Restore backup from 'dd' (possibly problem with softRAID HDDs) - linux

Question

I have 2 servers, Ubuntu 18.04. First is my main server which I want to backup. Second is tested server with Ubuntu 18.04 - KS-4 Server - Atom N2800 - 4GB DDR3 1066 MHz - SoftRAID 2x 2To SATA - when I want test my backup.
I make backup by 'dd' command and nextly I download this backup (wget) by server 2 (490gb, ~ 24hours downloading).
Now I want test my backup so I tried:
dd if=sdadisk.img of=/dev/sdb
I get:
193536+0 records in
193536+0 records out
99090432 bytes (99 MB, 94 MiB) copied, 5.06239 s, 19.6 MB/s
But nothing will change.
fdisk -l
Disk /dev/sda: 1.8 TiB, 2000398934016 bytes, 3907029168 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: dos
Disk identifier: 0x8efed6c9
Device Boot Start End Sectors Size Id Type
/dev/sda1 * 4096 1050623 1046528 511M fd Linux raid autodetect
/dev/sda2 1050624 3905974271 3904923648 1.8T fd Linux raid autodetect
/dev/sda3 3905974272 3907020799 1046528 511M 82 Linux swap / Solaris
GPT PMBR size mismatch (879097967 != 3907029167) will be corrected by w(rite).
Disk /dev/sdb: 1.8 TiB, 2000398934016 bytes, 3907029168 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: B3B13223-6382-4EB6-84C3-8E66C917D396
Device Start End Sectors Size Type
/dev/sdb1 2048 1048575 1046528 511M EFI System
/dev/sdb2 1048576 2095103 1046528 511M Linux RAID
/dev/sdb3 2095104 878039039 875943936 417.7G Linux RAID
/dev/sdb4 878039040 879085567 1046528 511M Linux swap
Disk /dev/md1: 511 MiB, 535756800 bytes, 1046400 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
Disk /dev/md2: 1.8 TiB, 1999320842240 bytes, 3904923520 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
lsblk -l
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sda 8:0 0 1.8T 0 disk
├─sda1 8:1 0 511M 0 part
│ └─md1 9:1 0 511M 0 raid1 /boot
├─sda2 8:2 0 1.8T 0 part
│ └─md2 9:2 0 1.8T 0 raid1 /
└─sda3 8:3 0 511M 0 part [SWAP]
sdb 8:16 0 1.8T 0 disk
├─sdb1 8:17 0 511M 0 part
├─sdb2 8:18 0 511M 0 part
├─sdb3 8:19 0 417.7G 0 part
└─sdb4 8:20 0 511M 0 part
I think the problem is with configuration of disks on server 2, specifically with 'Linux raid' between them. I searching how change its, I testing commands like 'mdadm...' but it's not working like I expected. So I have questions:
How change 'Linux raid' from 2 HDDS to 1 HDD with current system and 2 HDD clear, when I can test my backup properly?
It's generally possible to restore backup 490GB on 1.8TB?
I selected the best option to full linux backup?

Answer 1

To touch on question #3 "I selected the best option to full linux backup?"
You are using the correct tool, but not the correct items to use the dd command on.
It appears you created a .img file for your sda drive (which is normally only used to create a bootable disk and not a full backup of the drive.
If you want to create a full backup, for example of /dev/sda you would run the following:
dd if=/dev/sda of=/dev/sdb
But In your case your /dev/sda drive is a twice the size of your /dev/sdb drive so you might want to consider backing up important files using tar as well as compressing the backup using either gzip or bzip2.