Optimize Performance for Mac or VM?
Many of our users are probably familiar with the setting under VMware Fusion Preferences that lets users determine if they want to optimize for virtual machine disk performance, or optimize for performance of their Mac OS applications.
The way this works is that, by default, VMware Fusion “optimizes for virtual machine disk performance” with the tradeoff of using more of your Mac’s memory. Conversely, if you “Optimize for Mac OS application performance” VMware Fusion uses less of your Mac’s memory, but can decrease performance of your virtual machines—a tradeoff that some users are willing to take.
Apple Issue with Unbuffered IO
“Optimizing for Mac OS applications performance” works via enabling what is called “unbuffered IO.” The problem is that there is an Apple problem with unbuffered IO that can cause your virtual machine, or your entire OS, to hang at some points. If this occurs, your virtual machine disk can get corrupted, which, of course, we want to avoid at all costs.
Apple has now resolved this bug with Mac OS X 10.5.5. Feel free to update to Mac OS X 10.5.5 as appropriate to workaround this problem.