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Early 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.
The second is to create one or more device-mappers using the module parameter
"dm-mod.create=" through the kernel boot command line argument.
The format is specified as a string of data separated by commas and optionally
semi-colons, where:
- a comma is used to separate fields like name, uuid, flags and table
(specifies one device)
- a semi-colon is used to separate devices.
So the format will look like this::
dm-mod.create=<name>,<uuid>,<minor>,<flags>,<table>[,<table>+][;<name>,<uuid>,<minor>,<flags>,<table>[,<table>+]+]
Where::
<name> ::= The device name.
<uuid> ::= xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx | ""
<minor> ::= The device minor number | ""
<flags> ::= "ro" | "rw"
<table> ::= <start_sector> <num_sectors> <target_type> <target_args>
<target_type> ::= "verity" | "linear" | ... (see list below)
The dm line should be equivalent to the one used by the dmsetup tool with the
`--concise` argument.
Target types
============
Not all target types are available as there are serious risks in allowing
activation of certain DM targets without first using userspace tools to check
the validity of associated metadata.
======================= =======================================================
`cache` constrained, userspace should verify cache device
`crypt` allowed
`delay` allowed
`era` constrained, userspace should verify metadata device
`flakey` constrained, meant for test
`linear` allowed
`log-writes` constrained, userspace should verify metadata device
`mirror` constrained, userspace should verify main/mirror device
`raid` constrained, userspace should verify metadata device
`snapshot` constrained, userspace should verify src/dst device
`snapshot-origin` allowed
`snapshot-merge` constrained, userspace should verify src/dst device
`striped` allowed
`switch` constrained, userspace should verify dev path
`thin` constrained, requires dm target message from userspace
`thin-pool` constrained, requires dm target message from userspace
`verity` allowed
`writecache` constrained, userspace should verify cache device
`zero` constrained, not meant for rootfs
======================= =======================================================
If the target is not listed above, it is constrained by default (not tested).
Examples
========
An example of booting to a linear array made up of user-mode linux block
devices::
dm-mod.create="lroot,,,rw, 0 4096 linear 98:16 0, 4096 4096 linear 98:32 0" root=/dev/dm-0
This 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.
An example of multiple device-mappers, with the dm-mod.create="..." contents
is shown here split on multiple lines for readability::
dm-linear,,1,rw,
0 32768 linear 8:1 0,
32768 1024000 linear 8:2 0;
dm-verity,,3,ro,
0 1638400 verity 1 /dev/sdc1 /dev/sdc2 4096 4096 204800 1 sha256
ac87db56303c9c1da433d7209b5a6ef3e4779df141200cbd7c157dcb8dd89c42
5ebfe87f7df3235b80a117ebc4078e44f55045487ad4a96581d1adb564615b51
Other examples (per target):
"crypt"::
dm-crypt,,8,ro,
0 1048576 crypt aes-xts-plain64
babebabebabebabebabebabebabebabebabebabebabebabebabebabebabebabe 0
/dev/sda 0 1 allow_discards
"delay"::
dm-delay,,4,ro,0 409600 delay /dev/sda1 0 500
"linear"::
dm-linear,,,rw,
0 32768 linear /dev/sda1 0,
32768 1024000 linear /dev/sda2 0,
1056768 204800 linear /dev/sda3 0,
1261568 512000 linear /dev/sda4 0
"snapshot-origin"::
dm-snap-orig,,4,ro,0 409600 snapshot-origin 8:2
"striped"::
dm-striped,,4,ro,0 1638400 striped 4 4096
/dev/sda1 0 /dev/sda2 0 /dev/sda3 0 /dev/sda4 0
"verity"::
dm-verity,,4,ro,
0 1638400 verity 1 8:1 8:2 4096 4096 204800 1 sha256
fb1a5a0f00deb908d8b53cb270858975e76cf64105d412ce764225d53b8f3cfd
51934789604d1b92399c52e7cb149d1b3a1b74bbbcb103b2a0aaacbed5c08584
For setups using device-mapper on top of asynchronously probed block
devices (MMC, USB, ..), it may be necessary to tell dm-init to
explicitly wait for them to become available before setting up the
device-mapper tables. This can be done with the "dm-mod.waitfor="
module parameter, which takes a list of devices to wait for::
dm-mod.waitfor=<device1>[,..,<deviceN>]