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I'm using Ubuntu 10.04(64bit) for my desktop.
The machine has a 5GB RAM.

I want to use RAM disk(1G or 2G) but I don't know how can I do this.

Is there any opensource product of RAM disk?

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1 Answer 1

up vote 16 down vote accepted

The technology is built into the kernel, you don't need any extra tools. In fact, you already have a few RAM disks (which you shouldn't use, they're reserved for the system), which you can see by doing

grep -w tmpfs /proc/mounts

To set up a 2GB RAM disk mounted on /ramdisk, add the following line to /etc/fstab:

ramdisk  /ramdisk  tmpfs  mode=1777,size=2g

Then mount the disk with the command mount /ramdisk (this will be done automatically when you reboot).

The indicated size is a maximum, the disk only uses as much memory as the files that are on it.


You can change /tmp to be a RAM disk. In the /etc/fstab line above, put /tmp rather than /ramdisk, then reboot.

The first time you reboot after changing /tmp to be a RAM disk, the files that were in /tmp will be hidden. That's harmless, except that they're wasting a little disk space. You can clean them up (after you've rebooted with /tmp on the RAM disk) by doing

mount --bind / /mnt
rm -r /mnt/tmp/* /mnt/tmp/.*
umount /mnt

The mount --bind command makes /mnt a duplicate view of your root filesystem; but while the RAM disk now obscures /tmp on the root view, nothing is obscuring /mnt/tmp.

ADDED: You can switch /tmp to a RAM disk without rebooting, it's just a little more complicated. Add the line to /etc/fstab as above, then run the following commands:

mkdir /tmp.old
mount --bind /tmp /tmp.old
mount /tmp
cd /tmp
ln -s /tmp.old/* /tmp/.* .

Then delete /tmp.old after your next reboot.

The reason you can't just move files from /tmp.old to /tmp is that some critical programs have files open in /tmp, for example /tmp/.X11-unix/X0 which the X server listens on and every GUI program opens when it starts. Moving a file to a different filesystem means copying it and deleting the old one, so you would end up with the X server still listening on /tmp.old/.X11-unix/X0 but X clients contacting /tmp/.X11-unix/X0 in vain. On a server, you might get away with a move if you're careful.

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@giles: Is there a way to do this without change to fstab and rebooting? I'm assuming there's a corresponding mkfs method, followed by a mount but "man mkfs" wasn't very helpful. –  hotei Aug 14 '10 at 16:30
1  
@hotei: The reboot is only to switch /tmp over to tmpfs. It's not necessary, and I've edited my answer to explain how to do it, but it's more complicated. There's no mkfs involved because there's no underlying storage to prepare, the kernel handles everything. –  Gilles Aug 14 '10 at 18:43
    
@giles: Thanks for update. I was missing the part where "magic happens" during mount's reading of the the fstab line. I believe I understand why it works now. –  hotei Aug 15 '10 at 16:12
    
Can I ask if this results in a noticeable speedup? –  Robert Massaioli Oct 28 '10 at 21:55
    
@Robert: I think it did cause a noticeable speed up on my netbook's very slow SSD (mainly because syncs can last several seconds and /tmp on the disk increases the frequency of syncs), but this is an unreliable subjective impression. I've never tried to benchmark. –  Gilles Oct 28 '10 at 22:21

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