Re: Raid 1

[Andy Smith <andy@strugglers.net>]

Hi Mick,

On Sun, Jan 24, 2021 at 11:36:09AM +0000, mick crane wrote:
> I know I'm a bit thick about these things, what I'm blocked about is where
> is the OS.

Wherever you installed it.

> Let's say I have one PC and 2 unpartitioned disks.
> Put one disk in PC and install Debian on it.

I think you are fundamentally going about this the wrong way.

There are several concerns and I think you are mixing them up. If I
understand you correctly, you concerns are:

1. Your data and OS should be backed up.
2. Your data and OS should be available even if a disk dies

Concern #1 is totally separate from concern #2 and is achieved by
setting up a backup system, has very little to do with whether you
use RAID or ZFS or whatever. It is worth a separate thread because
it's separate project.

For concern #2, that being *availability* of data and OS, there's
many ways to do it. You seem to have settled upon ZFS for your data,
and OS separately by some other means. That's fine.

A ZFS mirror vdev is going to need two identically-sized devices.
And you want to keep your OS separate. This suggests that each of
your disks should have two partitions. The first one would be for
the OS, and the second one would be for ZFS.

If you are going to keep your OS separate, I don't see any reason
not to use mdadm RAID-1 for the OS even if you're going to use zfs
for your data. Yes you could just install the OS onto a single
partition of a single disk, but you have two disks so why not use
RAID-1? If a disk breaks, your computer carries on working, what's
not to like?

So personally I would just do the install of Debian with both disks
inside the machine, manual partitioning, create a single partition
big enough for your OS on the first disk and then another one the
same on the second disk. Mark them as RAID members, set them to
RAID-1, install on that.

Once it's up and running you can then go and create a second
partition that spans the rest of each disk, and then when you are
ready to create your zfs pool:

> "zpool create tank mirror disk1 disk2"

# zpool create tank mirror /dev/disk/by-id/ata-DISK1MODEL-SERIAL-part2 /dev/disk/by-id/ata-DISK2MODEL-SERIAL-part2

The DISK1MODEL-SERIAL bits will be different for you based on what
the model and serial numbers are of your disks. Point is it's a pair
of devices that are partition 2 of each disk.

> Can I then remove disk1 and PC will boot Debian from disk2 ?

This is only going to work if you have gone to the effort of
installing your OS on RAID. The easiest way to achieve that is to
have both disks in the machine when you install it and to properly
tell it that the first partition of each is a RAID member, create
them as a RAID-1 and tell the installer to install onto that.

As other mentioned, after it's installed you do have to manually
install the grub bootloader to the second device as well as by
default it only gets installed on the first one.

A word of warning: RAID is quite a big topic for the uninitiated and
so is ZFS. You are proposing to take on both at once. You have some
learning to do. You may make mistakes, and this data seems precious
to you. I advise you to sort out the backups first. You might need
them sooner than you'd hoped.

Cheers,
Andy

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