On a running Atomic system that you can boot into, you have two options for rollback. By default, atomic keeps the previously booted os tree; it will be available as a secondary bootloader entry. The first option is, while on a physical console, hold down shift to access the syslinux boot menu. While logged into a running system, atomic rollback will swap the bootloader order to the previous tree. The second option is, while logged into a running system, type atomic rollback which will swap the bootloader order to the previous tree.
There may be a time when you have an issue with the Atomic host and need to enter rescue mode to fix an issue. In this article, we will explore how to do just that.
Obtain the media and boot the system.
Boot your system to the Atomic ISO image and follow the instructions to enter rescue mode.
Arrow down to Troubleshooting, hit Enter
Arrow down to Rescue a RHEL Atomic Host system, hit Enter
Choose Continue to allow the installer to find and mount the existing file systems.
Now you should have a prompt.
Configure the environment in rescue mode.
- Activate the logical volumes.
Explore the existing partitions and LVM configuration. You should have home, root and swap set up as logical volumes. The boot partition will be /dev/sda1. Not the attributes before and after activating the volume. After activating the logical volume for root it will have an a attribute. This means it is now active.
# lvm lvs # lvchange -ay rah/root # lvm lvs
- Mount the filesystems to gain access to everything.
This path includes the deployment which contains the root filesystem that we need access to.
# mount /dev/mapper/rah-rhel /mnt/sysimage/
The next step is to mount the /boot/ partition. This article is assuming a default install. However, to confirm where the /boot partition is you can use lsblk and fdisk -l. lsblk will give you a good overview of how the disks are allocated. From that, you can also look at fdisk -l for the parition marked bootable and mount it as follows. In addition, since the root filesystem is mounted and you are chrooted into the proper environment you can confirm the disk UUID is correct by comparing what blkid reports and what’s in /etc/fstab.
# lsblk # cat /etc/fstab # blkid /dev/sda1 # mount /dev/sda1 /boot
After the /boot partition has been mounted, we can mount the rest of the filesystems. Note here that the path includes /mnt/sysimage/ostree/*. The last command uses systemd-nspawn to mount /dev/ and /proc/ appropriately inside the rescue environment.
# systemd-nspawn -D /mnt/sysimage/ostree/deploy/rhel-atomic-host/deploy/<CHECKSUM>/
At this point, you should have a functioning rescue environment.