Created How to use recovery console (markdown)

How-to-use-recovery-console.md

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+## Test Environment
+
+| Term                  | Definition                                       |
+|-----------------------|--------------------------------------------------|
+| RancherOS             |  v1.4.0  |
+| Platform              |  Virtualbox |
+| Root Disk             |  2GB  |
+| CPU                   |  1C   |
+| MEM                   |  2GB  |
+
+In order to demonstrate how to use the revery console, we choose a scene that the disk space is full and the OS cannot start.
+
+## Fill up the disk
+
+Start this VM to check disk usage:
+
+```
+/dev/sda1            ext4            1.8G    567.2M      1.2G  32% /opt
+/dev/sda1            ext4            1.8G    567.2M      1.2G  32% /mnt
+...
+...
+```
+
+Fill the remaining space with `dd`:
+
+```
+$ cd /opt/
+$ dd if=/dev/zero of=2GB.img bs=1M count=2000
+dd: writing '2GB.img': No space left on device
+1304+0 records in
+1302+1 records out
+
+$ ls -ahl
+total 1334036
+drwxr-xr-x    2 root     root        4.0K Jul 19 07:32 .
+drwxr-xr-x    1 root     root        4.0K Jul 19 06:58 ..
+-rw-r--r--    1 root     root        1.3G Jul 19 07:32 2GB.img
+```
+
+At this point you cannot reboot in the OS,  but you can reboot via Virtualbox:
+
+```
+$ shutdown -h now
+Failed to write to log, write /var/log/boot/shutdown.log: no space left on device
+[            ] shutdown:info: Setting shutdown timeout to 60 (rancher.shutdown_timeout set to 60)
+Failed to write to log, write /var/log/boot/shutdown.log: no space left on device
+Failed to write to log, write /var/log/boot/shutdown.log: no space left on device
+.[            ] shutdown:fatal: Error response from daemon: {"message":"mkdir /var/lib/system-docker/overlay2/7c7dffbed40e7b0ed4c68d5630b17a179751643ca7b7a4ac183e48a767071684-init: no space left on device"}
+Failed to write to log, write /var/log/boot/shutdown.log: no space left on device
+```
+
+After rebooting, you will not be able to enter the OS and there will be a kernel panic.
+
+![](https://ws1.sinaimg.cn/mw1024/006tNc79ly1ftf8071p5sj31kw0s14or.jpg)
+
+## Boot with recovery console
+
+When you can access the bootloader, you should select the `Recovery console` and  press `<Tab>` to edit:
+
+![](https://ws3.sinaimg.cn/mw1024/006tNc79ly1ftf7mpir3fj312u0i4a9z.jpg)
+
+You need add `rancher.autologin=tty1` to the end, then press `<Enter>`. If all goes well, you will automatically login to the recovery console.
+
+## How to recover
+
+We need to mount the root disk in the recovery console and delete some data:
+
+```
+$ mkdir /mnt/root-disk
+$ mount /dev/sda1 /mnt/root-disk
+
+# delete data previously generated using dd
+$ ls -ahl /mnt/root-disk/opt
+-rw-r--r--    1 root     root        1.3G Jul 19 07:32 2GB.img
+$ rm -f /mnt/root-disk/opt/2GB.img
+```
+
+After rebooting, you can enter the OS normally.
+
+
+
+