Published on: October 8, 2014 by George K.
Scenario:
Create ramdisk in Linux lets you directly store files in physical memory as if it was on your hard drive. RAM disks use the normal RAM in main memory as if it were a partition on a hard drive rather than actually accessing the data bus normally used for secondary storage such as hard disk
See below 20GB of the RAM memory has been used for RAM memory.
Output of df -h and free memory usage/availability
[root@a1 ~]# df -h Filesystem Size Used Avail Use% Mounted on /dev/sda3 1016G 41G 925G 5% / /dev/sda2 508M 37M 447M 8% /boot /dev/sda5 5.0G 33M 5.0G 1% /tmp /dev/sda6 1.8T 197M 1.7T 1% /home /dev/ram0 20G 15G 3.7G 81% /opt/bglens/ramcache
[root@a1~]# free -g total used free shared buffers cached Mem: 62 51 10 0 0 29 -/+ buffers/cache: 21 41 Swap: 7 0 7
Advantages of saving files in a ramdisk, rather than in a HD is accessing memory is faster than accessing a HD. (HD = HardDrive).
Major disadvantage is, since RAM memory is volatile, you will lose the data, when you reboot the system.
Below are the steps to create ramdisk of 20G in the above server.
Please make sure that there is a minimum of 24 GB memory in the server to give 20G as ramdisk. Here in above situation server had 64G.
Commands to create RAMDisk
# mkfs -q -i 8192 /dev/ram0 20971520 # mount /dev/ram0 /opt/bglens/ramcache # cp -rf /opt/bglens/data/posting.local/* /opt/bglens/ramcache
Here, there is a high chance that whenever you type the mkfs command as in above , you may get an output/error similar to below
Error for mkfs
mkfs.ext2: Filesystem larger than apparent device size Proceed anyway? (y,n)
If you give ‘n’, it will exit and you cant proceed with second step. But even if you say ‘y’, you won’t create it.
In such case, you would need to edit the boot loader configuration (usually grub.conf) and append the ‘ramdisk_size=20971520‘ to the kernel line. And reboot it. Here is a sample grub.conf
Also, please note that if we unmount the ram disk, it will still reserve RAM and won’t release it for system’s normal use until you reboot the system. It will get flagged so that the Linux kernel will not try to reuse the memory again. If you remount the ramdisk, your data will still be there and that RAM space, cannot be used by other application/systems
To use memory as storage and still allow the kernel to reuse the memory, you can use ramfs or the newer tmpfs. This is another option to ramdisk. It allocate memory dynamically and by allowing less-used pages to be moved onto swap space.
Example of a tmpfs mounting a directory
mount -t tmpfs tmpfs /dir
if you don’t fill up the space in the tmpfs, the kernel gets to use the free RAM for buffers and cache. If you delete a file, the kernel will reclaim the ram, and if you umount the directory, all data is lost.
Category : Howtos, Linux
George started his career in web hosting and Linux technical support in the year 2004 and is with SupportSages since 2009. He has keen interest in server optimizations, custom security solutions, hacked server recovery, cyber forensic and high availability fail over system design and implementation. George loves long drives and is passionate about art and literature.
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