# External Disk
`linuxkit run` has the ability to mount an external disk when booting. It involves two steps:

1. Make the disk available as a device
2. Mount the disk

## Make Disk Available
In order to make the disk available, you need to tell `linuxkit` where the disk file or block device is.

All local `linuxkit run` methods (currently `hyperkit`, `qemu`, and `vmware`) take a `-disk` argument:

* `-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.

If the _path` is specified it will use the disk at location _path_, if you do not provide `-disk `_path_, `linuxkit` assumes a default, which is _prefix_`-state/disk.img` for `hyperkit` and `vmware` and _prefix_`-disk.img` for `qemu`. 

If the disk at the specified or default `<path>` does not exist, `linuxkit` will create one of size `<size>`.

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.

**TODO:** GCP

## Mount the Disk
A disk created or used via `hyperkit run` will be available inside the image at `/dev/vda` with the first partition at `/dev/vda1`.

In order to use the disk, you need to do several steps to make it available:

1. Create a partition table if it does not have one.
2. Create a filesystem if it does not have one.
3. `fsck` the filesystem.
4. Mount it.

To simplify the process, two `onboot` images are available for you to use:

1. `format`, which:
    * checks for a partition table and creates one if necessary
    * checks for a filesystem on the partition and creates one if necessary
    * runs `fsck` on the filesystem
2. `mount` which mounts the filesystem to a provided path

```yml
onboot:
  - name: format
    image: "linuxkit/format:ba085fdcac31c383acee3b4b91d78eb7095e5ac3"
  - name: mount
    image: "linuxkit/mount:fe22dc5cbf109b4637b1caaafc76ccbf5140c3da"
    command: ["/mount.sh", "/var/external"]
```

Notice several key points:

1. format container
    * The format container needs to have bind mounts for `/dev`
    * The format container needs `CAP_SYS_ADMIN` and `CAP_MKNOD` capabilities
    * 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.
    * The default container config should be sufficient
2. mount container
    * 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.
    * The mount container needs to have bind mounts for `/dev` and `/var`
    * The mount container needs `CAP_SYS_ADMIN` capabilities
    * The mount container needs `rootfsPropagation: shared`
    * The default container config should be sufficient, though the `mount.sh` command needs to be specified

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`.