[RndTbl] setting up RAID 1 with mdadm

[Trevor Cordes]

On 2018-10-04 Gilbert E. Detillieux wrote:
> No, the LVM commands will not affect MD configuration at all.
> Strictly speaking, the mdadm.conf file (location may vary, depending
> on distro) isn't necessary.  Without it, the MD arrays will still be
> detected and assembled at boot time, but you may get different device
> names assigned to them (e.g. /dev/md127, instead of /dev/md0).

Not anymore: even without an mdadm.conf you can still get devices named
as you want them (/dev/md0, etc.).  You can set it at creation time or
after the fact somehow, though how to do it completely eludes my memory
at this moment.  But I know for a fact I'm right: my box has no
mdadm.conf and I have md1, md2, md3, md4 and md127 -- 1-4 were chosen,
127 is after I forgot how.  :-)

On 2018-10-05 Adam Thompson wrote:
> Without partitions, you'll find that you need an mdadm.conf file to
> instruct mdadm that there's an array there that needs to be started.
> Normally the Linux kernel looks for the magic "auto detect raid"
> partition type. -Adam

But you still don't need to specify the raid arrays (which is
redundant and easy to get out of sync with your changing hardware as
disks die/get replaced/arrays change), just the constituent devices:

DEVICE partitions
DEVICE /dev/sd[ab]

for example (you could use the by-name or by-uuid paths too if you
prefer).  You can even just put in all your drives, or /dev/sd[a-z] and
let the kernel sort it out, it'll just ignore the ones without raid
superblocks.

As mentioned: I strongly recommend using partitions as your basis, not
raw drives, because then it's easier to manipulate things in the
future, boot from them, add a MBR, absolutely no mdadm.conf, etc.  Not
critical, but handy.

If you are using partitions, you mark each as type fd (linux raid).

As Gilbert said, gdisk is pretty good.  And the newest versions of
fdisk support GPT also.  I just redid some of my RAID1's with GPT
(while online! no reboots! no USB boots!) instead of DOS partition
tables.  Worked out great.  gdisk gives extra options you may need for
boot partitions.  Always leave a 32M or so partition you can use as a
"BIOS boot partition" (YMMV with EFI).

My favorite thing about GPT, even if you aren't >2TB, is you can have
gobs of partitions without messing with DOS "extended partitions" which
are such a drag.  Grub2 fully supports GPT, and from what I can tell,
even ancient boxes will boot from GPT with MBR chaining to the "BIOS
boot partition".

Lastly, Linux md raid is the best thing ever.  I would almost never use
any hardware RAID (or other software raid like IMSM) if I can help it.
Your chances of recovering from big disasters is massively increased
with all the leeway and options mdadm gives you.  H/w RAID is such a
black box that if things don't go according to plan, you're usually SOL.

Very last lastly: go ahead and boot off or RAID1.  You don't need
anything special for RAID1, it just works.  Just put the grub-install
MBR on both disks (again, assuming non-EFI) in case one dies.  Make
partitions for at least your boot/root/swap and make RAID1 on top of
each.  It's literally as easy as what you've already done.  I'll also
mention I use RAID even on SSDs as they fail too, usually in horrific
ways.