boot.txt

Boot time creation of mapped devices
===================================

It is possible to configure a device mapper device to act as the root
device for your system in two ways.

The first is to build an initial ramdisk which boots to a minimal
userspace which configures the device, then pivot_root(8) in to it.

For simple device mapper configurations, it is possible to boot directly
using the following kernel command line:

dm="<name> <uuid> <ro>,table line 1,...,table line n"

name = the name to associate with the device
	after boot, udev, if used, will use that name to label
	the device node.
uuid = may be 'none' or the UUID desired for the device.
ro = may be "ro" or "rw".  If "ro", the device and device table will be
	marked read-only.

Each table line may be as normal when using the dmsetup tool except for
two variations:
1. Any use of commas will be interpreted as a newline
2. Quotation marks cannot be escaped and cannot be used without
   terminating the dm= argument.

Unless renamed by udev, the device node created will be dm-0 as the
first minor number for the device-mapper is used during early creation.

Example
=======

- Booting to a linear array made up of user-mode linux block devices:

  dm="lroot none 0, 0 4096 linear 98:16 0, 4096 4096 linear 98:32 0" \
  root=/dev/dm-0

Will boot to a rw dm-linear target of 8192 sectors split across two
block devices identified by their major:minor numbers.  After boot, udev
will rename this target to /dev/mapper/lroot (depending on the rules).
No uuid was assigned.