How to run RHEL5/Centos5 in Google Cloud

Google cloud instances run on top of KVM hypervisor. In newer KVM versions, such as the one used in Google Cloud, virtio-scsi is used instead of an older virtio-blk storage backend. The virtio-scsi disk devices looks like:

virtio-scsi vendor='Google' product='PersistentDisk' rev='1' type=0 removable=0

In RHEL6, a module is added in 2.6.32-244.el6 kernel. But virtio-scsi is not shipped in RHEL5 or Centos5.

* Tue Feb 28 2012 Aristeu Rozanski <[email protected]> [2.6.32-244.el6]
- [virt] virtio-scsi: add power management (Paolo Bonzini) [782027]
- [virt] virtio-scsi: add driver (Paolo Bonzini) [782027]

Legacy Centos5/RHEL5 servers moved using Lift and Shift to Google Cloud will fail to boot with an error similar to the following:

Unable to access resume device (LABEL=ROOT)
Creating root device.
Mounting root filesystem.
mount: could not find filesystem '/dev/root'
Setting up other filesystems.
Setting up new root fs
setuproot: Kernel panic - not syncing: Attempted to kill init!
moving /dev failed: No such file

Google has graciously backported the kernel code for 2.6.18* kernels. To fix the issue, we need to build and use virtio_scsi driver from backported code.

Build kernel module

Get the backported kernel code to a RHEL5/Centos5 system.

git clone https://github.com/GoogleCloudPlatform/compute-centos5-drivers.git

Get the src package for the kernel version (this might need .redhat.com access)

The following are other locations:
https://ftp.redhat.com/pub/redhat/linux/enterprise/5Server/en/os/SRPMS/
https://vault.centos.org/5.5/centosplus/x86_64/RPMS/

Install the package.

rpm -ivh kernel-2.6.18-194.el5.src.rpm

Build the kernel source code.

cd /usr/src/redhat/SPECS
rpmbuild -bp --target=x86_64 kernel.spec
cd /usr/src/redhat/BUILD/kernel-2.6.18/linux-2.6.18.x86_64
make oldconfig
make prepare
make scripts
make modules

Build the virtio_scsi module.

make M=/root/compute-centos5-drivers-master/third_party/virtio_scsi

Install module in the source system

The easiest way to install the module is to upload the module to Google Cloud Storage and use it to build initrd in the source system. Then the source system could be synced to Google Cloud.

Create a new bucket and upload the module:

# gsutil cp 2.6.18-194.el5-virtio_scsi.ko gs://rhel5test/

In the source system, download and copy the module to the relevant kernel module path:

wget https://storage.googleapis.com/rhel5test/`uname -r`-virtio_scsi.ko
cp `uname -r`-virtio_scsi.ko /lib/modules/`uname -r`/extra/virtio_scsi.ko

Run depmod to generate modules.dep and map files:

depmod -a

Create initrd file using the virtio modules:

mkinitrd -f --with=virtio_pci --with=virtio_scsi  --preload virtio --preload scsi_mod /boot/initrd-gcp-$(uname -r).img $(uname -r)

Update grub.conf with new initrd. For example,

title GCP Red Hat Enterprise Linux Server (2.6.18-194.el5)
        root (hd0,0)
        kernel /vmlinuz-2.6.18-194.el5 ro root=UUID=eff36034-b712-468f-8e6e-0816973a866d rhgb edd=off quiet console=ttyS0
        initrd /initrd-gcp-2.6.18-194.el5.img

Update the default parameter to boot from the new kernel:

default=1

Once the initrd is built and the grub configuration is made, the system can be synced to Google Cloud.

Both RHEL5 and Centos5 have reached End of Life. It is strongly suggested to move them to later operating system releases. The above workaround is a good method to migrate the systems to the cloud until the application is refactored to run in later operating system releases.