[CentOS] Bare drive RAID question, was RE: *very* ugly mdadm issue [Solved, badly]

Fri Sep 5 14:01:05 UTC 2014
Warren Young <warren at etr-usa.com>

On 9/5/2014 07:18, Richard Zimmerman wrote:
> Until I read this thread, I've never heard of building RAIDs on bare
> metal drives. I'm assuming no partition table, just a disk label?

I don't know what you mean by a disk label.  BSD uses that term for 
their alternative to MBR and GPT partition tables, but I think you must 
mean something else.

In Linux terms, we're talking about /dev/sda, rather than /dev/sda1, for 

> What is the advantage of doing this?

The whole idea of a RAID is that you're going to take a bunch of member 
disks and combine them into a larger entity.  On top of *that* you may 
wish to create partitions, LVMs, or whatever.

So the real question is, why do you believe you need to make each RAID 
member a *partition* on a disk, instead of just take over the entire 
disk?  Unless you're going to do something insane like:


...you're not going to get any direct utility from composing a RAID from 
partitions on the RAID member drives.

(Why "insane?"  Because now any I/O to /dev/md1 interferes with I/O to 
/dev/md0, because you only have two head assemblies, so you've wiped out 
the speed advantages you get from RAID-0 or -1.)

There are ancillary benefits, like the fact that a RAID element that 
spans the entire partition is inherently 4k-aligned.  When there is a 
partition table taking space at the start of the first cylinder, you 
have to leave the rest of that cylinder unused in order to get back into 
4k alignment.

The only downside I saw in this thread is that when you pull such a disk 
out of a Linux software RAID and put it into another machine, you don't 
see a clear Linux partition table, so you might think it is an empty 
drive.  But the same thing is true of a hardware RAID member, too.