Merge pull request #1776 from deitch/external-disk-docs

Add docs on how to use external disk

docs/external-disk.md

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+# 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. This depends on your run method.
+
+**Note:** With all of the options below, *always* give disk options before the _prefix_.
+
+### Mac HyperKit
+`linuxkit run hyperkit -disk <path_to_disk> -disk-size <size> <prefix>`
+
+* `-disk-size <size>`: size of disk in MB, e.g. `-disk-size 4096` will provide a 4GB disk.
+* `-disk <path>`: use the disk at location _path_, e.g. `-disk foo.img` will use the disk at `$PWD/foo.img`
+
+Several important points to note:
+
+1. if you do not provide `-disk `_path_, `linuxkit` assumes the default to be `$PWD/`_prefix_`-disk.img`.
+2. If the disk at `<path>`, or the default if `-disk` option is not provided, does not exist, `linuxkit` will create one of size `<size>`.
+
+### QEMU
+`linuxkit run qemu -disk <path_to_disk> -disk-size <size> <prefix>`
+
+* `-disk-size <size>`: size of disk in MB, e.g. `-disk-size 4096` will provide a 4GB disk.
+* `-disk <path>`: use the disk at location _path_, e.g. `-disk foo.img` will use the disk at `$PWD/foo.img`
+
+Several important points to note:
+
+1. if you do not provide `-disk `_path_, `linuxkit` assumes the default to be `$PWD/`_prefix_`-disk.img`.
+2. If the disk at `<path>`, or the default if `-disk` option is not provided, does not exist, `linuxkit` will create one of size `<size>`.
+
+### vmware
+`linuxkit run vmware -disk <path_to_disk> <prefix>`
+
+* `-disk <path>`: use the disk at location _path_, e.g. `-disk foo.img` will use the disk at `$PWD/foo.img`
+
+Unlike with qemu and hyperkit, `linuxkit run vmware` _currently_ will not create the disk for you if it does not exist. We intend to align the functionality in the near future. 
+
+## 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:a16f2bd94a83dd0cea4d490f710567a0cc60be33"
+    binds:
+     - /dev:/dev
+    capabilities:
+     - CAP_SYS_ADMIN
+     - CAP_MKNOD
+  - name: mount
+    image: "linuxkit/mount:ad138d252798d9d0d6779f7f4d35b7fbcbbeefb9"
+    binds:
+     - /dev:/dev
+     - /var:/var:rshared,rbind
+    capabilities:
+     - CAP_SYS_ADMIN
+    rootfsPropagation: shared
+    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.
+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`
+
+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`.