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Sunday, July 06, 2008

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Repairing Linux Using the Install System

I recently applied service to my Redhat 7.2 Linux/390 system, and I neglected to run zipl afterward to point to the new kernel. The kernel would not boot, I didn't have a backup, and my other Linux machines were older SUSE versions with incompatible disk layouts.

I did, however, still have my initial installation files, the initrd, kernel and parm files. This describes how I used those files to repair my Linux system.

My broken Linux used the following disk layout:

200 (dasda) - /

201 (dasdb) - swap

202 (dasdc) - /usr

203 (dasdd) - /home

1. Boot the Linux install system. For me this was running REDHAT EXEC on my Linux userid under VM.
   redhat
   For a system running in an LPAR, this could be done by IPLing from a tape drive.
2. Follow the install instructions and SSH in, but do not run 'loader'.
3. At this point, you have a functioning Linux, but the disks for the broken version are not accessible. To make them accessible, you have to make device nodes for each disk you need to access. Enter:
   mknod -m 660 /dev/dasda b 94 0
   mknod -m 660 /dev/dasda1 b 94 1
   mknod -m 660 /dev/dasdc b 94 8
   mknod -m 660 /dev/dasdc1 b 94 9
4. You will probably have to load the DASD drivers for your disks
   insmod dasd_mod dasd=200,202
   insmod dasd_eckd_mod
5. Now the file systems can be mounted.
   mount /dev/dasda1 /mnt
   mount /dev/dasdc1 /mnt/usr
6. Change the active root directory from the install system to the broken system.
   /mnt/usr/sbin/chroot /mnt /bin/bash
   cd /boot
7. Edit the zipl configuration file to point to the correct kernel version using your favourite editor.
   the /etc/zipl.conf
8. Run zipl to rewrite the boot files to the boot disk.
   /sbin/zipl
9. Get out of the chroot environment
   exit
10. Unmount your file systems so that they will be "clean" when you reboot
    cd /
    umount /mnt/usr /mnt
11. The install system will not shutdown, so just get a CP prompt on your 3270 session, and boot your fixed Linux. It should now come up normally.
    For a system running in an LPAR, this would be done by re-IPLing from the HMC.

A big thanks to Neale Ferguson, Mark Post, Guillaume Morin and, last but not least, Vic Cross for their help in fixing my Linux and refining this procedure.