X-Basics

Why OS X Doesn't Need a RAM Disk

Dan Knight - 2002.02.01

As noted elsewhere today, I've been using ramBunctious, a classic Mac OS RAM disk program, for years. Although it works in Classic mode under OS X, the RAM disks it creates are inaccessible to X, so I've retired a real workhorse program.

I like the idea of using a RAM disk, because these are files I may be accessing many times during the course of the day, but now I've learned that Mac OS X may make RAM disks a thing of the past. That's because X can use any available system memory to cache files - which helps explain why Classic under X had a higher disk score than Mac OS 9.2.2 all by itself in yesterday's tests.

As Ric Davis explained to me, X will intelligently cache files - the more often you access them, the more likely they'll be cached for lightning-fast access. Although that won't give the same speedy load times the first time a file is accessed, repeated access should be at pretty much RAM disk speed.

Benefits of not using a RAM disk include not locking up all that memory (128 MB in my case), not spending several seconds loading the RAM disk image at startup, and not having to worry about the computer crashing with unsaved contents on the RAM disk (which shouldn't be a problem with X anyhow).

That said, Gregory Youngs writes that all disk writes are cached under BSD/OS X, which means that a freeze, lockup, or power failure could cost you "saved" data that's only been cached and not yet written to the hard drive. Youngs further notes that the Unix sync command, which forces the OS to write all cached data to disk, doesn't appear to be available under OS X.

With the stability of Unix and a UPS (or portable), you minimize the chance of losing unwritten cached data, but there's still a risk. (It's probably less of a risk than we had with the regular freezes, bombs, and other problems that required a forced restart under the classic Mac OS).

Perhaps best of all, under OS X you don't have to fiddle with disk cache size for optimum performance - it's all done automatically by the OS.

I'd still like to see a RAM disk like ramBunctious (they're working on it), even if only to see if it's worth trying, but with caching this good, a RAM disk might not make any sense at all.

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