dm-init.rst (4353B)
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