[RndTbl] setting up RAID 1 with mdadm

[Gilbert E. Detillieux]

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 configuration 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.  Strictly 
speaking, the mdadm.conf file (location may vary, depending on distro) 
isn't necessary.  Without it, the MD arrays will still be detected and 
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 refer 
to your devices, then the actual assigned md device name doesn't matter.)

Gilbert

> This was the only part of DigitalOcean's procedure that I found to be 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 reboot, 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 files 
>> to the new device
>> sudo mv home home.orig; sudo mkdir home ; sudo chmod 777 home ; sudo 
>> mount /dev/mapper/datavg-datalv /home ; sudo cp -a /home.orig/* home
>> 8. edit fstab for startup config for this disk
-- 
Gilbert E. Detillieux		E-mail:	<gedetil at cs.umanitoba.ca>
Dept. of Computer Science	Web:	http://www.cs.umanitoba.ca/~gedetil/
University of Manitoba		Phone:	(204)474-8161
Winnipeg MB CANADA  R3T 2N2	Fax:	(204)474-7609