Re: [gentoo-user] duplicate HD drives

[Alan McKinnon]

On 13/09/2014 18:31, Joseph wrote:
> 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?


No.



-- 
Alan McKinnon
alan.mckinnon@×××××.com