1st of all Automated Tiering concept works with different drive types like SSD, SAS and MDL SAS. In your case all are SAS drives so this is anyway not applicable.
Now coming to your configuration, Global spare get set at the System level which means if you set one 600GB and one 1.2TB drive as Global spare and one 600GB drive failed. Then any one of the global spare may kick in and we can't control this but if one 1.2TB drive failed then obviously 1.2TB drive will kick in as Global spare. Rule is system look for same type and same size or bigger size drive compare to the drve which actually failed.
So in your case if you plan to have 4 x 600 drive as RAID1 disk group and 12 x 1.2TB drive as RAID6 disk group . Both same in both Pools. One 1.2TB drive as Global spare. Then as explained above if any 600GB drive failed then 1.2TB global spare will kick in and after reconstruction completed 600GB space will be wasted.
Coming to performance issue, yes it's not recommended to have different RAID type for same type of drives. If a disk group has a different capacity, then the number of pages stored on it will eventually differ compared to other disk group in that Tier, and would therefore also lead to unpredictable performance.
As Pages are striped across all the disk groups in that Tier, disk groups with fewer drives will underperform compared to the others, equally disk groups with more drives would get a little performance boost, which although not necessarily a bad thing in itself equates to reduced predictability.
Hope this helps!
Regards
Subhajit
I am an HPE employee
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