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Early 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 device for
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your system in two ways.
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The first is to build an initial ramdisk which boots to a minimal userspace
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which configures the device, then pivot_root(8) in to it.
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The second is to create one or more device-mappers using the module parameter
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"dm-mod.create=" through the kernel boot command line argument.
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The format is specified as a string of data separated by commas and optionally
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semi-colons, where:
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- a comma is used to separate fields like name, uuid, flags and table
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(specifies one device)
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- a semi-colon is used to separate devices.
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So the format will look like this::
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dm-mod.create=<name>,<uuid>,<minor>,<flags>,<table>[,<table>+][;<name>,<uuid>,<minor>,<flags>,<table>[,<table>+]+]
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Where::
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<name> ::= The device name.
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<uuid> ::= xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx | ""
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<minor> ::= The device minor number | ""
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<flags> ::= "ro" | "rw"
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<table> ::= <start_sector> <num_sectors> <target_type> <target_args>
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<target_type> ::= "verity" | "linear" | ... (see list below)
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The dm line should be equivalent to the one used by the dmsetup tool with the
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`--concise` argument.
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Target types
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============
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Not all target types are available as there are serious risks in allowing
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activation of certain DM targets without first using userspace tools to check
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the validity of associated metadata.
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======================= =======================================================
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`cache` constrained, userspace should verify cache device
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`crypt` allowed
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`delay` allowed
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`era` constrained, userspace should verify metadata device
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`flakey` constrained, meant for test
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`linear` allowed
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`log-writes` constrained, userspace should verify metadata device
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`mirror` constrained, userspace should verify main/mirror device
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`raid` constrained, userspace should verify metadata device
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`snapshot` constrained, userspace should verify src/dst device
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`snapshot-origin` allowed
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`snapshot-merge` constrained, userspace should verify src/dst device
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`striped` allowed
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`switch` constrained, userspace should verify dev path
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`thin` constrained, requires dm target message from userspace
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`thin-pool` constrained, requires dm target message from userspace
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`verity` allowed
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`writecache` constrained, userspace should verify cache device
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`zero` constrained, not meant for rootfs
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======================= =======================================================
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If the target is not listed above, it is constrained by default (not tested).
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Examples
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========
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An example of booting to a linear array made up of user-mode linux block
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devices::
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dm-mod.create="lroot,,,rw, 0 4096 linear 98:16 0, 4096 4096 linear 98:32 0" root=/dev/dm-0
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This will boot to a rw dm-linear target of 8192 sectors split across two block
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devices identified by their major:minor numbers. After boot, udev will rename
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this target to /dev/mapper/lroot (depending on the rules). No uuid was assigned.
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An example of multiple device-mappers, with the dm-mod.create="..." contents
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is shown here split on multiple lines for readability::
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dm-linear,,1,rw,
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0 32768 linear 8:1 0,
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32768 1024000 linear 8:2 0;
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dm-verity,,3,ro,
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0 1638400 verity 1 /dev/sdc1 /dev/sdc2 4096 4096 204800 1 sha256
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ac87db56303c9c1da433d7209b5a6ef3e4779df141200cbd7c157dcb8dd89c42
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5ebfe87f7df3235b80a117ebc4078e44f55045487ad4a96581d1adb564615b51
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Other examples (per target):
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"crypt"::
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dm-crypt,,8,ro,
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0 1048576 crypt aes-xts-plain64
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babebabebabebabebabebabebabebabebabebabebabebabebabebabebabebabe 0
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/dev/sda 0 1 allow_discards
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"delay"::
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dm-delay,,4,ro,0 409600 delay /dev/sda1 0 500
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"linear"::
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dm-linear,,,rw,
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0 32768 linear /dev/sda1 0,
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32768 1024000 linear /dev/sda2 0,
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1056768 204800 linear /dev/sda3 0,
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1261568 512000 linear /dev/sda4 0
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"snapshot-origin"::
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dm-snap-orig,,4,ro,0 409600 snapshot-origin 8:2
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"striped"::
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dm-striped,,4,ro,0 1638400 striped 4 4096
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/dev/sda1 0 /dev/sda2 0 /dev/sda3 0 /dev/sda4 0
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"verity"::
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dm-verity,,4,ro,
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0 1638400 verity 1 8:1 8:2 4096 4096 204800 1 sha256
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fb1a5a0f00deb908d8b53cb270858975e76cf64105d412ce764225d53b8f3cfd
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51934789604d1b92399c52e7cb149d1b3a1b74bbbcb103b2a0aaacbed5c08584
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