Re: [gentoo-user] Tips for fresh install with GRUB2+RAID1+LVM2

[Peter Humphrey]

On Sunday 17 May 2015 12:48:58 Nuno Magalhães wrote:

> I want to use mdadm to create a RAID1 with 2 SATA disks. From what i
> gather, i'll need (bootable)  0xFD partitions, i'll use full disk for
> them and no separate /boot (unless required). Is GPT required or can i
> stick to MBR? Is fdisk safe? Seems usable to me.

Use this guide:

https://wwwold.gentoo.org/doc/en/gentoo-x86+raid+lvm2-quickinstall.xml

> Is 0.90 metadata mandatory for RAID1? What's the recommended version?

You need 0.90 metadata for any volume that needs to be bootable, unless you 
use what Dale calls an init thingy. I don't do that, and this is my setup:

/boot on /dev/md1 = /dev/sd[ab]1 in RAID1 with metadata 0.90.
/ on /dev/md5 = /dev/sd[ab]5 in RAID1 with metadata 0.90.
[the rest] on /dev/md7 = /dev/sd[ab]7 in RAID1 with metadata > 1.0.

> I've seen manuals that use mknode, but discovered that mdadm --create
> will create a /dev/md for me.

I didn't know that. Interesting.

> (Later i want to get rid of systemd-udev and use eudev instead.)

I use openrc, not systemd. It still works well and has less complication - and 
less typing!

> Also, why so many tty* and similar in /dev? Is there a way to make it
> less crowded? Just a whim...

I tried removing them but found they'd been re-created after the next boot. Ho 
hum.

> Does GRUB2 (rather grub2-install) handle well RAID1 stuff or does it
> get confused?

Don't know. I use legacy-grub. The last I looked, grub-2 couldn't handle the 
choice of kernels I have.

> What's the difference between dodmraid and domdadm? I don't want to
> use fakeraid, so is it safe to just use domdadm?

I think so, yes. As far as I know, dmraid is fake raid.

> Then i want to use LVM on the /dev/md and setup my / in an LV. This
> seems straightforward (as far as creation goes). Any tips here? I
> remember you could tune LVM LVs to the underlying RAID stripe.

I just followed the quick-installation guide.

> I intend to use XFS for /. Incidentally, if i later decide to "fork
> out" /usr (or some other subdirectory) into it's own LV, is it "just"
> a mater of copying its contents and updating /etc/fstab? Or should i
> just do it now and expand the LVs if later required (especially if i
> want to use different filesystems)?

I can't help you with XFS. I know that ext4 in an LV in a VG in a PV on RAID1 
works reliably, even though it does look complex when I write it like that.

> The problem will be (has been) getting it to boot.
>
> If i have 2 kernels in /boot, shouldn't grub2-mkconfig generate 2
> entries in the GRUB boot menu (not counting the recovery entries)?
>
> I read somewhere that genkernel is a bad choice for creating the
> kernel because it un/sets a CONFIG_ flag that interferes with RAID/LVM
> (sorry, i should've noted where/what). I use make menuconfig anyway,
> it's fun. I do use genkernel to create the initramfs, i'm assuming
> "genkernel --lvm --mdadm --bootloader=grub --install initramfs" would
> suffice?

Again, legacy grub here. But if you're using an initramfs, from what I've seen 
you don't need to specify metadata 0.90.

--->8

> While the Handbook is awesome, it doesn't cover this and all the
> information i find is either outdated or conflicting.

I reinstalled yesterday while following the guide I cited.

Damn. I've just checked and something has renamed my /dev/md7 to 
/dev/md127. Again. It's just too bad. I shall have to stop it when I get a 
quiet moment and reassemble it into /dev/md7. Actually, I know what caused 
it but I didn't notice at the time.

HTH.

-- 
Rgds
Peter