summaryrefslogtreecommitdiff
path: root/Documentation/device-mapper/dm-init.rst
diff options
context:
space:
mode:
Diffstat (limited to 'Documentation/device-mapper/dm-init.rst')
-rw-r--r--Documentation/device-mapper/dm-init.rst125
1 files changed, 125 insertions, 0 deletions
diff --git a/Documentation/device-mapper/dm-init.rst b/Documentation/device-mapper/dm-init.rst
new file mode 100644
index 000000000000..e5242ff17e9b
--- /dev/null
+++ b/Documentation/device-mapper/dm-init.rst
@@ -0,0 +1,125 @@
+================================
+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