A lot of SSDs on the market connect to the motherboard via PCI-E and have read/write speeds of less than 1000 Mb/s. Why don't they use SATA 6Gb/s? I assumed from the name that it supports speeds up to 6 Gb/s...
Am I missing some limitations SATA has?
There's a few pieces that combine to make PCI-E a better alternative.
When you access a file over SATA, it can either read or write, but not both at the same time while PCI-E can perform several operations simultaneously. Imagine reading a log file, adding a new line and saving it to the drive. Add more services that are writing logs and it doesn't scale well over SATA.
The second is latency, usually for high performance computing, like Hoov said. Data sent to a drive must pass through a SATA controller, then over to the drive where the SSD controller formats it correctly for the drive. Using PCI-E removes a step as the SATA Controller is pulled from the line and the processor is talking straight to the SSD controller.
In my understanding, because of a more direct connection to the CPU PCI-Express SSDs can have less latency. A SATA controller would be connected to PCI-Express and is another layer introducing its overhead. That might be one reason. Another reason for shorter latency might be related to protocols which are better at handling the high IO/s loads of SSDs. SATA was designed when SSD weren't as common as now.
