HPE Storage Users Group

We currently have 8 x 400GB SSD's for our Tier 0 and balanced AO seems to want to move additional information up to tier 0 despite the current disks pulling only about 350 IOPS each so I'm thinking that I'd be best served by buying 8 of the new 920GB eMLC drives and letting the box add them to my existing CPG's. What effect does mixing capacities like this have? I assume it allocates enough spare chunklets from the new disks to make the total spare pool large enough to cover the loss of one of the new drives, but does that process respect cage level redundancy? I'd hate to buy 8* 920GB drives and lose as high a percentage as I did with the initial 400's.

Yes sparing will be taken into account for the new SSD's so you will lose some space dependent on the algorithm you've opted for, typically 1 or 2 largest disks in a tier. There's no problem mixing the drives in the same CPG, but obviously the 400GB drives will fill before the 920's, which means potentially not all data will come from all SSD drives. If you're using AO then some will come from all SSD's and some, above the 400GB drives capacity will come only from the 920GB's. Cage level availability will be enforced at the CPG, assuming Raid 5 3+1 then you need a minimum 8 disks to ensure your can sustain CLA beyond the capacity of the current 400GB drives. I'd probably look at the new eMLC 480GB's as there's less of a capacity jump and you can add more for the money. Which spreads things wider and provides a better balance, but the 920's are still a valid option.

For what it's worth the list price on the 920 GB drives is cheaper than the 400 GB drives. Not sure what you would actually pay.