CentOS - Dec 2020 - CentOS and mdadm

Hi,

I'm regularly using software RAID for my CentOS storage servers, either with
RAID 1 or RAID 6 depending on the number of disks.

Here's two questions about mdadm I've always been wondering about.

1. I'm not sure about the --metadata option. Let's say I want to create
a
simple RAID 1 setup, I'm usually doing it like this:

# mdadm --create /dev/md/boot --level=1 --raid-devices=2 \
  --metadata=1.2 /dev/sda1 /dev/sdb1

If anyone can explain the exact meaning of --metadata, I'd be grateful.

Oh, and is it OK to use --metadata=1.2 in CentOS 7.x and 8.x ?

2. I'm regularly encountering the mdadm --misc option in various tutorials
on
the Internet. I looked it up in mdadm's manpage, but it doesn't seem to
be
documented.

Any idea what this option does ?

Cheers,

Niki

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On 12/1/20 3:48 AM, Nicolas Kovacs wrote:
> If anyone can explain the exact meaning of --metadata, I'd be grateful.

I think the man page is pretty clear on that one.? There are two 
different versions of the metadata block, and the second one (1.x) can 
be stored at different locations depending on your purpose. For example, 
if you want to MD RAID (mirror) your EFI System partition, you can store 
the metadata that identifies the partition as an MD RAID component at 
the end of the partition. The UEFI firmware will ignore the metadata and 
read the partition as if it were not a member (and that's OK as long as 
it's read-only).? However, a partition with metadata at the end would be 
difficult to resize, so that's not the preferred location for most 
components.

> Oh, and is it OK to use --metadata=1.2 in CentOS 7.x and 8.x ?

Yes.

> 2. I'm regularly encountering the mdadm --misc option in various
tutorials on
> the Internet. I looked it up in mdadm's manpage, but it doesn't
seem to be
> documented.
>
> Any idea what this option does ?

It's one of the major modes, and assumed if no other mode is specified.? 
See the MODES and OPTIONS sections near the beginning of the mdadm man page.

I'm guessing that --misc used to be required in order to use any of the 
options specified in the "For Misc mode" section.