Hello,   You are mostly correct.  By default space will be split with most of it on the SATA member. The Advanced Performance Load Balancer looks at the relative steady state latency of the members.  If the latency of one is over a threshold, it will exchange bust "hot" pages on the busier array, for idle or "cold" pages on the less busy member.  it won't move the entire volume to the SAS.  iSCSI tends to be very bursty.  If you look in SANHQ you can see that over time.    You also do have the option of creating a separate pool for the new member and moving volumes there.  You will always have the option to merge pools if need be.   Going the other way is more difficult and time consuming.   You can also assign critical volumes to the RAID10 member, by setting the volume preference to RAID10.  That will change the balance for all the other volumes.  
Hi, Since November 2011 we've been using a PS6100XV configuration with 24x 600GB SAS 15k rpm disk in raid-10 (we've close to 50%, random, writes) which is performing perfectly in our VMware setup. As we've started a datacenter migration we are using up disk space pretty fast. The current prognoses is that we have a "full" member (300 GB unallocated) while averaging around 1200-1500 IOps. At this point we would have enough IOps but are short on storage so the obvious option is to add a SATA member. After some consulting with Dell they did the calculations and confirmed we could add a PS6100E with 24x NearLine SAS 7.2K disks without getting into trouble with the new writes which will be divided by the load balancer based on available disk space of each member. The new member will provide us with some 20 TB space where we currently have just under 6TB. This will mean that new writes will be split 1:4 between the SAS:SATA array if I'm correct. As I would like to understand what will happen in de background I would like to know if I understand/assume (deadly I know, that’s why I'm here ;)) things correctly; When I'm writing 1000 MB new data initially 250 MB will end up on the SAS unit and 750 MB on the SATA. While time passes the group notices I'm using 500 MB of that whole dataset pretty heavy and it moves 250 MB from the SATA to the SAS since he's faster. Next the 500 MB (heavy active) data on the SAS gets overwritten, as the data is already on the SAS unit, the SATA unit doesn't have to do a thing.  Besides knowing if the above it correct I would love to hear other users experience with a similar setup and possible interesting case specific reading material on the matter stated above. With kind regards, Henk Kraal
What to expect with a PS6100 XV+E group