Bristol Digest, Vol 577, Issue 2

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Today's Topics:

1. Re: Two identical disks - system boots to the wrong one
(Jamie Lokier)


----------------------------------------------------------------------

Message: 1
Date: Tue, 18 Nov 2014 23:54:42 +0000
From: Jamie Lokier <jamie@shareable.org>
To: andrew@1dtv.com, Bristol and Bath Linux User Group
<bristol@mailman.lug.org.uk>
Subject: Re: [bristol] Two identical disks - system boots to the wrong
one
Message-ID: <20141118235442.GB6266@jl-vm1.vm.bytemark.co.uk>
Content-Type: text/plain; charset=us-ascii

Hi Andrew,

It's because the BIOS setting doesn't hide the drive from Linux, and
filesystems are searched by their UUIDs. It's generally unhelpful to have
two filesystems with the same UUID appearing to Linux.

One quick thing you can do is change the filesystem UUIDs using 'tune2fs'
(for ext2/3/4) or the appropriate filesystem tool.

Then change the GRUB config file in /boot/grub/grub.cfg and /etc/fstab, for
any mentions of the filesystem UUIDs, to the ones which match the disks
where those files occur.

If any entries aren't using UUIDs but are, for example, /dev/sda1 (or the
GRUB equivalent), that's not really good. You can change them to match the
disk in your tests, but the effect will be different if a drive really fails
or is unplugged. UUID is better, so use that.

RAID1 is generally good with two HDDs or two SSDs. It keeps everything
synchronised and will boot if either drive fails or is removed.

Though if you're unlucky with a drive not failing but "turning weird"
(usually meaning slow) then it's not a guaranteed solution.

With a HDD and SDD, RAID1 may not have the optimal performance
characteristics that you want. You may want to explore mdadm
--write-mostly, --write-behind, and a write-intent bitmap thay may be
internal, or located only on the SSD.

If you use RAID1 for booting, you will need to install GRUB separately on
each disk (GRUB writes extra sectors, to just one drive at a time), and make
sure the GRUB config includes the RAID1 module. It can be tricky to verify
the installation really worked, so you'll need to try unplugging each drive
to check it boots from both.

A variation (which I prefer because GRUB won't get stuck if the other disk
is semi-broken) is to install GRUB with a non-RAID boot configuration, on a
RAID1 partition, with RAID metadata at the end of the partition so the same
partition is accepted as a non-RAID read-only filesystem to GRUB.

Best,
-- Jamie



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