Does OS X throttle hard disk performance on the boot drive?

Question

Does OS X throttle hard disk performance on the boot drive?
I was upgrading my boot drive on a Mac Pro (1,1) from the original Seagate 7200.9 250gb drive to a new Western Digital Caviar Black model 1tb drive.
What surprised me was that disk benchmarking tools (both AJ System Test and Xbench) showed that the new drive had a 30% decrease in speed tests (primarily writes) when actually serving as the boot drive. Put back in the original drive, and while not as extreme as the new drive, it also shows slower write speeds while serving as the boot drive vs serving as an extra internal drive.
In Xbench, the writes are both sequential and random unchached writes, AJ System test is less descriptive, but the write speeds on the new drive are painfully slow to watch.
Hardware-wise, I'd think this new WD
Is there something in the OS that impacts this? Is it something that can be tuned or tweaked? I feel like I'm not getting the most bang for my buck on the new drive (or the old for that matter).
That said, the new drive reads more than twice as fast as the old, and I can feel the difference significantly on boot up (login screen appears faster) and all my startup items load up much faster after logging in.

Answer 1

R C-R wrote:
Keep in mind that benchmarks don't really mean much unless test conditions are the same for each test. Benchmark each drive used as the boot drive & you get a relative comparison of their performance as a boot drive.
Which was really all I was after in the first place, and more as a curiosity than anything else, but it was just too unreal to see the drive lose half its read/write speed just because it was the boot drive. I'd expect, and accept, some relative drop in performance, but there seems like there was something wrong.
My problem gets stranger at this point. I was about to give up on using this drive, it was actually painfully slow to use and I'd gone back to my original drive. I thought I'd give it one more chance as part of some external drive enclosure stuff I was doing.
I put the drive back in Bay 1 on my Mac Pro, and it took forever to boot up, and when I ran benchmarks on it, they were only 5-10% of what I'd seen as a scratch drive, yes, that's 5% total. I actually got a score of 5.04 on Xbench. As a scratch drive, the thing gets in the 90's, and as boot drive it was getting 40s to 50s.
Anyways, after avoiding temptation to throw the thing against the wall, I tried putting it into an eSATA external dock, and booted from it. I am typing this from that boot session right now. The system booted up lightening fast, and benchmarks in Xbench are comparable to when it's a scratch drive, and AJA System Test just showed read/write speeds of 98 MB/s. This is what I would expect.
So now my issue appears to be that my Mac Pro, for some reason, is throttling, make that strangling, this particular hard drive when it is placed in Bay 1 and used as a boot drive. Some kind of hardware issue on the internal SATA bus? Incompatibility with this drive and Mac Pros? It's gonna take some more research or testing I guess.
Very frustrating. Reminiscent of my days with Beige Boxes running WindowsXP; certainly not the "it just works" experience I've grown to love since switching a few years ago.
I'll post back when I find out more.
Thanks again for the followup, and interestingly, I didn't get an email notification of an update to the thread. Odd.