Re: Booting from RAID1

[Michael Evans <mjevans1983@gmail.com>]

On Sun, Dec 20, 2009 at 7:37 AM, Leslie Rhorer <lrhorer@satx.rr.com> wrote:
>> >> Oh, and you'll also very likely want grub to read the /boot
>> >> filesystem, which is why it must be on a partition followed by the
>> >> raid header, instead of a partition containing a raid header and raid
>> >> protected partition.  That use is OK since grub operates read-only.
>> >
>> >        I think I follow you, here.  IOW, partition the drive, create the
>> md
>> > target, and then format the RAID array, right?  Or are you saying one
>> should
>> > format the partition and then create the RAID array on top of it?
>> >
>> >
>>
>> It works much more cleanly if you create the RAID array first, and
>> then use the container it provides;
>
>        Now I definitely don't follow you.  Are you saying one should create
> an array from the raw drive, partition it, format the first partition, then
> create secondary arrays from the second and third partition, and finally
> format the second and third array?
>
>> this way the end of your file-system does not overlap the raid super-
>> block.
>
>        I can follow that - there's a danger of wiping or moving the mdadm
> superblock if it is contained in the filesystem, but it seems to me
> partitioning the drive and then creating the array from a partition, as I
> first suggested, will accomplish that just fine.
>
>

It is an option to partition it after the fact; however generally it's
better to partition it first, and then apply whatever raid level you
like to each partition.

You misunderstand on the second half; the danger is that both the
'filesystem' and 'mdadm superblock' attempt to use the whole device.
That may appear to work for a while, but there is danger that the
filesystem could grow and store data in the same place the mdadm
superblock is; depending on if the filesystem or mdadm then filesystem
within the device was used this will either fail, or destroy the raid
information block.
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