dm-init.rst

  1================================
  2Early creation of mapped devices
  3================================
  4
  5It is possible to configure a device-mapper device to act as the root device for
  6your system in two ways.
  7
  8The first is to build an initial ramdisk which boots to a minimal userspace
  9which configures the device, then pivot_root(8) in to it.
 10
 11The second is to create one or more device-mappers using the module parameter
 12"dm-mod.create=" through the kernel boot command line argument.
 13
 14The format is specified as a string of data separated by commas and optionally
 15semi-colons, where:
 16
 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.
 20
 21So the format will look like this::
 22
 23 dm-mod.create=<name>,<uuid>,<minor>,<flags>,<table>[,<table>+][;<name>,<uuid>,<minor>,<flags>,<table>[,<table>+]+]
 24
 25Where::
 26
 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)
 33
 34The dm line should be equivalent to the one used by the dmsetup tool with the
 35`--concise` argument.
 36
 37Target types
 38============
 39
 40Not all target types are available as there are serious risks in allowing
 41activation of certain DM targets without first using userspace tools to check
 42the validity of associated metadata.
 43
 44======================= =======================================================
 45`cache`			constrained, userspace should verify cache device
 46`crypt`			allowed
 47`delay`			allowed
 48`era`			constrained, userspace should verify metadata device
 49`flakey`		constrained, meant for test
 50`linear`		allowed
 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
 57`striped`		allowed
 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
 61`verity`		allowed
 62`writecache`		constrained, userspace should verify cache device
 63`zero`			constrained, not meant for rootfs
 64======================= =======================================================
 65
 66If the target is not listed above, it is constrained by default (not tested).
 67
 68Examples
 69========
 70An example of booting to a linear array made up of user-mode linux block
 71devices::
 72
 73  dm-mod.create="lroot,,,rw, 0 4096 linear 98:16 0, 4096 4096 linear 98:32 0" root=/dev/dm-0
 74
 75This will boot to a rw dm-linear target of 8192 sectors split across two block
 76devices identified by their major:minor numbers.  After boot, udev will rename
 77this target to /dev/mapper/lroot (depending on the rules). No uuid was assigned.
 78
 79An example of multiple device-mappers, with the dm-mod.create="..." contents
 80is shown here split on multiple lines for readability::
 81
 82  dm-linear,,1,rw,
 83    0 32768 linear 8:1 0,
 84    32768 1024000 linear 8:2 0;
 85  dm-verity,,3,ro,
 86    0 1638400 verity 1 /dev/sdc1 /dev/sdc2 4096 4096 204800 1 sha256
 87    ac87db56303c9c1da433d7209b5a6ef3e4779df141200cbd7c157dcb8dd89c42
 88    5ebfe87f7df3235b80a117ebc4078e44f55045487ad4a96581d1adb564615b51
 89
 90Other examples (per target):
 91
 92"crypt"::
 93
 94  dm-crypt,,8,ro,
 95    0 1048576 crypt aes-xts-plain64
 96    babebabebabebabebabebabebabebabebabebabebabebabebabebabebabebabe 0
 97    /dev/sda 0 1 allow_discards
 98
 99"delay"::
100
101  dm-delay,,4,ro,0 409600 delay /dev/sda1 0 500
102
103"linear"::
104
105  dm-linear,,,rw,
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
110
111"snapshot-origin"::
112
113  dm-snap-orig,,4,ro,0 409600 snapshot-origin 8:2
114
115"striped"::
116
117  dm-striped,,4,ro,0 1638400 striped 4 4096
118  /dev/sda1 0 /dev/sda2 0 /dev/sda3 0 /dev/sda4 0
119
120"verity"::
121
122  dm-verity,,4,ro,
123    0 1638400 verity 1 8:1 8:2 4096 4096 204800 1 sha256
124    fb1a5a0f00deb908d8b53cb270858975e76cf64105d412ce764225d53b8f3cfd
125    51934789604d1b92399c52e7cb149d1b3a1b74bbbcb103b2a0aaacbed5c08584
126
127For setups using device-mapper on top of asynchronously probed block
128devices (MMC, USB, ..), it may be necessary to tell dm-init to
129explicitly wait for them to become available before setting up the
130device-mapper tables. This can be done with the "dm-mod.waitfor="
131module parameter, which takes a list of devices to wait for::
132
133  dm-mod.waitfor=<device1>[,..,<deviceN>]