RAM Disks

The Linux kernel needs to mount a root file system before it can execute any processes. For rescue diskettes and CD's it is often quite natural to load the root file system into a RAM disk at boot time and mount the root file system on the RAM disk. This has the following advantages:

• The RAM disk can be stored compressed on the disk, so more programs can be stored on a diskette.
• The diskette or CD can be removed from the drive after the RAM disk is loaded, freeing the drive for other purposes, such as restoring backups or loading other utilities.
• Programs load faster from a RAM disk than from a diskette.

Living without a RAM disk

For a Linux installation on a hard disk it is quite natural not to use a RAM disk, but for a Linux installation that is booted from a diskette (without depending on a root file system elsewhere) there is only one valid reason not to use a RAM disk: you don't have enough RAM for the RAM disk. As a rule of thumb, if you have less than 6-8MB of RAM. you will probably need to boot without a RAM disk, though it can be achieved with 4MB if you really strip things down.

If you boot from a diskette without a RAM disk, with the root file system on a diskette, the kernel will always prompt for an extra diskette, so the kernel and the root file systems can live on separate diskettes. The root file system cannot be compressed. It is of course possible to put both on a single diskette. In that case the boot diskette contains the root file system and in that root file system the kernel lives, as would be the case with most hard disk installations. On the boot diskette you need to install a boot loader that can live inside that file system, which rules out SYSLINUX, but both GRUB and LILO should do.

Installing Linux on a machine from diskette without a RAM disk is a tricky process, which involves the following steps:

• Boot from a diskette without a RAM disk. The diskette may not be removed from the drive.
• Partition the hard disk, initialize the swap partition with mkswap and create a file system on the hard disk.
• Copy the contents of the root file system from the diskette to the hard disk.
• Reboot and select the root device on the hard disk (that you had just copied). This time the diskette drive is free.
• Copy all your data to the hard disk partition.

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.

Loading a RAM disk at boot time

The feature to load a RAM disk from a diskette at boot time has existed almost from the beginning (it existed in 1992). Since then, some features have been added or modified. It is possible to load a RAM disk image from the boot diskette at a specific offset and to load it from a different diskette.

There are two ways to specify the RAM disk parameters.

• Use the rdev command to set the boot parameters. See section 3.2 for details.
• Set the ramdisk_start, load_ramdisk and prompt_ramdisk parameters from the kernel command line. This is how to do it in LILO:

  root=/dev/fd0
  append ramdisk_start=0 load_ramdisk=1 prompt_ramdisk=1
  

It is possible to put a file system on the first part of the diskette (and install LILO there) and to use the rest of the diskette for the compressed RAM disk image. Use a size argument to the mkfs command to specify the number of blocks for the file system.

It is also possible to put a (non-compressed) file system on the boot diskette and load the entire boot diskette as a RAM disk at boot time. Even the kernel, the LILO second stage boot loader and the map file would be loaded into the RAM disk. Advantage is that the same diskette can be used with or without a RAM disk.

The initial RAM disk

The initial RAM disk is loaded by the boot loader. There are two uses for the initial RAM disk:

• The primary purpose of the initial RAM disk is mostly of interest to maintainers of Linux distributions. The initial RAM disk makes it possible to install a minimal kernel image on the hard disk without the file system or hard disk device drivers compiled in. At boot time these drivers are loaded as modules, not from the hard disk, but from an initial RAM disk. After doing that, the kernel would switch to the real root device (the hard disk). Without an initial RAM disk you would need the hard disk drivers (there are many possible SCSI host adapters) and the file system drivers compiled into the kernel, so you either have a huge kernel or many different kernels to choose from.

  Such an initial RAM disk needs a file system with the following:

  • A /dev directory with some essential devices.
  • Some directories for mount points.
  • A shell with some binaries. The busybox program would be all that is needed.
  • The required kernel modules.
  • A startup script that loads the required kernel modules.
• The initial RAM disk can also be used for rescue, install or special-purpose diskettes, much as you would use the other type of RAM disk. You simply do not use the feature that you can switch back to the real root device. This is the type of diskette described in my ``Getting Linux into Small Machines'' article.

There is no reason why you should not be able to install an initial RAM disk from a second diskette, but most boot loaders do not support this. This is a limitation of the boot loader, not of Linux. Using the pause command, GRUB can do this, as the following part of a configuration file shows:

title Linux with RAM disk on separate diskette
kernel (fd0)/boot/vmlinuz 
pause Insert the second diskette.
initrd (fd0)/boot/initrd.gz