Dear Aunt TUAW: Can I boot from a RAM disk?
Dear Aunt TUAW,
I would like to run Mac OS X 10.6.7 off a RAM disk, for better performance/security while browsing the World Wide Web (I don't want to save the image with all those gnarly cookie crumbs to disk).
I understand that a family living down the street from me is able to run Linux from a RAM disk -- can you help me out?
Your loving niece,
Auntie immediately consulted with Unkie Mike after receiving your letter. He responded as follows:
Running OS X off a RAM disk isn't supported (and hasn't been on the Mac for a long, long time -- since Mac OS 9, in fact). Linux is a different beast.
You can run a browser in private or 'incognito' mode if you're concerned about security, or use a virtualization tool like Parallels or VMware to run a virtual Mac on your Mac. However, to run 10.6.7 virtualized you'd need the server version (expensive).
You can get most of the performance benefits of a RAM disk with an SSD boot drive, which we've covered on the site.
Further, Unkie uncovered the following information from a post on the MacRumors forum. Attributed to a "former Apple Employee," this seems to describe why RAM disks are a poor match to OS X.
That thing is snake oil, as are the vast majority of ramdisk products on OS X. Designing a ramdisk that works well on OS X is remarkably difficult because backing a block device into wired memory causes the contents to be double buffered above it in the Unified Buffer Cache.
For transient data on systems with no VM pressure, what will happen is the file is created, the object backing it sits in the UBC, it might get synched to disk, but unless you are running low on ram it stays in ram as well. That is why you see no speed increases, all you are doing is eliminating the background asynch writeout, and you are wasting a lot of ram to do it. More importantly, if you are actually creating a wired ram disk you are eating a ton of kernel address space which can be an issue if you have a lot of memory (large page tables) or several video cards.
Getting back to your actual issue, it sounds to Auntie as if you're looking for a secure browsing solution rather than a RAM disk boot solution. For that, take a look at this Macworld hints post that shows you how to store your Safari cache on a RAM drive.
Hope this helps.
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