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298f4aab32
These are not needed, but we are inconsistent. Been waiting for a quiet moment to fix this since I noticed while doing a presentation... Signed-off-by: Justin Cormack <justin.cormack@docker.com>
64 lines
3.0 KiB
Markdown
64 lines
3.0 KiB
Markdown
# External Disk
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`linuxkit run` has the ability to mount an external disk when booting. It involves two steps:
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1. Make the disk available as a device
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2. Mount the disk
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## Make Disk Available
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In order to make the disk available, you need to tell `linuxkit` where the disk file or block device is.
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All local `linuxkit run` methods (currently `hyperkit`, `qemu`, and `vmware`) take a `-disk` argument:
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* `-disk path,size=100M,format=qcow2`. For size the default is in GB but an `M` can be appended to specify sizes in MB. The format can be omitted for the platform default, and is only useful on `qemu` at present.
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If a _path_ is specified `linuxkit` will use the disk at location _path_. If you do not provide `-disk ` _path_, `linuxkit` assumes a default path, which is _prefix_`-state/disk.img`.
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If the disk at the specified or default _path_ does not exist, `linuxkit` will create one of size _size_.
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The `-disk` specification may be repeated for multiple disks, although a limited number may be supported, and some platforms currently only support a single disk.
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**TODO:** GCP
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## Mount the Disk
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A disk created or used via `hyperkit run` will be available inside the image at `/dev/vda` with the first partition at `/dev/vda1`.
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In order to use the disk, you need to do several steps to make it available:
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1. Create a partition table if it does not have one.
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2. Create a filesystem if it does not have one.
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3. `fsck` the filesystem.
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4. Mount it.
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To simplify the process, two `onboot` images are available for you to use:
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1. `format`, which:
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* checks for a partition table and creates one if necessary
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* checks for a filesystem on the partition and creates one if necessary
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* runs `fsck` on the filesystem
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2. `mount` which mounts the filesystem to a provided path
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```yml
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onboot:
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- name: format
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image: linuxkit/format:84a997e69051a1bf05b7c1926ab785bb07932954
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- name: mount
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image: linuxkit/mount:b24bd97ae43397b469dbaadd80f17f291c817bdf
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command: ["/mount.sh", "/var/external"]
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```
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Notice several key points:
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1. format container
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* The format container needs to have bind mounts for `/dev`
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* The format container needs `CAP_SYS_ADMIN` and `CAP_MKNOD` capabilities
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* The format container only needs to run **once**, not matter how many external disks or partitions are provided. It finds all block devices under `/dev` and processes them.
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* The default container config should be sufficient
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2. mount container
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* The mount container `command` is `mount.sh` followed by the desired mount point. Remember that nearly everything in a linuxkit image is read-only except under `/var`, so mount it there.
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* The mount container needs to have bind mounts for `/dev` and `/var`
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* The mount container needs `CAP_SYS_ADMIN` capabilities
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* The mount container needs `rootfsPropagation: shared`
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* The default container config should be sufficient, though the `mount.sh` command needs to be specified
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With the above in place, if run with the current disk options, the image will make the external disk available as `/dev/vda1` and mount it at `/var/external`.
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