rmechaber (5/1/2012)
But I don't understand why a hardware-based RAID controller doesn't simply pipe the same data off to 2 disks simultaneously, reducing the write-time penalty to something much less than 100%? I'd expect there to be some time penalty for the controller to do that, but wouldn't that be significantly less than double the single-disk write time?
RAID controllers do indeed write to multiple drives simultaneously. The issue isn't whether they're serial or parallel - the issue is that we're just asking the drives to do more writes. Let's say we've got ten hard drives, and each of them can sustain 100 writes per second - a total of 1,000 writes per second. If we configure RAID in a manner that requires two writes for every thing SQL Server needs to get to disk, that means we can push 500 transactions per second. (In reality, it's way less than that since we've gotta write both the data and log file, but let's talk theoretically for a second.) If we configure RAID in a way that requires a 6 write penalty instead of 2, that means we can only do 1000/6 = 167 transactions per second.