added 21*900GB SAS drives which are unbound and want to add them to our existing 2 storage pools. There are no hot spares configured so far. How many drives should be left free and will there be any impact on performance after adding these drives to these 2 pools.  pools are configrued with RAID 5(4+1) configuration.
We have VNX5200 in our environment. we added 21 900GB SAS drives which are 
unbound and want to add them to our existing 2 storage pools