Evaluating hdd speed: data transfer rate vs internal data rate

Question

I have two Seagate disks a 2T st2000lm007 (2.5) and a 3T ST3000DM008 (3.5). I have a choice between mounting one as a USB 3.0 external and the other as an internal.

Is there an advantage either way? I will use one drive for frequently accessed and NFS share data (system is on SSD) and the other for archive/backup.

I paste some performance stats from CNET below. I've read that Transfer Rate determines drive performance and see that in these stats both drives have the same Transfer Rate but that the ST3 has a significantly better Internal Data Rate so I'm not sure how to read these.

ST3
Internal Data Rate 210 MBps
Seek Time 8.5 ms (average)
Drive Transfer Rate 600 MBps (external)

ST2
Internal Data Rate 100 MBps
Seek Time 13 ms (average)
Drive Transfer Rate 600 MBps (external)

Answer 1

You will likely get marginally faster performance by mounting the 3.5 inch drive internally and the 2.5 inch drive externally. Connecting directly to SATA is typically more efficient the USB, so it makes sense the faster drive gets the benefit of being internal.

Also, by having the 3.5 inch drive internal you should be able to power the 2.5 inch drive over USB meaning 1 less power cable.

Answer 2

I'm not sure how to read these [interface data transfer rate vs internal data rate].

It is not difficult, but the final answer on "performance" is impossible to provide.

Performance of a peripheral mass storage depends on the chain of interfaces, buffer (cache) sizes along the data path, and overall data access pattern - short defragmented files versus few contiguous large files (like ISO images or DVD movies).

The datapath chain is (1)System MEMORY - (2)interconnect fabric - (3)host controller - (4)SATA interface - (5)drive buffer (disk cache) - (6)media controller - (7)magnetic head channel (aka "internal data rate"). Overall performance is bounded by weakest link in the chain, although the size of internal disk cache might appear as a big factor on scattered short files.

The SATA3 interface data rate (6Gbps, or 600 MBps) is the marketing term. It is the raw bit-based performance of the SATA3 interface. Every SATA3 device can be slapped with this moniker, which doesn't have much meaning. When performace of all other links in the datapath is higher than 600MBps (like in some SSDs), the SATA interface can deliver about 550-560 MBps sustained, the rest gets wasted in protocol overhead.

The "internal data rate" usually refers to electronics within (5)-(6)-(7) blocks and may refer to the internal controller clock rate, but eventually it is limited by magnetic flying head channel. Current best MR head technology (~2019) allows for 200-220 MBps sustained data rate from a single head to platters.

So a faster "internal" drive will yield better results on long data transfers, and a drive with bigger buffer will do better on short scattered files.

If you consider the use of USB, you will have ... (3)USB host controller - (3a) USB bus - (3b) USB to SATA bridge - (4) SATA interface etc. So two extra links is added into the datapath, with own limitations. USB 3.0 sub-chain will limit SATA to about 430 MBps, but since the internal data rate is only ~200, you won't see much of a difference.

In short, evaluation of HDD performance is a tricky business.