How to create software raid on working ubuntu server with gpt?

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i have this structure of hdd on my server:

fdisk -l
Disk /dev/sda: 2.7 TiB, 3000592982016 bytes, 5860533168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: EA206948-7BBC-4AD9-8835-40746D07F575

Device     Start       End   Sectors    Size Type
/dev/sda1     40      2048      2009 1004.5K BIOS boot
/dev/sda2   4096 409602047 409597952  195.3G Linux RAID


Disk /dev/sdb: 2.7 TiB, 3000592982016 bytes, 5860533168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: D2514683-E466-41C0-82AD-44635CA12AE8

Device     Start       End   Sectors    Size Type
/dev/sdb1     40      2048      2009 1004.5K BIOS boot
/dev/sdb2   4096 409602047 409597952  195.3G Linux RAID


Disk /dev/md2: 195.3 GiB, 209714085888 bytes, 409597824 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes

How you can see total size of my hdd-s – 2,7T. Also, i have program raid for /. This is /dev/sda2 and /dev/sdb2. How i can create other partition with /dev/sda3 and /dev/sdb3 with size 500GB on both disks and make Linux RAID 1, like i am using on /dev/sda2 and /dev/sdb2 ?
I tried with using of fdisk. I made two partitions with 500GB for each. I made mdadm:
mdadm --create /dev/md3 --metadata=0.90 --level=mirror --raid-devices=2 /dev/sda3 /dev/sdb3
and i added ARRAY /dev/md3 UUID=3a5d7bb4:7f8f4e1f:a4d2adc2:26fd5302 to /etc/mdadm/mdadm.conf. also i added my /dev/md3 to fstab, but after reboot, my system was not loaded. Where i have error ? How can i do this operation ? How can i do to have for /dev/sda3 and /dev/sdb3 Linux Raid and not Linux filesystem

Answer

It seems /dev/sda3 is missing a filesystem (fs).

Use mkfs to create the fs of your choice and edit /etc/fstab accordingly.

Attribution
Source : Link , Question Author : Piduna , Answer Author : Thomas Hofmann

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