[RndTbl] setting up RAID 1 with mdadm

Gilles Detillieux grdetil at scrc.umanitoba.ca
Thu Oct 18 14:19:20 CDT 2018

Thanks, everyone, for all the good advice. I decided it was worth the 
effort to rebuild the RAID array using properly typed partitions. gdisk 
was instantly familiar, having used Linux's older fdisk more times than 
I care to remember.

On 10/06/2018 05:53 AM, Trevor Cordes wrote:
> 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.
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Gilles R. Detillieux              E-mail: <grdetil at scrc.umanitoba.ca>
Spinal Cord Research Centre       WWW:    http://www.scrc.umanitoba.ca/
Dept. of Physiology and Pathophysiology, Rady Faculty of Health Sciences,
Univ. of Manitoba  Winnipeg, MB  R3E 0J9  (Canada)

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