Clearly, even the latest 2.5” Ultrastar C15K147 doesn’t stand a chance against the 3.5” Ultrastar 15K600 if you look at performance. This is because of the 3.5" model's higher data density and more aggressive performance. However, the 2.5” drive shines when it comes to power consumption, requiring half or even less power than the 3.5” drives, while performance differences aren’t even remotely close to the 50% or higher difference in power consumption.
In general, 2.5,” 15,000 RPM SAS drives will deliver 25% less performance than 3.5” drives, but efficiency increases at least twofold. Our results here are specific to Hitachi's drives, but we would expect similar results with Fujitsu or Seagate drives, although the performance results would have been different.
The performance "issue" with 2.5" drives can be managed through adjusting the drive count. Two 2.5” SAS drives in RAID will outperform one fast 3.5” SAS drive in every workload. At the same time, the power required won’t be higher and capacities may be similar.
If you take our results to the next level, you can easily imagine that a simple RAID with four 3.5” drives could be replaced by six to ten 2.5” drives. These would not consume more power than the 3.5” models, but resulting performance would be 50% to 2.5x higher, depending on the specific configuration. Meanwhile, power consumption will remain below the level of the 3.5” RAID.
All that’s left to consider is cost, and this has to be contemplated carefully. Performance storage may offer the best cost effectiveness when using a small number of SSDs, but as soon as you need capacity, flash memory might quickly become too expensive. That's when you’ll turn to arrays with 15K drives. For best price/capacity, 3.5” drives at 7,200 RPM are probably the most sensible choice unless you also need a certain performance level. In that case, you'll find best compromises with 2.5” hard drives.