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Diffstat (limited to 'Documentation/device-mapper/dm-init.txt')
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diff --git a/Documentation/device-mapper/dm-init.txt b/Documentation/device-mapper/dm-init.txt new file mode 100644 index 000000000000..8464ee7c01b8 --- /dev/null +++ b/Documentation/device-mapper/dm-init.txt @@ -0,0 +1,114 @@ +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 |