[RndTbl] setting up RAID 1 with mdadm

Adam Thompson athompso at athompso.net
Fri Oct 5 08:56:13 CDT 2018

On October 5, 2018 7:09:50 a.m. CDT, Scott Toderash <scott at 100percenthelpdesk.com> wrote:
>In my case it stayed as md0 but in theory I think it could assign 
>anything, as you say.
>On 18-10-04 01:33 PM, Gilbert E. Detillieux wrote:
>> On 04/10/2018 1:23 PM, Gilles Detillieux wrote:
>>> Thanks, Scott.
>>> One of the steps in the tutorial is to save the MD RAID
>>> in /etc/mdadm/mdadm.conf. They suggest using "sudo mdadm --detail 
>>> --scan | sudo tee -a /etc/mdadm/mdadm.conf", which I did. In your 
>>> approach, that step isn't done. Is this a detail that pvcreate looks
>>> after for you, either by adding to that file itself, or saving the 
>>> setup elsewhere? 
>> No, the LVM commands will not affect MD configuration at all.
>> speaking, the mdadm.conf file (location may vary, depending on
>> isn't necessary.  Without it, the MD arrays will still be detected
>> assembled at boot time, but you may get different device names 
>> assigned to them (e.g. /dev/md127, instead of /dev/md0).
>> If you want consistent device names, it's best to have the mdadm.conf
>> file.  (If you're going to use UUID's or logical volume names to
>> to your devices, then the actual assigned md device name doesn't
>> Gilbert
>>> This was the only part of DigitalOcean's procedure that I found to
>>> a bit kludgy. I was surprised that there wasn't something right in 
>>> mdadm to manage the saving of the configuration more automatically. 
>>> Adding the fstab entry was as I'd expect for any file system type. 
>>> Other than that, things were pretty plug-and-play, with no messing 
>>> around with systemctl or anything like that required. After a
>>> the RAID array was back in action just as it should be.
>>> On 10/04/2018 10:07 AM, Scott Toderash wrote:
>>>> Here are my notes from the last time I build a Linux RAID on LVM. 
>>>> This was on 16.04LTS
>>>> I think my approach was slightly different. The RAID device is 
>>>> created on the LVM devices.
>>>> 1. create partitions of type Linux RAID Autodetect on both disks
>>>> fdisk -l /dev/sdb
>>>> fdisk -l /dev/sdc
>>>> 2. create a RAID array called md0 using mdstat
>>>> sudo mdadm --create /dev/md0 --level=mirror --raid-devices=2 
>>>> /dev/sdb1 /dev/sdc1
>>>> 3. add the md0 raid device to the LVM pool
>>>> sudo pvcreate /dev/md0
>>>> 4. create a volume group called datavg
>>>> sudo vgcreate datavg /dev/md0
>>>> 5. create a logical volume called datalv within the volume group
>>>> sudo lvcreate --name datalv --size 1.8T datavg
>>>> 6. format the newly created logical volume
>>>> sudo mkfs.ext4 /dev/datavg/datalv
>>>> 7. move home files to a temporary location, create a new home and 
>>>> mount the newly formatted device there, copy the original home
>>>> to the new device
>>>> sudo mv home home.orig; sudo mkdir home ; sudo chmod 777 home ;
>>>> mount /dev/mapper/datavg-datalv /home ; sudo cp -a /home.orig/*
>>>> 8. edit fstab for startup config for this disk
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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.
Sent from my Android device with K-9 Mail. Please excuse my brevity.
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