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

Fri Sep 5 16:54:15 UTC 2014
Les Mikesell <lesmikesell at gmail.com>

On Fri, Sep 5, 2014 at 9:01 AM, Warren Young <warren at etr-usa.com> wrote:
> 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:
>     /dev/md0
>        /dev/sda1
>        /dev/sdb1
>     /dev/md1
>        /dev/sda2
>        /dev/sdb2
> ...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.)

Well, to exactly the same extent that putting multiple partitions and
filesystems on a non-raid drive is insane for those reasons...   And
you generally can't avoid this if you want to boot from the same disks
where you store data with mirroring.   And the very nice up side is
that you can now pull your drives out, put them in different bays, add
others, etc. and the system will still assemble the right partitions
into the right raid devices and mount them correctly.  Or at least it
would in the < 2TB days...

> 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.

Isn't it possible to duplicate that when you make a single partition
and use the partition as a raid member?   And get autoassembly if it
is less than 2TB?    I consider it a real loss that autoassembly
doesn't work on large drives.  People will almost certainly lose data
in some scenarios as a result.

> 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.

I've always liked software raid1 just because you can recover the data
from any single drive on any machine with a similar interface.   But,
I guess that's why we have backups...

   Les Mikesell
     lesmikesell at gmail.com