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I thinking to buy my old Mac an SSD, but after asking around to some friends, and looking to different disks, some of my friends said that the SSD's read/write speed won't matter because my CPU won't be able to handle the increase in the speed. To be clear, they said the SSD would increase performance but should rather buy a 450 MB/s SSD then a 550 MB/s one. I am using a late 2011 MacBook Pro with i5-2435M, and the ARK page for it doesn't mention anything related to disk read/write speeds. So should I go for a faster SSD or will the money spent be wasted?

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  • Yes, his laptop has SATA 3.0. SATA 3.1 and 3.2 don't change the speed available at the traditional SATA interface. SATA 3.1 does not define any new transfer speeds a all, only new form factors and a few other features. 3.2 does define up to 2 GB/s (!) transfers but only over a new "SATA Express" interface, which basically brings a couple of PCIe lanes out to a new type of connector, giving the drive the option of being directly connected to PCIe instead of going through the SATA controller. I guarantee you that a 2011 laptop does not have that. :) Jul 30, 2015 at 20:32

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Apologies, but... your friends don't know what they're talking about. Your CPU speed does not directly limit disk transfer speed (not since we got away from PIO modes on Parallel ATA).

This page shows that your machine has a 6 Gbit/s SATA interface. Every 550 MB/s SSD I can find also has a 6 Gbit/s SATA interface. So, your machine will not limit the SSD's performance. (And if your machine had a 3 Gbit/s SATA interface the SSD would still be far faster than the hard drive you're replacing.)

The actual SATA transfer speed of e.g. 6 Gbit/s (600 MB/s) is implemented not by the CPU, but by the RAM itself, the memory controller, and a DMA controller that's part of the computer's SATA interface (the last two are part of the chipset). The RAM in that machine is apparently 1333 MHz DDR3; this has a peak transfer speed of 10.7 gigabytes per second, almost 20 times the SATA rate.

I will agree, though, that the difference your perceived performance between those two drives will probably not be significant unless you are hitting the disk a LOT during your normal use of the machine. And if you're hitting the disk a lot it may be a sign of not enough RAM. SSDs don't really achieve those peak transfer rates that often anyway. Check your performance measurement tools (I'm unfamiliar with Mac OS, can't advise you there) and see if it looks short on RAM. If it is, and if you can put more RAM in the machine, you will probably get more improvement for your money by getting the less-expensive SSD and using the money saved for the RAM upgrade.

Another point is that if the CPU is very busy with a lot of compute-bound tasks, then it might not be able to keep I/O requests flowing to the SSD. Once the SSD completes a request it (via the SATA interface and the PCIe bus) sends the CPU an interrupt that says "that one's done". It's the CPU's job to then tell the drive what to do next. If the CPU is very busy with other things at that time then there might be a delay before it gets that done, and during that delay the drive will accomplish nothing. A faster CPU or a less busy one will keep the drive busier.

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    n.b.: 6 Gbit/s SATA = SATA 3.0. Since SATA uses 10 bits/byte on the serial channel this does equate to 600 MB/s. Jul 30, 2015 at 20:33

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