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!
Do you have an app in mind? EMC has quite a lot of best practices documents up on powerlink and/or support.emc.com. Your best bet is searching up there for a recommendation document.
But all those recommendations come from the random/sequential I/O patterns and the fact whether it's write or read. So if you know what your logs and/or db is going to do, you can do the math real quick, using the previous posts in this thread.
Personal experience: Our OLTP apps are all sharing the same RAID5 pool, but that's due to the fact the $$$-aspect has been a key driver lately . All things considered, performance is good at the moment, so i wouldn't change that for something more expensive. The massive amount of drives absorbs pretty much every peak and FAST VP / FAST Cache smoothen it out even more.
The only pain point is a LUN that's asking for 10000 IOps at 99,9% random small read. Response times are still good for that LUN, but you can imagine the impact on a CX4 at that point.
Given a bag of coins, I'd probably invest in a number of SSD drives, since the Tier1 drives from the pool are getting hammered. I'd first use it to create a new RAID5/10 group, dedicated for that greedy LUN, and try and see if the pool->RG switch brings down the SP utilization a bit. If that doesn't bring much improvement, I'd move those drives into the pool.