How to know what speed SATA 3 drive runs in SATA 2?

I am looking to buy a SSD drive. My laptop can do a max of SATA 2. So initially I was looking at SATA 2 drives but I can get SATA 3 drives for about the same prices. So now I am looking at SATA 3 drives(why not right?, more future proof and it is newer tech). I know that the drive won't do any more than SATA 2 speeds since my laptop is limiting it. But my question is, when you see the specs of the SATA 3 drives, they list speeds when they are hooked up to a SATA 3 interface. But how to know what kind of speeds they will do when hooked up to a SATA 2 interface?

Do they run at the SATA 2 max speed, possibly slower than SATA 2 drives, or some other number in between?

Thanks.

 

I have a 128GB Crucial M4 running on a SATA II interface and here's what the speeds look like, most drives i've seen hooked on SATA II reach 250-260MB/s in sequential:

 

You will probably not notice any difference between a SATA3 on SATA3 vs SATA2 in normal use. The sequential will be cap at sata2, other should be the similar.

tljo , your 4k write seem lacking for m4 Oo?

 

SATA 2.0 = 3.0 Gbps = 275MB/s

Which means with a laptop with SATA 2 this is the "top speed". So if a SSD have the specs "500MB Sequential Read" and "350MB Sequential Write", your notebook is only capable of transfering 275MB anyway. So you get 275MB in sequential read and write

It can vary slightly though like Tijo mention since this is the theoretical limit

 

I've never seen a drive actually hit 275 MB/s on CDM and AS SSD sequential R/W tests under SATA 3.0 Gbps. Sequential read can get pretty close, but sequential write tops out at about 225-240 MB/s, even if the drive is capable of 350+ MB/s under SATA 6.0 Gbps.

 

Slightly old picture, but here is my 830 on a SATA 2 port:

 

Stupid series 5 mobile chipset problem that i didn't bother to fix with the tweaks found on the forum. That is all.

 

yeah, and even with all the tweaks possible max one can get from h/pm55 is barely 2/3 of max m4 performance

 

Yup, on my desktop, the reads/writes are where they should be.

 

would there be a noticeable difference going from an intel 160gb x25 g2 to a samsung 256gb 830? This on a Sata II connection.

 

The extra capacity would make a difference in writes, the random would probably be slightly higher overall, but that's about it. No real reason to do the swap unless you need the extra capacity.