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What is AHCI? How does it improve SSD speed?


So you finally decided to address that huge bottleneck in your computer. Upgrading to an SSD for your boot drive. But when you finally get up and running, something seems a little off. Maybe your file transfers are slower than you expected.

Welcome to AHCI

Underwhelming SSD performance could be caused by a somewhat, obscure setting in your UEFI BIOS. You see most modern hard drives or SSDs can be set to operate in either IDE mode or AHCI mode. But what in the heck does that mean, well there are multiple ways your system has of talking to drives. IDE or parallel ATA is the standard that was used for a long time. Back when we were all using those huge gray ribbon cables to connect hard drives to motherboards. Drives with the newer smaller SATA or serial ATA connectors, that most of us use today can also utilize a newer standard called advanced host controller interface or AHCI.

AHCI enables cool features like hot swapping, so you don’t have to turn off your computer if you want to unplug an external drive from an E-SATA port. Or use that super neato hot swap bay at the top of your case. A less obvious but also the important advantage is support for native command queuing (NCQ). Which affects the way that your drives handle multiple requests who fetch data at one time. On hard drives using NCQ can mean the drives read and write head, doesn’t have to move around as much to get a certain piece of information. Speeding up your access times, but how does AHCI also help speed up SSDs.

Well although the benefits of NCQ on SSDs are small or negligible for home users. Many users do report greatly increased performance especially for large file transfers in AHCI mode. Compared to IDE mode due to AHCI just plain being designed with more modern drives in mind.

Step aside for NVM-e

Samsung 960 Evo NVM-e SSD

Samsung 960 Evo NVM-e SSD

So then why would you ever want to use IDE mode? Can we just get rid of that option? Well if you’re running an older operating system, such as Windows XP or earlier. There’s a good chance that it won’t have native AHCI support, meaning it won’t play nicely with your drives. If your motherboard is set to AHCI mode. Unless you do some software trickery when you install the OS. This might also be the case if you’re trying to run software from a bootable USB stick or an optical disk. And if you have a boot drive that you’ve been using an IDE mode but then switch over to AHCI, You might get a boot error. Forcing you to switch back to IDE. Though fortunately if you’re using Windows there’s a registry tweak that you can use to fix this more permanently.

Of course, AHCI isn’t the end game for drive protocols and the sexy new one nvm Express is starting to gain popularity. While you won’t be able to use it with garden-variety SATA drives. It’s being implemented in high-performance PCI Express SSDs. Since NVM-e was designed from the ground up for solid-state storage. It can enable speeds several times faster than what you’d get with AHCI on a standard SATA interface. So, you might want to look into those if you’ve got some money to burn and milliseconds to count when you’re trying to frantically open Google Chrome.