[RndTbl] setting up RAID 1 with mdadm

Gilles Detillieux grdetil at scrc.umanitoba.ca
Thu Oct 4 13:23:42 CDT 2018

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? 
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
> On 18-10-04 09:42 AM, Gilbert E. Detillieux wrote:
>> I think the tutorial is complete enough. The only thing I'd do 
>> differently is create appropriate partition tables on the raw disks 
>> (GPT if the disks are >2TB or if you need to use GPT for other 
>> reasons, but the older MS-DOS partition tables would be fine 
>> otherwise), set up one partition on each drive and tag them with the 
>> applicable partition type to indicate they're MD RAID partitions, and 
>> use the device names for the partitions rather than the raw drives in 
>> the mdadm commands.  While this isn't necessary, I think it would 
>> help in post-mortem recovery, and in keeping your sanity when you're 
>> (or someone else is) trying to figure out what you did a few years 
>> later.
>> LVM has some nice features to offer, e.g. if you anticipate wanting 
>> to add more capacity to this file system in the future, or you want 
>> to split a large array into multiple file systems. But for simple use 
>> cases, I wouldn't bother.  If you do use LVM, don't use its RAID 
>> features; use LVM over top of MD.
>> If you go with GPT partition format, and don't want to deal with the 
>> arcane syntax of parted commands, there are alternatives: gparted for 
>> the full-GUI, Partition-Magic-like experience, or gdisk for a simple 
>> fdisk-like, retro, text-menu-based interface.
>> Gilbert
>> On 04/10/2018 8:33 AM, Gilles Detillieux wrote:
>>> Circumstances finally forced me to bite the bullet and learn 
>>> something I had been putting off for too long: setting up a RAID 
>>> array under Linux. I'm almost embarrassed, now that I've done it, 
>>> that I waited so long because it was way simpler than I'd imagined. 
>>> I thought I'd have to figure out all kinds of magic with LVM, 
>>> parted, and mdadm, but I found this tutorial that showed a simple 
>>> set of mdadm commands to set up RAID 1:
>>> https://www.digitalocean.com/community/tutorials/how-to-create-raid-arrays-with-mdadm-on-ubuntu-16-04 
>>> This was actually one of the simplest scenarios: the system had an 
>>> SSD for the root/boot partition, and two 2 TB hard drives for data 
>>> storage (/dev/sdb & sdc), so I just needed to set up RAID 1 and not 
>>> worry about the intricacies of booting from RAID. So, software RAID 
>>> seemed like the quick & easy way to go with a minimum of fuss.
>>> Now the nagging question: is it really this simple, or does the 
>>> tutorial above oversimplify and omit some important steps? Can 
>>> someone with ample RAID and mdadm experience advise or provide tips 
>>> on anything else I should do or lookout for?
>>> Thanks,
>>> Gilles

Gilles R. Detillieux              E-mail: <grdetil at scrc.umanitoba.ca>
Spinal Cord Research Centre       WWW:    http://www.scrc.umanitoba.ca/
Dept. of Physiology and Pathophysiology, Rady Faculty of Health Sciences,
Univ. of Manitoba  Winnipeg, MB  R3E 0J9  (Canada)

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