Think twice about tweaking Mac hardware from Windows

A Boot Camp tip circulating around the Web could be bad for your data.

I noticed a tip in several blogs last week about tweaking hard drive memory settings from within Windows when running under Boot Camp. As far as I can see, the tip originates from a blogger named LamboMan from Bristol, England.

His suggestion concerns Boot Camp performance on the Mac Pro workstations. Intel Macs use Extensible Firmware Interface for boot services, rather than the old PC BIOS. However, Windows wants a BIOS, so that's emulated.

According to LamboMan, the current BIOS emulation for the Mac Pro doesn't support some modern memory handling available on hard drives, slowing performance.

But, when it comes to all Mac systems, disk access is very slow on them. This is because data transfers between memory and hard drives are currently done with PIO, or Programmed Input/Output. This is not as fast as the more modern technology known as DMA, or Direct Memory Access. DMA is much faster than PIO. Luckily, there is a fix. If you go into the Windows Device Manager, and look under the advanced settings tab, there will be an option to an Enable DMA.

Now, I would tell you that this is something that you shouldn't do. I won't do this.

However, to make doubly sure, I checked in with a couple of storage engineers who have a lot of experience with Macs and PCs. They both declined attribution.

One engineer, a technical lead for a storage company, said he wasn't "comfortable with the change in the setting."

"As Apple doesn't document well the differences between different Mac hardware implementations, this is one of those things that might work for some, but not for others. Apple has a bad habit of keeping the overall name of the Mac the same, while changing the insides," the engineer explained.

Apple's often revises logic boards or other components of a product or line, with little or no notice given to users or even to developers. The name is the same but there are differences under the hood. Perhaps there's a technical note buried somewhere. Most of the time this is a good thing, which fixes a problem. But a change can lead to erratic performance if you veer off the recommended settings. Sometimes, on some Macs, but not always.

Of course, it's fine to change this setting on a PC. But a Boot Camp Mac isn't a PC through and through. The engineer said that Apple may have a good reason to use programmed I/O.

He said if you must try this out, do it on a machine that doesn't matter to your workflow and with data that you can afford to lose.

Another storage engineer, now a consultant, suggested that Apple's avoidance of DMA for Boot Camp is a sign of a workaround for some intermittent hardware QA issue.

"The odds are that Apple is working around bugs here. I can't say for certain, but that is what usually happens," the consultant said.

He pointed to past problems with some other storage ASICs, such as FireWire and USB controllers. These issues were later resolved with firmware updates for Macs and for drives in external storage systems.

So, if your data is important to you, perhaps stay away from this "performance" tweak.


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