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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:
vroot,,,ro,
0 1740800 verity 254:0 254:0 1740800 sha1
76e9be054b15884a9fa85973e9cb274c93afadb6
5b3549d54d6c7a3837b9b81ed72e49463a64c03680c47835bef94d768e5646fe;
vram,,,rw,
0 32768 linear 1:0 0,
32768 32768 linear 1:1 0
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
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