Boot time creation of mapped devices
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===================================
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It is possible to configure a device mapper device to act as the root
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device for your system in two ways.
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The first is to build an initial ramdisk which boots to a minimal
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userspace which configures the device, then pivot_root(8) in to it.
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For simple device mapper configurations, it is possible to boot directly
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using the following kernel command line:
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dm="<name> <uuid> <ro>,table line 1,...,table line n"
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name = the name to associate with the device
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after boot, udev, if used, will use that name to label
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the device node.
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uuid = may be 'none' or the UUID desired for the device.
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ro = may be "ro" or "rw". If "ro", the device and device table will be
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marked read-only.
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Each table line may be as normal when using the dmsetup tool except for
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two variations:
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1. Any use of commas will be interpreted as a newline
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2. Quotation marks cannot be escaped and cannot be used without
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terminating the dm= argument.
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Unless renamed by udev, the device node created will be dm-0 as the
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first minor number for the device-mapper is used during early creation.
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Example
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=======
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- Booting to a linear array made up of user-mode linux block devices:
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dm="lroot none 0, 0 4096 linear 98:16 0, 4096 4096 linear 98:32 0" \
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root=/dev/dm-0
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Will boot to a rw dm-linear target of 8192 sectors split across two
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block devices identified by their major:minor numbers. After boot, udev
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will rename this target to /dev/mapper/lroot (depending on the rules).
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No uuid was assigned.
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