1 ================================
2 Early creation of mapped devices
3 ================================
5 It is possible to configure a device-mapper device to act as the root device for
6 your system in two ways.
8 The first is to build an initial ramdisk which boots to a minimal userspace
9 which configures the device, then pivot_root(8) in to it.
11 The second is to create one or more device-mappers using the module parameter
12 "dm-mod.create=" through the kernel boot command line argument.
14 The format is specified as a string of data separated by commas and optionally
17 - a comma is used to separate fields like name, uuid, flags and table
18 (specifies one device)
19 - a semi-colon is used to separate devices.
21 So the format will look like this::
23 dm-mod.create=<name>,<uuid>,<minor>,<flags>,<table>[,<table>+][;<name>,<uuid>,<minor>,<flags>,<table>[,<table>+]+]
27 <name> ::= The device name.
28 <uuid> ::= xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx | ""
29 <minor> ::= The device minor number | ""
30 <flags> ::= "ro" | "rw"
31 <table> ::= <start_sector> <num_sectors> <target_type> <target_args>
32 <target_type> ::= "verity" | "linear" | ... (see list below)
34 The dm line should be equivalent to the one used by the dmsetup tool with the
40 Not all target types are available as there are serious risks in allowing
41 activation of certain DM targets without first using userspace tools to check
42 the validity of associated metadata.
44 ======================= =======================================================
45 `cache` constrained, userspace should verify cache device
48 `era` constrained, userspace should verify metadata device
49 `flakey` constrained, meant for test
51 `log-writes` constrained, userspace should verify metadata device
52 `mirror` constrained, userspace should verify main/mirror device
53 `raid` constrained, userspace should verify metadata device
54 `snapshot` constrained, userspace should verify src/dst device
55 `snapshot-origin` allowed
56 `snapshot-merge` constrained, userspace should verify src/dst device
58 `switch` constrained, userspace should verify dev path
59 `thin` constrained, requires dm target message from userspace
60 `thin-pool` constrained, requires dm target message from userspace
62 `writecache` constrained, userspace should verify cache device
63 `zero` constrained, not meant for rootfs
64 ======================= =======================================================
66 If the target is not listed above, it is constrained by default (not tested).
70 An example of booting to a linear array made up of user-mode linux block
73 dm-mod.create="lroot,,,rw, 0 4096 linear 98:16 0, 4096 4096 linear 98:32 0" root=/dev/dm-0
75 This will boot to a rw dm-linear target of 8192 sectors split across two block
76 devices identified by their major:minor numbers. After boot, udev will rename
77 this target to /dev/mapper/lroot (depending on the rules). No uuid was assigned.
79 An example of multiple device-mappers, with the dm-mod.create="..." contents
80 is shown here split on multiple lines for readability::
84 32768 1024000 linear 8:2 0;
86 0 1638400 verity 1 /dev/sdc1 /dev/sdc2 4096 4096 204800 1 sha256
87 ac87db56303c9c1da433d7209b5a6ef3e4779df141200cbd7c157dcb8dd89c42
88 5ebfe87f7df3235b80a117ebc4078e44f55045487ad4a96581d1adb564615b51
90 Other examples (per target):
95 0 1048576 crypt aes-xts-plain64
96 babebabebabebabebabebabebabebabebabebabebabebabebabebabebabebabe 0
97 /dev/sda 0 1 allow_discards
101 dm-delay,,4,ro,0 409600 delay /dev/sda1 0 500
106 0 32768 linear /dev/sda1 0,
107 32768 1024000 linear /dev/sda2 0,
108 1056768 204800 linear /dev/sda3 0,
109 1261568 512000 linear /dev/sda4 0
113 dm-snap-orig,,4,ro,0 409600 snapshot-origin 8:2
117 dm-striped,,4,ro,0 1638400 striped 4 4096
118 /dev/sda1 0 /dev/sda2 0 /dev/sda3 0 /dev/sda4 0
123 0 1638400 verity 1 8:1 8:2 4096 4096 204800 1 sha256
124 fb1a5a0f00deb908d8b53cb270858975e76cf64105d412ce764225d53b8f3cfd
125 51934789604d1b92399c52e7cb149d1b3a1b74bbbcb103b2a0aaacbed5c08584