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How to Create and Use a RAM Disk

Last updated 2010-01-19 - Send Feedback
Related: how-to, macOS, memory, Photoshop

In Mac OS X Snow Leopard 10.6, the 2.1GB limit on RAM disk size has been eliminated: any size RAM disk can be created, at least using the 64-bit kernel.

Just as important, 8GB memory modules are now available from OWC, which allows up to 64GB of memory to be used in an 8-core Mac Pro Nehalem, or 32GB in a quad-core model.

Speed of a RAM disk

In theory, a RAM disk should be ultra-fast.

Testing with DiskTester shows that in Mac OS X 10.6.2, a RAM disk can be written at ~1400MB/sec and read at about 2100MB/sec. That’s far faster than anything but the gnarliest RAID array, but is actually only a fraction of RAM speed. Still, it’s way more than fast enough, hardly any program can use even 1/4 that speed.

When a RAM disk is appropriate

You should also have at least 16GB of memory if you want to use a RAM disk.

A RAM disk is appropriate only for temporary working space (scratch). See caveats below.

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RAM disk caveats (read first!)

There are some problematic issues to understand with RAM disks that really undermine the usefulness and benefits, at least on Mac OS X 10.6.x.

However, if you work within the limitations, there are still benefits to be had. Please see Optimizing Photoshop — Using a RAM Disk.

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Try a RAM disk yourself

Download the diglloyd script if you’d like to “play” (download available to any diglloydTools customer).

You should be comfortable using Terminal. Do not email for support on this, you’re on your own, unless you want to pay for consulting help.

The script is named crd (“create ram disk”).

Example

Caution: don’t create a RAM disk larger than your system memory (less 2GB), or you’ll have severe virtual memory paging.

Create a 16GB (16384MB) RamDisk1:

llcMP:diglloyd-crd lloyd$ ./crd 16384 RamDisk1
Copyright 2010, DIGLLOYD INC. All Rights Reserved
See our performance tools at https://macperformanceguide.com/Software-Main.html
This script is designed for Mac OS X Snow Leopard 10.6.x only!.
Do not use it with OS X Leopard (10.5) or earlier.
...
Creating 16384 MB ram disk named RamDisk1
hdid -nomount ram://33554432
New block device: /dev/disk12
Creating volume named: RamDisk1
Initialized /dev/rdisk12 as a 16 GB HFS Plus volume
Mounting in /Volumes/RamDisk1
Volume RamDisk1 on /dev/disk12 mounted
RamDisk 'RamDisk1' created, 16384MB

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