Re: My attempt to understand dmraid

On Thu, Oct 11, 2007 at 10:57:39AM -0700, Sean Bruno wrote:
> I am trying to delve into dmraid/device-mapper and learn how the magic
> happens and I came up with what I think is the "way it works."  Please
> critique and correct me where I am wrong or over-simplify the operation
> of the current dmraid architecture as it is used under Fedora/redhat
> systems.
> 
> Assumptions:
>   dmraid compatible device(sil3118)
>   Hard Drives of equal size(SATA in my case)
>   Installation of Fedora Core 7(x86_64)
> 
> 1.
> The initial setup occurs on the RAID controller where I selected RAID1
> with my two disks.  This initialized the drives for use with
> dmraid/device-mapper by placing the SiL metadata on the drives and set
> them to a initialized state.

Well, it just puts vendor specific metedata on the drives making up
a RAID set. Vendors typically ship M$ drivers and a managent tool to use
such ATARAID sets on Windows.

> 
> 2.
> The system boots off the first hard drive(sda) and loads the kernel from
> there.

As part of the initrd actions, a mirrored mapping gets activated for
your RAID1 set.

> 
> 3.
> Booting Fedora Core 7, /etc/rc.sysinit loads the fake RAID1 module
> "dm-mirror".  This actually works with the software RAID code in md to
> manage the RAID1.

No need for MD. dm-mirror is a selve-contained device-mapper mirroring
implementation.

> The system then detects that the drives are a RAID1
> set by invoking '/sbin/dmraid  -ay -i -p -t' This detects the RAID1 set
> and creates /dev/mapper/silXXXXXXX through the device-mapper libraries.

Like I said, activation happens in the initrd.

> 
> 
> What is unclear to me is where the system mounts the filesystems
> on /dev/mapper/silXXXXXX and how the dm-mirror module manages drive
> failues.  Can someone fill in the pieces for me?

/etc/fstab is filled during installation.

A failed drive is being ignored by dm-mirror until you replace it
using the ATARAID bios (sil in your case).
Online replacement for dmraid/device-mapper is still in the works.

Regards,
Heinz

> 
> Sean
> 
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Heinz Mauelshagen                                 Red Hat GmbH
Consulting Development Engineer                   Am Sonnenhang 11
Storage Development                               56242 Marienrachdorf
                                                  Germany
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