There's no technical reason to segregate the drives, it's more of an individual management or internal political decision. You can separate them but then you have to begin planning which drives you need to add where to support individual workloads, a bit like a traditional array where you have discrete raid groups / disk groups / aggregates etc. That's typically where the desire to physically partition things comes from. Since the capacity differential between 900GB and 1200GB is minimal in reality you just need to keep an eye on the physical capacity consumption on the smaller (see System reporter daily reports) drives and plan ahead.
Chunklets are distributed evenly across all drives of a given class (FC) regardless of their capacity. Meaning the 1200GB drives will only have to do more work once the 900GB drives are 100% full, so as stated above aggregating both 900GB & 1200GB in the same CPG will provide the best performance. In a combined CPG performance is spread across all disks up to the point the 900GB drives are full, at which point the 1200GB drives will service net new writes (not necessarily overwrites) and over time a higher proportion of reads vs the 900GB drives. Add drives of either capacity to the CPG and you have additional capacity (relative to drive type) and more performance (equivalent IOps), both are 10K RPM anyway.
Since data is wide striped, performance is aggregated across all spindles, so rather than trying to place data in a silo. You would be better just adding more 1200GB drives going forward, which is also a no brainer from a planning perspective. With this model over time you will also end up with more 1200GB spindles anyway, meaning they will provide better aggregate performance than the original 900GB drives would have on their own.
One thing to keep in mind given the drive sizes you're dealing with and possibly a reason to separate, is whether you need to consider the use of raid 6 on the 1.2TB drives. In which case you could go raid 6 across a common 900/1200 CPG or filter with raid 5 on 900 and raid 6 on the 1.2TB.
BTW if you decide you want to change any of this at any point in the future, assuming you have some space available, you can simply re-tune to a new CPG layout with DO.
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