[TriLUG] SLES9 RAID and EVMS

[Ken Mink]

On 12/8/05, Pat Regan <thehead at patshead.com> wrote:
>
> Ken Mink wrote:
> > If I'm wrong about this, I know someone will correct me.
> >
> > I just want to review so you can correct me on the setup. You've got 8
> > drives; 1 for /boot and swap and 7 in a RAID5 array. You've been able to
> > partition for /boot and swap and have successfully created the RAID5
> array
> > which is /dev/md0. Now you want to carve up /dev/md0 for filesystems
> and/or
> > other volumes?
> >
>
> It seems like a waste to use 1 whole drive for /boot (I am assuming they
> are identical drives).  I actually don't know what the current state is
> as far as grub/lilo booting from RAID5, but I am pretty sure they can
> boot from a RAID 1.  There is nothing preventing you from running a
> crazy 8 drive RAID 1 as your boot partition.  That way you can install
> lilo/grub on each drive and it will be able to boot no matter what
> happens.
>
> > /dev/md0 is logically the same as a single partition. The word logically
> is
> > a clue for you. You need to use LVM(Logical Volume Manager) to create
> > logical volumes inside md0. You can then use the logical volumes for
> > filesystems and such.
> >
>
> Technically, there is nothing preventing him from creating a file system
> directly on top of /dev/md0.


True. I assumed from the wording that he was looking to create more than one
filesystem/volume. To create more then one filesystem in md0, you need LVM.


> If you're unfamiliar with logical volumes, they're incredibly useful and
> > allow for all sorts of disk configurations. I don't have a good link for
> a
> > guide. Maybe someone else could provide one.
> >
>
> LVM rocks.  However, I think it is (usually) a reasonable idea to create
> more than one md device on each set of drives.  Slicing a set of drives
> up gives you more flexibility if you want to add or remove drives from a
> RAID 5.  I can probably explain better by example.
>
> If you partition the raw drives into, say, 4 partitions.  /dev/md0 being
> a RAID 5 of the first partition of each drive, /dev/md0 the second
> partition, up to /dev/md3.
>
> This gives you 4 RAID 5 arrays.  You can then take each of the md
> devices and add them all to the save volume group.  So far, this seems
> pretty pointless.
>
> Lets say this was a 4 drive array.  Next year, you decide to add 2 more
> drives...  In this case, as long as your VG is 25% empty (or can be made
> 25% empty), you can manage to extend the RAID 5 from 4 drives to 6.
>
> First, you'd partition your 2 new drives to match the other 4.  Then you
> would remove 1 of the md devices from the VG and destroy that md
> device.  You would then recreate that md device with 6 partitions
> instead of 4, and add it back to the VG.  Repeat 3 more times and you're
> done.


It took me a bit to work out exactly how. Before removing one of the md
physical volumes, who'd have to pvmove all the physical extents to the new
physical volume(the new md device). Once all the PEs were moved to the new
larger volumes, who'd be free lvextend to use more space.

This would work. For the largest md device size there must be that much free
on the disks.

Nice,
Ken


I don't know how clearly I have explained, and I would be more than
> happy to clarify anything :).
>
> > I hope this was helpful and correct,
> > Ken
>
> Me too :)
>
> Pat
>
>
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