Re: [gentoo-user] How to get raid

[Volker Armin Hemmann]

Am Mittwoch, 4. Januar 2012, 22:45:45 schrieb Jeff Cranmer:
> On Thu, 2012-01-05 at 04:01 +0100, Volker Armin Hemmann wrote:
> > the short one:
> >
> > partition one disk with (c)fdisk. Use sfdisk to transfer the partition
> > scheme to the other disks.
> >
> > run mdadm --create /dev/md0 level=whatever you want --raid-
> > devices=thenumberofdevices /dev/sdXY /dev/sdZY ...
> >
> > mdadm --detail --scan >> /etc/mdadm.conf
> >
> > done
>
> OK, but there is active data on the disks, so I don't want to partition
> them.  They should already partitioned, and running fdisk will erase the
> data.

first rule:

always mount a scratch monkey

In your case: always backup data.

There is a way to preserve the data on one disk, create a raid5 with one disk 
missing, then copying the data onto the raid and add the disk. 

But that is high risk stuff.

>
> If I run mdadm --create /dev/md0 level=5
> --raid-devices=3 /dev/sdb /dev/sdc /dev/sdd, will that erase data
> already on the disks?
>
> Prior to running this command, there is no /dev/md entry.  Is this
> correct?

yes. You might have to create the nodes with mknod - my memory is sketchy 
there.


> Looking further by using fdisk, it appears that sdc has a linux
> partition on sdc1 starting at sector 34, and a GPT partition of size 0+
> at /dev/sdc4, sector 0.  Nothing else is on that disk (no sdc2 or sdc3).
>
> sdd and sdb report invalid partition table flags and do not appear to
> have active partitions.  Does this make sense?

if you used fakeraid before, yes. But that means: without the original 
fakeraid everything on that disks is inaccessible... and you need to partition 
them.

>
> Is it possible that I ordered the disks incorrectly when I installed
> them, and by simply swapping disks b and c at the raid I can get things
> to start making sense?  Is there an order to a set of RAID5 disks?  I
> thought any two of three RAID5 disks could be recovered, regardless of
> which one dies?

no.
First, the order of the disks is irrelevant, but the most important thing:

with Raid5 ONE disk out of an array might fail. No matter how many disks - two 
fail and everything is lost.

>
> > there is a reason why I never ever touch genkernel.
> >
> > you should forget that crap. You don't need to copy around anything. If
> > your root is not on some fancy setup, you don't need initramfs.
> >
> > Just make a nice kernel, put it in /boot. Done.
>
> OK.  The OS disk is non-RAID (120GB SSD), so I don't need any fancy
> options in my kernel. All the domdadm and dodmraid stuff is needed just
> when your OS disk is raided.  Correct?

yes


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