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How to use a Ramdisk for Linux

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If this document changes, it will be available at http://www.tcu-inc.com/mark/articles/Ramdisk.html .

Index:

Introduction to RamDisk
How to use RamDisk
Changing the size of the ramdisks
Example of how to use a RamDisk for a webserver.
Comments

Introduction to RamDisk

Hello! This is a brief article about how to setup a RamDisk on a RedHat 6.0 system. It should be very similar for other Linux distributions.

What is a RamDisk? A RamDisk is a portion of memory that you allocate to use as a partition. Or, in other words, you are taking memory, pretending to treat it as a hard drive, and you are saving your files to it. Why would you want to use a RamDisk? Well, if you know that certain files you have are constantly going to be used, putting the files into memory will increase the performance of your computer since your memory is faster than your hard drive. Things like web servers with lots of data can be sped up in the this way. Or, if you are insane, and you have a PII 550 Mhz computer with 1 gig of memory and an old 500 meg hard drive, you can use it just to increase your hard drive space. Then again, if you want an almost diskless machine, it might not be that crazy afterall.

Here are some more resources to help you.

http://metalab.unc.edu/LDP/HOWTO/Kernel-HOWTO.html
http://metalab.unc.edu/LDP/HOWTO/mini/LILO.html
/usr/src/linux/Documentation/ramdisk.txt

How to use RamDisk

Well, it is very easy to use a ramdisk. First of all, the default installation of RedHat 6.0 comes with ramdisk support. All you have to do is format a ramdisk and then mount it to a directory. To find out all the ramdisks you have available, do a "ls -al /dev/ram*". This gives you the preset ramdisks available to your liking. These ramdisks don't actually grab memory until you use them somehow (like formatting them). Here is a very simple example of how to use a ramdisk.

mkdir -p /tmp/ramdisk0
mkfs -t ext2 /dev/ram0
mount /dev/ram0 /tmp/ramdisk0

Those three commands will make a directory for the ramdisk to be located at, format a ramdisk (default being 4 megs), and mount the ramdisk to the directory "/tmp/ramdisk0". Now you can treat that directory as a pretend partition! Go ahead and use it like any other directory or as any other partition.

What are some catches? Well, when the computer reboots, it gets wiped. Don't put any data there that isn't copied somewhere else or if it is critical data. If you make changes to that directory, and you need to keep the changes, figure out some way to back them up.

Changing the size of the ramdisks

Well, how do I change the size of the ramdisks? Well, personally, I could only find two ways to change them. Neither of the options will let you change the size of the ramdisks after the computer has started. That sucks.

Here is the hard way first. Look at this file :
/usr/src/linux/drivers/block/rd.c
then edit it, change this line
int rd_size = 4096; /*Size of the ramdisks */
by changing the number to whatever size you want in kilobytes. Then compile the kernel, install the kernel, and reboot the computer.

Okay, now the easy way. Add this line to your lilo.conf file:
ramdisk=10000
and it will make the default ramdisks 10 megs after you type the "lilo" command and reboot the computer. Here is an example of my /etc/lilo.conf file.

boot=/dev/hda
map=/boot/map
install=/boot/boot.b
prompt
timeout=50
image=/boot/vmlinuz-2.2.5-15
        label=linux
        root=/dev/hda2
        read-only
        ramdisk=10000

Actually, I got a little over 9 megs of usable space.

Example of how to use a RamDisk for a webserver.

Okay, here is an example of how to use 3 ramdisks for a webserver. Let us say you are 99% confident that your default installation of Apache for RedHat 6.0 won't use more than 9 megs for its cgi-scripts, html, and icons. Here is how to install one.
First, issue this command to move the real copy of home directory for the webserver to a different place. Also, make the directories to mount the ramdisks to.

mv /home/httpd/ /home/httpd_real
mkdir /home/httpd
mkdir /home/httpd/cgi-bin
mkdir /home/httpd/html
mkdir /home/httpd/icons

Then, add these commands to your /etc/rc.d/rc.local file.


        ### Make the ramdisk partitions
/sbin/mkfs -t ext2 /dev/ram0
/sbin/mkfs -t ext2 /dev/ram1
/sbin/mkfs -t ext2 /dev/ram2

        ### Mount the ramdisks to their appropriate places

mount /dev/ram0 /home/httpd/cgi-bin
mount /dev/ram1 /home/httpd/icons
mount /dev/ram2 /home/httpd/html

        ### Copying real directory to ramdisks
tar -C /home/httpd_real -c . | tar -C /home/httpd -x

        #### Restarting the webserver
/etc/rc.d/init.d/httpd restart

I would reboot your computer, even though you really don't have to if you just issue this command to start the rc.local file again:

/etc/rc.d/rc.local

Comments

Please remember one thing, BACKUP YOUR DATA if you change it and you need it. When the computer reboots, any changes are lost. A cron job should do it. Have it check every 5 minutes and see if any files have changed and backup any changes. There are better ways of doing this, but I won't get into it. Another thing you could do is make your changes to the real directory, and then copy over the changes to the ramdisks. That is much safer.
One thing to note, some motherboards for IBM PC compatible systems only cache memory below 256 megs (or even 128 megs). If you use a lot of ram, take that into consideration.
A cool use of this would be to have a computer with 1 gig of memory and then use 256 megs for "/tmp". If you have lots of processes that use "/tmp", it should help speed up your system. Also, anything in /tmp would get lost when the computer reboots, which can be a good thing.
At first, I tried to use lilo.conf to change the sizes of the ramdisks, but it didn't work. After Rob Funk asked me why I was trying to do it the hard way (recompiling the kernel and such), I tried it again, and it worked. I don't know what I did wrong the first time. Thanks Rob!

Mark Nielsen works for The Computer Underground as a file clerk and as a professional (suit and tie) consultant at 800linux.com. In his spare time, he does volunteer stuff, like writing these documents for The Linux Gazette and linux.com. This document was edited using Nedit and ispell.