[TriLUG] SLES9 RAID and EVMS

[Pat Regan]

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.

> 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.

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
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 189 bytes
Desc: OpenPGP digital signature
URL: <http://www.trilug.org/pipermail/trilug/attachments/20051208/70dd1bde/attachment.pgp>