[nSLUG] RAID and partitioning

D G Teed donald.teed at gmail.com
Tue May 8 12:50:14 ADT 2007

Another recent innovation in mdadm is the ability to grow the raid device.
Example and user experiences with this:

I'm thinking I'll prefer this over LVM or EVMS for something like email
with capability to grow.

On 5/6/07, Bill Davidson <billdavidson at eastlink.ca> wrote:
> Hi:
> One thing about Linux that continues to amaze me is the ongoing
> innovation.  Some people criticize Linux, saying "It's just an inferior
> copy of an old operating system," but it's actually a hotbed of
> innovation.  Run-time loadable I/O schedulers, the new kexec system
> call, it goes on and on.
> So anyway, a while ago I wrote:
> > Thanks for that, it was exactly the kind of help I was looking for.  I
> > see now that had I read a little further in the RAID HOWTO I would have
> > seen:
> >
> > "RAID devices cannot be partitioned, like ordinary disks can....The
> > solution to the partitioning problem is LVM" !
> So yesterday I booted this old server with a Knoppix 5.1 CD and was
> reviewing the mdadm man page, when I noticed this option under the
> "--create" section:
>        -a, --auto{=no,yes,md,mdp,part,p}{NN}
>               Instruct mdadm to create the
> device  file  if  needed,  possibly
>               allocating an unused minor number.  "md" causes a
> non-partition-
>               able array to be used.  "mdp", "part" or "p" causes a
> partition-
>               able array (2.6 and later) to be used.  "yes" requires the
> named
>               md device to have a 'standard' format, and
> the  type  and  minor
>               number will be determined from this.  See DEVICE NAMES
> below.
> [snip]
>               For  partitionable arrays, mdadm will create the device file
> for
>               the whole array and for the first  4  partitions.
> A  different
>               number  of partitions can be specified at the end of this
> option
>               (e.g.  --auto=p7).  If the device name ends
> with  a  digit,  the
>               partition  names  add  a 'p', and a number, e.g.
> "/dev/home1p3".
>               If there is no trailing digit,
> then  the  partition  names  just
>               have a number added, e.g. "/dev/scratch3".
> [snip]
>        While entries in the /dev directory can have any format you like,
> mdadm
>        has an understanding of 'standard' formats which it uses
> to  guide  its
>        behaviour when creating device files via the --auto option.
>        The  standard  names  for  non-partitioned  arrays (the only sort
> of md
>        array available in 2.4 and earlier) either of
>               /dev/mdNN
>               /dev/md/NN
>        where NN is a number.  The standard names for partitionable
> arrays  (as
>        available from 2.6 onwards) is one of
>               /dev/md/dNN
>               /dev/md_dNN
>        Partition  numbers  should  be  indicated by added "pMM" to these,
> thus
>        "/dev/md/d1p2".
> So I tried it.  I created a RAID5 array using sdb, sdc, and sdd, and
> using the "-ap" option, and used "/dev/md_d0" as the device name.  This
> created /dev/md_d0 with major number 253, minor 0, plus /dev/md_d0p1
> through /dev/md_d0p4 with minor numbers 1 through 4, plus for some
> reason /dev/md_d1 with minor 64 and /dev/md_d1p1 through /dev/md_d1p4
> with minor numbers 65-68.
> Then I ran fdisk on /dev/md_d0 and created a partition.  I seem to
> recall that fdisk complained on exit, but the partition was created.
> Then I made a filesystem on /dev/md_d0p1 and mounted it, copied some
> files to it, unmounted it, shut down and restarted the machine, ran
> "mdadm --assemble...", remounted the partiton, and everything was
> there.
> I assume that this is still fairly experimental behaviour, but it is
> pretty darned cool.
> Of course, after all that, it is still not suitable for what I was
> thinking of, because no boot loaders know about RAID5.  And besides, LVM
> makes a lot more sense if only because volumes can be resized as needed.
> So...  I have (or my daughter's school has) this box with 5 identical
> 36GB ultra-160 SCSI drives.  The on-board Adaptec AIC-7899 controller
> has two channels, so I put three drives on one channel and two on the
> other.  I'm thinking I'll create two partitions on each drive, one small
> one (say 128MB) and one large one.  I'll use the small partitions on sda
> (id 0 on channel A) and sdd (id 0 on channel B) as a RAID1 array to
> mount at /boot.  I'll use the other three small partitions for swap.
> Then I'll use all five large partitions for a RAID5 array and create
> logical volumes on that for everything else.
> Now I just have to pick a distro...
> Oh, and does anybody have a line on a Voltage Regulator Module for a
> Dell Poweredge 1400SC?
> --
> Bill Davidson <billdavidson at eastlink.ca>
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