Grub and MBR manipulation question
Posted by OuldeFauder on 2011/11/19 4:17:34
I have a machine that I would like to use for testing. Elrepo kernels etc. To do this I will use several drives on the same machine, using DD (dcfldd) to reinstall any system that gets messed up. Replication the complete drive with DD causes it to boot the drive that you IF from and if that is a failed system your only option is to reinstall Centos as the MBR is copied from the source drive which points back to the original source drive. I have tried changes to grub but I always end up with booting the failed the system. My bios allows for selecting the drive to boot from but the MBR for that drive which is the drive which has been DD'ed from the source by DD so if you boot sdb it will actually end up running sda, the failed system. Grub lists the drives as HD0, HD1, etc but Centos disk utility list drives as sda etc. What is the correlation? Can this be done with grub or is the MBR so completely separate that this is not possible? Can the MBR be changed without the use of assembler, C++ etc? I would like to "DD if =/dev/sda of=/dev/sdb bs 4096 conv=notrun,noerror" and if a problem ocuurs after changes to sda boot sdb and do" DD if /dev/sdb of=//dev/sda bs=4096 conv=notrun,noerror" after telling my bios to boot sdb which I can do from the bios boot menu. In other words, I boot to sda and get a failure, I reboot to sdb and from Centos run DD, then boot to a now working sda. getinfo.sh disk as follows.
Information for disk problems.
Information for disk problems.
== BEGIN uname -rmi ==
2.6.32-71.29.1.el6.x86_64 x86_64 x86_64
== END uname -rmi ==
== BEGIN rpm -qa \*-release\* ==
rpmforge-release-0.5.2-2.el6.rf.x86_64
centos-release-6-0.el6.centos.5.x86_64
elrepo-release-6-4.el6.elrepo.noarch
adobe-release-x86_64-1.0-1.noarch
== END rpm -qa \*-release\* ==
== BEGIN cat /etc/redhat-release ==
CentOS Linux release 6.0 (Final)
== END cat /etc/redhat-release ==
== BEGIN getenforce ==
Enforcing
== END getenforce ==
== BEGIN free -m ==
total used free shared buffers cached
Mem: 1878 689 1188 0 29 298
-/+ buffers/cache: 362 1515
Swap: 3775 0 3775
== END free -m ==
== BEGIN cat /etc/fstab ==
#
# /etc/fstab
# Created by anaconda on Thu Nov 17 22:08:04 2011
#
# Accessible filesystems, by reference, are maintained under '/dev/disk'
# See man pages fstab(5), findfs(8), mount(8) and/or blkid(8) for more info
#
/dev/mapper/vg_alice-lv_root / ext4 defaults 1 1
UUID=f03b9fef-ff56-4377-b43c-bac3520bf4e0 /boot ext4 defaults 1 2
/dev/mapper/vg_alice-lv_home /home ext4 defaults 1 2
/dev/mapper/vg_alice-lv_swap swap swap defaults 0 0
tmpfs /dev/shm tmpfs defaults 0 0
devpts /dev/pts devpts gid=5,mode=620 0 0
sysfs /sys sysfs defaults 0 0
proc /proc proc defaults 0 0
== END cat /etc/fstab ==
== BEGIN df -h ==
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/vg_alice-lv_root
50G 4.2G 43G 9% /
tmpfs 940M 100K 939M 1% /dev/shm
/dev/sdc1 485M 50M 411M 11% /boot
/dev/mapper/vg_alice-lv_home
21G 172M 19G 1% /home
== END df -h ==
== BEGIN fdisk -l ==
Disk /dev/sda doesn't contain a valid partition table
Disk /dev/dm-0 doesn't contain a valid partition table
Disk /dev/dm-1 doesn't contain a valid partition table
Disk /dev/dm-2 doesn't contain a valid partition table
Disk /dev/sda: 799.9 GB, 799937658880 bytes
255 heads, 63 sectors/track, 97253 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000
Disk /dev/sdb: 250.1 GB, 250059350016 bytes
91 heads, 10 sectors/track, 536700 cylinders
Units = cylinders of 910 * 512 = 465920 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x0001589b
Device Boot Start End Blocks Id System
/dev/sdb1 * 3 536700 244197376 7 HPFS/NTFS
Disk /dev/sdc: 80.0 GB, 80026361856 bytes
255 heads, 63 sectors/track, 9729 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000d406e
Device Boot Start End Blocks Id System
/dev/sdc1 * 1 64 512000 83 Linux
Partition 1 does not end on cylinder boundary.
/dev/sdc2 64 9730 77637632 8e Linux LVM
Disk /dev/sdd: 320.1 GB, 320072933376 bytes
255 heads, 63 sectors/track, 38913 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00084381
Device Boot Start End Blocks Id System
/dev/sdd1 * 1 64 512000 83 Linux
Partition 1 does not end on cylinder boundary.
/dev/sdd2 64 38914 312057856 8e Linux LVM
Partition 2 does not end on cylinder boundary.
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