CENTOS - RAMDISK

Ramdisk is very good to have if you want something to stay in memory. Files in memory makes it so you can access them with out having to access hard drive all the time. Perfect candidates would be things which do not change eg. web images or downloadable files, etc. If you have Linux Kernel 2.4 or later, you already have support of ramdisk built in. You can check if ramdisk is setup by doing:

# dmesg | grep RAMDISK
RAMDISK driver initialized: 16 RAM disks of 16384K size 1024 blocksize

You should get above output on CentOS and RHEL. Other linux flavors will have similar output as well. If you would like to see how they are named and what you would need to refer to, do the following:

# ls -l /dev/ram*
lrwxrwxrwx 1 root root 4 Apr 24 12:05 /dev/ram -> ram1
brw-rw---- 1 root disk 1, 0 Apr 24 12:05 /dev/ram0
brw-rw---- 1 root disk 1, 1 Apr 24 12:05 /dev/ram1
brw-rw---- 1 root disk 1, 10 Apr 24 12:05 /dev/ram10
brw-rw---- 1 root disk 1, 11 Apr 24 12:05 /dev/ram11
brw-rw---- 1 root disk 1, 12 Apr 24 12:05 /dev/ram12
brw-rw---- 1 root disk 1, 13 Apr 24 12:05 /dev/ram13
brw-rw---- 1 root disk 1, 14 Apr 24 12:05 /dev/ram14
brw-rw---- 1 root disk 1, 15 Apr 24 12:05 /dev/ram15
brw-rw---- 1 root disk 1, 2 Apr 24 12:05 /dev/ram2
brw-rw---- 1 root disk 1, 3 Apr 24 12:05 /dev/ram3
brw-rw---- 1 root disk 1, 4 Apr 24 12:05 /dev/ram4
brw-rw---- 1 root disk 1, 5 Apr 24 12:05 /dev/ram5
brw-rw---- 1 root disk 1, 6 Apr 24 12:05 /dev/ram6
brw-rw---- 1 root disk 1, 7 Apr 24 12:05 /dev/ram7
brw-rw---- 1 root disk 1, 8 Apr 24 12:05 /dev/ram8
brw-rw---- 1 root disk 1, 9 Apr 24 12:05 /dev/ram9
lrwxrwxrwx 1 root root 4 Apr 24 12:05 /dev/ramdisk -> ram0

All those ramdisks listed have same size. In above example, they are all 16MB. Let us change that so we have more space allowed. Note that I say allowed and not allocated. We allocate space in one of the later steps by formatting one of the drives above. Let us set it up so we have 128 MB. Since this has to be in multiples of 1024, we will setup Ramdisk to have 131072K.

vi /etc/grub.conf

Find first line which looks similar to following:

kernel /vmlinuz-2.6.9-42.0.10.EL ro root=/dev/VolGroup00/LogVol00

add ramdisk_size=131072 to the end of the line. Now your line should look like:

kernel /vmlinuz-2.6.9-42.0.10.EL ro root=/dev/VolGroup00/LogVol00 ramdisk_size=131072 Save and exit grub.conf. At this point you have it configured to have ramdisk with new size but it does not take effect until you reboot your system. Once you have rebooted your system, we can start doing rest of configurations.

mke2fs -m 0 /dev/ram0

This will format the ram0 ramdrive for us to use. At this point, kernel will allocate space for you. Let us setup Ramdisk mount point so we can use it. We will also have it be owned by user “sunny” so that user can read/write to that mount.

mkdir /home/ramdisk
mount /dev/ram0 /home/ramdisk
chown sunny.sunny /home/ramdisk

At this point you should be able to type: mount and see your new Ramdisk drive mounted on /home/ramdisk

Remember that everything you put on this drive will be gone if you reboot your server. If you unmounted the Ramdisk drive and remounted it, your files will still be there. It is because your system has that much ram set aside for your Ramdisk and will not use it for anything else. If you would like to setup Ramdisk the same next time you boot up, add these lines to your /etc/rc.local files.

mke2fs -m 0 /dev/ram0
mount /dev/ram0 /home/ramdisk
chown sunny.sunny /home/ramdisk

Source from : http://crazytoon.com/2007/06/01/ramdisk-how-do-you-install-and-set-up-ramdisk-under-linux-centos-rhel-fedora/