3.5'' Vs. 2.5'' SAS HDDs: In Storage, Size Matters

3.5” Test HDDs: Hitachi Ultrastar 15K450, 15K600

Hitachi Ultrastar 15K450

The Ultrastar 15K450 is no longer Hitachi's most current model, as it was replaced by the Ultrastar 15K600 a few months ago. We decided to include it, though, because the 450GB generation has been deployed in large quantities by all hard drive makers, and it's closest to the 147GB, 15,000 RPM drive we used for data density comparison.

Hitachi offers 300GB and 450GB capacity points and utilizes a SAS 3Gb/s interface along with 16MB of cache memory. The 450GB flagship is based on four platters; only three are required for 300GB. Smaller capacities are not available. Power requirements at idle for the 15K450 are lower than on the 15K600, but slightly higher under load.

The 450GB drive is also the hottest of the three, showing why differences between one product generation and the next can be significant. Let’s move on to the current 600GB drive.

Hitachi Ultrastar 15K600

The Ultrastar 15K600 is Hitachi’s top model, available in 300, 450, and 600GB capacities. Once again, it takes four platters to reach the top capacity, while the other two models come with two and three platters. We utilized the 600GB unit in our tests.

Hitachi quadrupled the cache capacity from 16MB to 64MB, and it ugraded the SAS interface from 3 Gb/s to 6 Gb/s. This doesn’t have a direct impact on performance, as the drive's platter I/O throughput is physically limited to a maximum of 195 MB/s. However, SAS 6Gb/s allows for higher peak performance into and off the cache memory, and it's utilized to connect storage applications to host adapters.

The drive requires a bit more power than the 450GB predecessor when it idles, but it's better under load. Since performance increases quite a bit compared to the Ultrastar 15K450, power efficiency both for I/O performance per watt, as well as throughput per watt, makes impressive steps forward. However, the 2.5” drive still does much better.