Setting up fakeraid with mdadm / dmraid

Setting up fakeraid with mdadm / dmraid

[Christoph Pleger]

Hello,

I have a machine with an LSI Megaraid Sofware RAID fakeraid, which uses
ddf format. About two weeks ago, when I wanted to install the machine, I
configured a RAID 1 array in the BIOS config utility and then booted the
machine from PXE with an NFSROOT. mdadm and dmraid are installed in the
NFSROOT, dracut is used for initrd generation. As dracut prefers mdadm
over dmraid, mdadm was chosen for RAID management.

But though mdadm detected the RAID (it created container device /dev/md127
and raid device /dev/md126), it destroyed it - that is, though I did not
perform any action on the raid disks, at the next boot, the BIOS RAID
utility had "forgotten" about the RAID configuration it had created
before.

Why are the possible reasons for that?

Regards
  Christoph

Re: Setting up fakeraid with mdadm / dmraid

[John Stoffel]

Christoph> I have a machine with an LSI Megaraid Sofware RAID
Christoph> fakeraid, which uses ddf format. About two weeks ago, when
Christoph> I wanted to install the machine, I configured a RAID 1
Christoph> array in the BIOS config utility and then booted the
Christoph> machine from PXE with an NFSROOT. mdadm and dmraid are
Christoph> installed in the NFSROOT, dracut is used for initrd
Christoph> generation. As dracut prefers mdadm over dmraid, mdadm was
Christoph> chosen for RAID management.

First off, I would strongly suggest that you turn off the FakeRAID
entirely, just expose both disks to the OS and have the OS do the
mirroring.  FakeRAID was designed when CPUs were much slower, but RAID
HW ASICs were also expensive.  So you got the worst of both worlds!
*grin*

It's also almost entirely black magic to support these things, since
the vendors don't generally share the details on the on-disk format
that I'm aware of.  I'm probably wrong in the details.

But again, please just use the controller in JBOD (Just a Bunch of
Disks) mode, and let mdadm mirror the disks for you.  

Christoph> But though mdadm detected the RAID (it created container
Christoph> device /dev/md127 and raid device /dev/md126), it destroyed
Christoph> it - that is, though I did not perform any action on the
Christoph> raid disks, at the next boot, the BIOS RAID utility had
Christoph> "forgotten" about the RAID configuration it had created
Christoph> before.

Christoph> Why are the possible reasons for that?

Crappy implementation of semi-secret on-disk format?   It could be
that mdadm over-wrote something on the disk, or you partitioned it and
then overwrote something the FakeRAID wanted kept around.

In general, just don't go the fake RAID route, it's not worth the
hassle.

John

Re: Setting up fakeraid with mdadm / dmraid

[Jes Sorensen]

"Christoph Pleger" <Christoph.Pleger@cs.tu-dortmund.de> writes:
> Hello,
>
> I have a machine with an LSI Megaraid Sofware RAID fakeraid, which uses
> ddf format. About two weeks ago, when I wanted to install the machine, I
> configured a RAID 1 array in the BIOS config utility and then booted the
> machine from PXE with an NFSROOT. mdadm and dmraid are installed in the
> NFSROOT, dracut is used for initrd generation. As dracut prefers mdadm
> over dmraid, mdadm was chosen for RAID management.
>
> But though mdadm detected the RAID (it created container device /dev/md127
> and raid device /dev/md126), it destroyed it - that is, though I did not
> perform any action on the raid disks, at the next boot, the BIOS RAID
> utility had "forgotten" about the RAID configuration it had created
> before.
>
> Why are the possible reasons for that?

The LSI uses DDF and requires mdmon etc. to be launched. It is not clear
from your posting what you created, but you would have to update dracut
to include mdmon etc. after creating the RAID.

Jes

Re: Setting up fakeraid with mdadm / dmraid

[Christoph Pleger]

Hello,

> Christoph> I have a machine with an LSI Megaraid Sofware RAID
> Christoph> fakeraid, which uses ddf format. About two weeks ago, when
Christoph> I wanted to install the machine, I configured a RAID 1
Christoph> array in the BIOS config utility and then booted the
> Christoph> machine from PXE with an NFSROOT. mdadm and dmraid are
Christoph> installed in the NFSROOT, dracut is used for initrd
> Christoph> generation. As dracut prefers mdadm over dmraid, mdadm was
Christoph> chosen for RAID management.

> It's also almost entirely black magic to support these things, since the
vendors don't generally share the details on the on-disk format that I'm
aware of.  I'm probably wrong in the details.

The same fakeraid works quite well in Linux when using dmraid instead of
mdadm, so the ddf format used is obviously not secret. I just would like
to switch to mdadm because its documentation states that it supports the
ddf format and because many sites on the internet recommend to move from
dmraid to mdadm for fakeraid management.

> But again, please just use the controller in JBOD (Just a Bunch of
Disks) mode, and let mdadm mirror the disks for you.

The purpose of my NFSROOT is to partition disks, create filesystems and
unpack a base system to the disks. This should support a great variety of
machines, so the installation system has to be quite general. Though it is
not the case here, what if Linux had to be installed on the RAID together
with another OS? I think, that then all installed OSes should have the
same view on the RAID - the view which if offered by the fakeraid.

And if I had not intended to reinstall the machine anyway, but wanted to
use the NFSROOT as a rescue system, my data had been lost without
intention. So, even if mdadm does not support ddf fakeraids (correctly),
it should at least not destroy data.

> Crappy implementation of semi-secret on-disk format?   It could be that
mdadm over-wrote something on the disk, or you partitioned it and then
overwrote something the FakeRAID wanted kept around.

Like I wrote before, I did not perform any action on the disks. No
mounting, no partitioning, no mkfs. Just booting with an NFSROOT which has
mdadm and dmraid installed.

Regards
  Christoph