115 lines
4.0 KiB
Plaintext
115 lines
4.0 KiB
Plaintext
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
|