Re: [gentoo-user] duplicate HD drives

[Joseph]

On 09/13/14 08:07, Kerin Millar wrote:
>On 13/09/2014 04:17, Joseph wrote:
>> On 09/12/14 23:52, Neil Bothwick wrote:
>>> On Fri, 12 Sep 2014 15:53:19 -0600, Joseph wrote:
>>>
>>>> I have two identical HD in a box and want to duplicate sda to sdb
>>>> I want sdb to be bootable just in case something happens to sda so I
>>>> can swap the drives and boot.
>>>>
>>>> Do I boot from USB and run:
>>>> dd if=/dev/sda of=/dev/sdb bs=512 count=1
>>>
>>> If you remove the cunt argument as already mentioned, this will copy the
>>> whole drive, but it will be incredibly slow unless you add bs=4k. It also
>>> only copies it once, as soon as you start using sda, sdb will be out of
>>> date. Set up a RAID-1 array with the two drives, then install GRUB to the
>>> boot sector of each drive, using grub2-install and you will always be
>>> able to boot in the event of a failure of either drive.
>>>
>>>
>>> --
>>> Neil Bothwick
>>
>> I'll be interested in setting up RAID-1. Is it hard?
>> I've never done it and I know there is plenty of information on line
>> about RAID-1
>>
>> I'm not going to grub2 anytime soon.  This machine has BIOS and the HD
>> has MBR partition.
>> With recent problem I had with my other older box (that has BIOS) and
>> grub2 I'm not going to play with it.
>>
>> Is it hard to set it UP RAID-1
>
>No, it is not. However, to keep things simple, observe the following:
>
>   * create the array with the --metadata=0 option (using mdadm)
>   * mark the partitions belonging to the array as type FD
>   * enable CONFIG_MD_AUTODETECT in the kernel
>
>Doing so will ensure two things. Firstly, that the legacy version of
>grub is able to read the kernel. Unlike grub2, it does not intrinsically
>understand RAID. Using the original metadata format prevents that from
>being an issue; grub can be pointed to just one of the two partitions
>belonging to a RAID-1 array and read its filesystem.
>
>Secondly, using the original metadata format means that, once the kernel
>has loaded, it is able to assemble the array by itself. Therefore, you
>may have your root filesystem on the array and mount it without having
>to use an initramfs.
>
>In terms of partitioning, you could just create one big partition on
>each drive, join them into an array, and make that the basis of a root
>filesystem. As much as Gentoo has enshrined the concept, a dedicated
>boot filesystem is simply not necessary and swap can be created as a
>file. Alternatively, you could follow the handbook style and create
>three arrays for boot, swap and root.
>
>There is a trick to achieving bootloader redundancy. Let's say that you
>have set up array /dev/md0, with /dev/sda1 and /dev/sdb1 as its members,
>and that /dev/md0 contains a singular root filesystem. In the grub
>shell, one would run these commands:
>
>   grub> device (hd0) /dev/sda
>   grub> root (hd0,0)
>   grub> setup (hd0)
>   grub> device (hd0) /dev/sdb
>   grub> root (hd0,0)
>   grub> setup (hd0)
>
>The magic here is that the bootloader will still be able to function,
>even if a disk is removed or broken.
>
>Finally, even though your disks are not exactly the same size, it does
>not matter. If there is a discrepancy among the devices that mdadm is
>given to create an array with, it will size the array according to the
>lowest common denominator. If you prefer, you can manually ensure that
>the partitions are the exact same size on both disks.
>
>--Kerin

If I do:
fdisk /dev/sda
t 1 fd

Won't it destroy data on /dev/sda?

-- 
Joseph