================================ 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=,,,,[,
+][;,,,,
[,
+]+] Where:: ::= The device name. ::= xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx | "" ::= The device minor number | "" ::= "ro" | "rw"
::= ::= "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=[,..,]