A good question from victor over on Twitter...
So for starters, the bigger the RG is, the more data disks you'll have compared to parity disks. In the new situation you'll have the nett capacity of 14 drives to allocate to servers. In the older 6+2 groups you would have to make two groups and still only end up with the capacity of 12 disks. Using 4 parity disks instead of a total of 2. This is just the financial aspect...
Performance wise, a bigger RAID group (obviously ) contains more drives, which will yield you more performance. However, watch out for the downside of this: rebuild times will be longer with larger RAID groups. Or, if you're rebuilding on ASAP rate, the impact on your storage (bus, disks, SP for parity calculations) will be larger.
If you want the performance and low rebuild impact, keep the RG's small and use striped metaLUNs to get you the amount of disks (=IOPs) you need. But you'll pay for it with $$
For RAID10 or RAID1 writing, no readback is performed. If you change a block, it's written to the primary disk and the secondary disk: 2 write I/O's in total.
This is different from RAID5 where you need to read the old block, the parity (that's 2 reads), then write the new block and the new parity (that's 2 writes).
RAID6 builds on RAID5: you read the old block and the old parity blocks (dual parity), so 3 reads in total. And then you write it all back: data + parity blocks. 3+3=6!
As discussed on twitter I've been using this DIY heatmap to visualize the CLARiiON performance on one view: http://www.penguinpunk.net/blog/emc-diy-heatmaps/
Check out the sample output file: http://www.penguinpunk.net/blog/wp-content/uploads/2011/12/heatmap.html
Thanks everyone, we had some great fun and good questions!