Rackspace Cloud Essentials - Rescue Mode on Linux Cloud Servers
When should I use Rescue Mode?
If your Linux system has become non-bootable or is suffering from critical system errors, you can use Rescue Mode to recover your system data. These problems may be caused by file system corruption, boot file corruption, or other configuration errors. Normally, if your system encounters any problem during the boot process, you would boot in to a maintenance mode environment known as Single User Mode that would allow you to login with your root password and check for any errors. Unfortunately, using Single User Mode has its share of problems:
Your system is read-only and you cannot make corrective changes.
- Most services such as networking are disabled. This would prevent you from copying your data to another server.
- You would have to access your server using the Console, which is slower than using a traditional SSH login.
To avoid having to use Single User Mode, you can bring your server up in Rescue Mode through the Cloud Control Panel.
What is Rescue mode?
Rescue mode grants the root user full access to your non-bootable server’s filesystem. You can use it to modify problems in configuration files or to copy data from your Cloud Server to a remote location. Rescue Mode through the Cloud Control Panel is similar to booting into single-user mode with networking enabled.
Getting your server into Rescue mode
- Log in to the Cloud Control Panel and select Servers at the top.
- In the list of servers, select the action cog next to the desired server and then select Enter Rescue Mode.
- Once you click Enter Rescue Mode, read the information on the pop-up window, and then press the button marked Enter Rescue Mode.
- Copy the temporary password. You can use an SSH client to connect to your server using the public IP address and this temporary root password to login to Rescue Mode.
- Click on Dismiss Password.
- The green bar to the left of the server name will turn yellow during the process of building into Rescue Mode and then red when the process has completed. This may take several minutes.
Connecting to your server in Rescue Mode
- Once your server has entered rescue mode, you can use an SSH client to connect to your server using the public IP address and the temporary root password to login to Rescue Mode.
Troubleshooting your server in Rescue Mode
- Before you can access the files on your server you'll need to mount the server's file system. To do that you'll need to look at your partitions to determine your file system's device. NOTE: If you plan on using fsck on this filesystem, DO NOT MOUNT the fiesystem. Once you've logged into your server in Rescue Mode, run the command:
You'll get output that looks similar to what is below:
- Look at the different disk names that are found. A disk entry looks like:
Disk /dev/sdb1: 2147 MB
- This shows us the device and the size of the disk. Here is a description of the different disks in this screenshot:
- The first block, the one with the size of about 2GB, is the rescue mode filesystem.
- The second block, the one in the screenshot with a size of 10.2GB, is the server's file system. Its size will be different depending on the size of your server, of course.
- If a third block is displayed, it is your swap volume (only on older cloud servers).
- Once you've identified the block for your server's file system check out that part after "Disk" that looks like a file path. In the example above, the device is:
- That could also be "/dev/sdb1" or "/dev/xvdb1". It can be different depending on the distribution image used to build your server. Now that you know your file system's device you can assign it a directory and mount it for access. Plug your file system device into the following command in place of "/dev/diskdevice":
mount /dev/diskdevice /mnt
- For example, for /dev/sda1 the command would be:
mount /dev/sda1 /mnt
- Now you can access your files through the /mnt directory. Just remember that you'll need to put "/mnt" in front of the usual paths you'd use to get to files. For example, if you have a problem in the /etc/fstab file you need to fix, you'd actually access that file at:
- If you were to just edit "/etc/fstab" while in rescue mode you'd change the fstab for the rescue mode file system, not your normal file system.
Exiting Rescue Mode
- That’s it! Once you are done troubleshooting your system, you can exit Rescue Mode by clicking the link labeled Exit Rescue Mode in the Cloud Control Panel under your Server Details page.
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