RAM disk devices

A RAM disk is just another block device, except that the buffers are never written to a real device, so the data exists only in RAM. The standard kernel comes with 16 RAM disk devices. Initially no memory is allocated to a RAM disk, but buffers will be added dynamically as data is written to it. The maximum size of a RAM disk is 4MB by default and can be modified at boot time with the ramdisk_size command line option. In LILO you could add the following command to the configuration file to double the size:

append ramdisk_size=8192

Note that this specifies a maximum size, smaller RAM disks take less space.

See the kernel source file drivers/block/rd.c how the RAM disk is initialized. There are three possibilities:

• A RAM disk is not initialized at boot time by default. On a system that was booted without a RAM disk, you can put data into the RAM disk block device and mount it. The following commands create an ext2 file system and mount that:

  mke2fs /dev/ram2 2880
  mount /dev/ram2 /mnt
  

• A RAM disk can be loaded at boot time from a device. In this case the kernel checks if the data is a valid gzip data stream or a valid file system (only a few types allowed). Compressed RAM disk images are decompressed by the kernel.
• A RAM disk can be loaded at boot time from the memory area where the boot loader put the initial RAM disk image. It is loaded in almost the same way (the read routines pull the data from memory instead of a device), but the main initialization code will treat the initial RAM disk differently with regard to mounting.