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I tried this command to create a ramdisk

sudo hdik -nomount ram://4194304

but it failed with the message hdik: attach failed: error 0x6e = 110

It works only if I decrease the size to about 1 GB. Is there any ramdisk limit in OS X or did I miss something?

Using Mac OS X Mountain Lion with 8 GB RAM.

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It seems there's some kind of limitation when using hdik. You can try using hdiutil to create the ramdisk.

diskutil erasevolume HFS+ "ram disk" `hdiutil attach -nomount ram://4194304`
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Thanks, it works. Maybe one question more. Once the ram disk is created and populated with data, the diskimage-helper process in the Activity Monitor shows more or less the memory usage of ramdisk. Since I use ramdisk as temporary file location, the temporary files are regularly cleaned up. Why does the diskimage-helper however still show the maximal memory usage? Is there any way to "free" the memory hold by diskimage-helper without unmounting it? – elgcom Aug 22 '12 at 21:00

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