by Mark Nielsen
About the author:
The author (home page) works at The Computer Underground, Inc. as a file clerk and as a professional consultant at 800linux.com. In his spare time, he does volunteer stuff, like writing these documents. This document was edited using Nedit and ispell.
This article shows how to use RAM as a virtual harddisk.
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 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.
# create a mount point: mkdir /tmp/ramdisk0 # create a filesystem: mke2fs /dev/ram0 # mount the ramdisk: mount /dev/ram0 /tmp/ramdisk0Those three commands will make a directory for the ramdisk , format the ramdisk (create a filesystem), 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.
The default size of the ramdisk is 4Mb=4096 blocks. You saw what
ramdisk size you got while you were running mke2fs.
should have produced a message like this:
mke2fs 1.14, 9-Jan-1999 for EXT2 FS 0.5b, 95/08/09 Linux ext2 filesystem format Filesystem label= 1024 inodes, 4096 blocks 204 blocks (4.98%) reserved for the super user First data block=1 Block size=1024 (log=0) Fragment size=1024 (log=0) 1 block group 8192 blocks per group, 8192 fragments per group 1024 inodes per groupRunning
df -k /dev/ram0 tells you how much of that you can really use
(The filesystem takes also some space):
>df -k /dev/ram0 Filesystem 1k-blocks Used Available Use% Mounted on /dev/ram0 3963 13 3746 0% /tmp/ramdisk0
What are some catches? Well, when the computer reboots, it gets wiped. Don't put any data there that isn't copied somewhere else. If you make changes to that directory, and you need to keep the changes, figure out some way to back them up.
Compiling the ramdisk a loadable module has the advantage that you can
decide at load time what the size of your ramdisks should be.
Okay, first the hard way. Add this line to your lilo.conf file:
ramdisk_size=10000 (or ramdisk=10000 for old kernels)
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 label=linux root=/dev/hda2 read-only ramdisk_size=10000Actually, I got a little over 9 megs of usable space as the filesystem takes also a little space.
When you compile ramdisk support as loadable module then you can decide at load time what the size should be. This is done either with an option line in the /etc/conf.modules file:
options rd rd_size=10000or as a command line parameter to ismod:
insmod rd rd_size=10000Here is an example which shows how to use the module:
umount /tmp/ramdisk0 .
insmod rd rd_size=20000
mount /dev/ram0 /tmp/ramdisk0
mv /home/httpd/ /home/httpd_real mkdir /home/httpd mkdir /home/httpd/cgi-bin mkdir /home/httpd/html mkdir /home/httpd/iconsThen, add these commands to the start procedure in your /etc/rc.d/init.d/httpd.init (or where ever the httpd gets started on your system):
### 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 (the ### data on the ramdisks is lost after a reboot) tar -C /home/httpd_real -c . | tar -C /home/httpd -x ### After this you can start the web-server.
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