[RndTbl] setting up RAID 1 with mdadm

Scott Toderash scott at 100percenthelpdesk.com
Fri Oct 5 07:08:02 CDT 2018

Good point. I didn't do that step, but I found when I reboot the machine 
it sorts everything out on its own anyway. For safety, it is probably 
better to include that step.

On 18-10-04 01:23 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? 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

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