HDD on SATA I (1.5 Gbps)

I am planning to buy an SSD for my laptop, but my budget isn't high enough to afford something like a 1 TB SSD; the most I can get on my budget is 240 GB.  So I am planning to go for an SSD, which I will plug into my laptop's HDD port; and I will use my old HDD (1 TB) in my optical drive slot using a caddy (I don't use the ODD).

But then I remembered that my ODD SATA port is 1.5 Gbps or 187.5 megabytes per second (SATA I).  I ran a benchmark on my laptop's HDD and it is barely touching 95 R/W speed on SATA III. Would I have any performance loss?

3 Answers

Yes, there will be some performance drop. Negligible, IMHO. Sata I is not only speed, it's also lack of certain features introduced with later revisions.

But, speaking from experience - it doesn't matter. No matter how fast the HDD is, it will be crawling compared to SSD. And, unless you're working with large files constantly, only then you will still have a bottleneck there; any other use and it merely annoys just a tiny little bit.

However, this will be mostly uninterrupted by OS, so I'm fairly confident your sustained transfer will hit max and stay there, without having to constantly switch to service request from system - like paging file...

Just to show you: I have SSD internally and for the time being my HDD is on USB. And 2.0 at that (ports are 3.0, but bungled drivers cause them to not work as that). And I don't mind. At all... Not since my Win7 boots in 22 seconds to login and there are no lags when starting, using and closing programs (including games).

Brief answer:
You are not going to loose noticeable speed when you install the older laptop drive on a SATA I/SATA-1.5Gpbs port.

Somewhat more verbose:

95MB/sec sustained linear read/write is about the normal max for a HDD. It is actually pretty good for a laptop drive.

Lets compare that with SATA. SATA-1.5 has a signal speed of 1.5Gbit/sec. That is not equal to an actual data transfer speed of 187MB/sec. To balance electricity 10/8 encoding is used. That leaves a theoretical max speed of 150MB/sec.

In practise this is somewhat lower. 135MB/sec seems a good guess for max realistic sustained transfer speed. (Guess since it depends on a lot of things, such as the SATA controller used).

That is still well above the actual speed you measured for your drive, so the ODD SATA port should not be a bottleneck.

Some notes:

You have a laptop with both SATA-I and SATA-III? Are you sure that the ODD SATA port really is SATA-I or is the link speed just SATA-I. (E.g. a SATA-II or a SATA-III controller and a SATA-I optical drive would cause the port to work at SATA-I speeds. When you replace that ODD with a SATA-II HDD the link speed should change to the new lowest shared speed. Which is likely 3.0Gbit/sec.

If you are doing a lot of small IO requests (e.g. semi random 4k writes) then you probably will gain some speed when using a faster link since you would send them faster to the drive and the drive could reorder them. But after the drives cache fills it will still need to slow down in order to actually write the data. So you may experience different speed in benchmarks. I do not expect that to be noticeable in real life though.

Transfer speed is a matter of latency introduced by SATA controller - older and slower SATA I controller will result in slower transfers.

It happens, because disk transfer consists of several requests and responses between SATA controller and disk controller, where each one has some latency, not just linear transfer. These latency times sum up and result in slower transfers.

I hope this explains your problem without digging into electric signal encoding details.