Re: My attempt to understand dmraid

On Fri, Nov 09, 2007 at 07:02:19PM -0800, Sean Bruno wrote:
> On Mon, 2007-10-22 at 12:16 +0200, Heinz Mauelshagen wrote:
> > 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.
> > 
> I thought(from reading the code), that dm-mirror is a module that uses
> MD functions(it's built on top of the s/w raid code).

No, it's a distinct device-mapper RAID1 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.
> > 
> Specifically, I was looking at the redhat startup script /etc/rc.sysinit.  Is this how the system figures out that there's a RAID device to deal with?

The dmraid call in rc.sysinit would activate any additional ATARAID sets,
which haven't been activated before in the initrd.

Heinz

> 
> 
> Sean

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Heinz Mauelshagen                                 Red Hat GmbH
Consulting Development Engineer                   Am Sonnenhang 11
Storage Development                               56242 Marienrachdorf
                                                  Germany
[email protected]                            PHONE +49  171 7803392
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