Mdadm

There are a few options for software RAID on Gnu Linux. Among them is BtrFS and ZFS (the latter of which has licensing issues, so use discretion before using)., however today I will focus on mdadm. This is historically the oldest software raid, therefore should be better vetted, although its performance may be less of that of the first two mentioned - for simple servers, mdadm might be the most stable choice. However, after testing it in practice I found that it was not stable on 1 out of 2 machines. The general rule that 'software RAID is not trustworthy' might apply here. Generally, go with hardware RAID.

Creation of RAID

Will not be covered here (yet). You must create the partition tables. Create the raid with mdadm. mkfs.ext4 on the raid partition. Add mdadm to grub config. Reinstall grub. Details may be provided later.

Usage

Status / Details

# sudo mdadm -D /dev/md127

/dev/md127:
        Version : 1.2
  Creation Time : Fri Feb  1 01:00:25 2019
     Raid Level : raid1
     Array Size : 57638912 (54.97 GiB 59.02 GB)
  Used Dev Size : 57638912 (54.97 GiB 59.02 GB)
   Raid Devices : 3
  Total Devices : 2
    Persistence : Superblock is persistent

    Update Time : Fri Feb  1 02:40:44 2019
          State : clean, degraded
 Active Devices : 2
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 0

           Name : devuan:root
           UUID : 83a8dc03:802a4129:26322116:c2cfe1d4
         Events : 82

    Number   Major   Minor   RaidDevice State
       -       0        0        0      removed
       1       8       17        1      active sync   /dev/sdb1
       2       8       33        2      active sync   /dev/sdc1

(here you can see, one was removed. It auto removes, when unplugged)

Add Drive to Array

This assumes you have already created the RAID. This is for adding to an existing array.

# mdadm --add /dev/md127 /dev/sda1

NOTE: If you setup 2 hdds, in a raid, and want to add a third, if you just --add, it will show up as a spare...

if you do mdadm --grow /dev/md127 -raid-devices=3 then the third might be active sync (what we want) note that the --grow, seems to allow for parameter changes after you have already created the raid. you can also specify the exact same command, raid-devices=3 in the setup of the raid (see install doc). Note that if you lose a drive, you can simply add it.

NOTE: don't worry about mkfs.ext4 on the raid members, after initial setup. The RAID will manage that.

NOTE: if you have a new drive and need to copy the hdd partition tables: https://unix.stackexchange.com/questions/12986/how-to-copy-the-partition-layout-of-a-whole-disk-using-standard-tools or aka

(FOR MBR ONLY)
Save:
sfdisk -d /dev/sda > part_table

Restore:
sfdisk /dev/NEWHDD < part_table

(FOR GPT:)
# Save MBR disks
sgdisk --backup=/partitions-backup-$(basename $source).sgdisk $source
sgdisk --backup=/partitions-backup-$(basename $dest).sgdisk $dest

# Copy $source layout to $dest and regenerate GUIDs
sgdisk --replicate=$dest $source
sgdisk -G $dest

Add HDD back to RAID (to fix failed drive)

# mdadm --add /dev/md127 /dev/sda1
mdadm: added /dev/sda1
# sudo mdadm -D /dev/md127
/dev/md127:
        Version : 1.2
  Creation Time : Fri Feb  1 01:00:25 2019
     Raid Level : raid1
     Array Size : 57638912 (54.97 GiB 59.02 GB)
  Used Dev Size : 57638912 (54.97 GiB 59.02 GB)
   Raid Devices : 3
  Total Devices : 3
    Persistence : Superblock is persistent

    Update Time : Fri Feb  1 02:41:43 2019
          State : clean, degraded, recovering
 Active Devices : 2
Working Devices : 3
 Failed Devices : 0
  Spare Devices : 1

 Rebuild Status : 0% complete

           Name : devuan:root
           UUID : 83a8dc03:802a4129:26322116:c2cfe1d4
         Events : 92

    Number   Major   Minor   RaidDevice State
       3       8        1        0      spare rebuilding   /dev/sda1
       1       8       17        1      active sync   /dev/sdb1
       2       8       33        2      active sync   /dev/sdc1

You can check on the rebuild partway:

 Rebuild Status : 6% complete

           Name : devuan:root
           UUID : 83a8dc03:802a4129:26322116:c2cfe1d4
         Events : 103

    Number   Major   Minor   RaidDevice State
       3       8        1        0      spare rebuilding   /dev/sda1
       1       8       17        1      active sync   /dev/sdb1
       2       8       33        2      active sync   /dev/sdc1

You can also watch it via /proc/

watch -n1 cat /proc/mdstat

Every 1.0s: cat /proc/mdstat
server: Fri Feb  1 02:43:24 2019

Personalities : [raid1] [linear] [multipath] [raid0] [raid6] [raid5] [raid4] [raid10]
md127 : active raid1 sda1[3] sdb1[1] sdc1[2]
      57638912 blocks super 1.2 [3/2] [_UU]
      [==>..................]  recovery = 11.2% (6471936/57638912) finish=13.2min speed=64324K/sec

unused devices: <none>

WARNING: Reinstall grub on the new drive again as well afterwards.

Email Notifications

Test emails on mdadm.. first configure email however you prefer (i currently use ssmtp).

edit /etc/mdadm/mdadm.conf; put the email recipient in mailadd variable

then

# mdadm --monitor --scan --test --oneshot

should send an email

(Be careful that updates don't break this file. I see it's generated by mkconf.)

https://ubuntuforums.org/showthread.php?t=1185134 for more details on email sending

External Links

The section about degraded disks: https://help.ubuntu.com/lts/serverguide/advanced-installation.html.en

General partition tips: https://github.com/zfsonlinux/zfs/wiki/Debian-Stretch-Root-on-ZFS

Configuring Email w/ssmtp: https://wiki.zoneminder.com/SMS_Notifications