Migrating a Linux boot partition to RAID-1

• The disk might really have failed. See if you can access the disk using cfdisk.
• The partition that you added might not be the same as the device listed in /etc/raidtab One common mistake is to omit the partition number when adding the device. For example, if this is your /etc/raidtab:

  raiddev                /dev/md0
  raid-level             1
  nr-raid-disks          2
  nr-spare-disks         0
  persistent-superblock  1
  device                 /dev/hda1
  raid-disk              0
  device                 /dev/hdc1
  raid-disk              1

  you may have run /sbin/raidhotadd /dev/md0 /dev/hdc instead of /sbin/raidhotadd /dev/md0 /dev/hdc1
  See if you can add the correct partition by running the latter command. If it fails then you will need to re-partition the physical disk (/dev/hdc) and then re-run raidhotadd with the correct parameters. The correct partition information can be determined by running cfdisk /dev/hda A reboot may be neccessary before running these commands.

You can determine the correct size for the RAID partition by doing the following:

• Run cfdisk /dev/hda where /dev/hda is your new disk.
• Type uEnter to change the unit to sectors.
• Type pEnter to display the partition table
• Make a note of the size of a sector in bytes.
• Type qEnter to exit the program
• Run cfdisk /dev/hdc where /dev/hdc is your old disk.
• Type uEnter to change the unit to sectors.
• Type pEnter to display the partition table
• Make a note of the total number of sectors on the disk as well as the size of a sector in bytes.
• Type qEnter to exit the program
• Determine the size (in sectors) to use for the RAID partitions on the old and new drives.
  • If the sector size is the same on the old and new drives then the partition size on both disks is the same:
    partition_size_new   =   disk_size_old - 1
    partition_size_old   =   disk_size_old - 1
    For example, if disk_size_old=10000, sector_size_old=512 and sector_size_new=512 then:
    partition_size_new   =   10000 - 1   =   9999
    partition_size_old   =   10000 - 1   =   9999
  • If the sector size of the old drive is larger than the the sector size on the new drive then the partition size on the new disk will be larger than the total number of sectors on the old disk:

    partition_sizenew   =	( disk_sizeold - 1 )	(	sector_sizeold	)
    			sector_sizenew

    partition_size_old   =   disk_size_old - 1
    For example, if disk_size_old=10000, sector_size_old=1024 and sector_size_new=512 then:
    

    partition_sizenew   =	( 10000 - 1 )	(	1024	)	=   ( 9999 ) ( 2 )   =   19998
    			512

    partition_size_old   =   10000 - 1   =   9999
    Please note that since I do not have drives with differing sector sizes, I was unable to test this configuration. Proceed at your own risk (and let me know if it works correctly).
  • If the sector size of the old drive is smaller than the the sector size of the new drive then the partition size on the new drive will be smaller than the partition size of the old disk:

    partition_sizenew   =   floor	(	( disk_sizeold - 1 )	(	sector_sizeold	)	)	- 1
    				sector_sizenew

    partition_sizeold   =	(	( partition_sizenew + 1 )	(	sector_sizenew	)	)	- 1
    				sector_sizeold

    For example, if disk_size_old=10000, sector_size_old=512 and sector_size_new=1024 then:
    

    partition_sizenew   =   floor	(	( 10000 - 1 )	(	512	)	)	- 1	=   floor( 9999 × 0.5 ) - 1	=   floor( 4999.5 ) - 1	=   4999 - 1	=   4998
    				1024

    partition_sizeold   =	(	( 4998 + 1 )	(	1024	)	)	- 1	=   ( 4999 × 2 ) - 1	=   9998 - 1	=   9997
    				512

    Please note that since I do not have drives with differing sector sizes, I was unable to test this configuration. Proceed at your own risk (and let me know if it works correctly).
• Follow the main instructions for setting up the array with the following deviations:
  • When you use fdisk to set up the RAID partition on the new drive, enter 1 for the start sector of the partition and partition_size_new as the end sector.
  • After you have set up the array and you can boot off it, but before you run raidhotadd to add the old drive to the array, you will need to re-partition the old drive:
    • Clear the partition table on the old drive by running dd if=/dev/zero of=/dev/hdc bs=512 count=1 where /dev/hdc is the old drive.
      You may wish to make a backup of the partition table before deleting it. You can do this by running dd if=/dev/hdc of=/root/hdc_partition_table bs=512 count=1 where /dev/hdc is the old drive.
    • Run fdisk /dev/hdc where /dev/hdc is the old drive.
    • Type pEnter to view the partition table. You should see no partitions listed.
    • Type uEnter to change the units to sectors.
    • Type cEnter to leave DOS compatibility mode.
    • Type nEnter to create a new partition.
    • Type pEnter to select a primary partition.
    • Type 1Enter to select the first partition.
    • Type 1Enter to set the first sector of the partition.
    • Enter partition_size_new and then hit Enter to set the last sector of the partition.
    • Type wEnter to write the partition table to disk and exit the program.
  • Add partition to array using raidhotadd as per the main instructions.