Q: Merge partition (CentOS 7) I have CentOS 7 at VM and I want to merge /dev/sda3 with /dev/sda4 (recently created PV). Output of fdisk -l listed below:
Disk /dev/sda: 24.7 GB, 24696061952 bytes, 48234496 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 label type: dos
Disk identifier: 0x0009c749

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *        2048      411647      204800   83  Linux
/dev/sda2          411648     1460223      524288   82  Linux swap / Solaris
/dev/sda3         1460224    41058303    19799040   83  Linux
/dev/sda4   *    41058304    48234495     3588096   8e  Linux LVM

But when Im trying to check VG it leads to nothing:
[root@server1 ~]# vgs
[root@server1 ~]# vgdisplay

Output of df -h:
Filesystem      Size  Used Avail Use% Mounted on
/dev/sda3        19G   17G  640M  97% /
devtmpfs        2.8G     0  2.8G   0% /dev
tmpfs           2.8G     0  2.8G   0% /dev/shm
tmpfs           2.8G  8.8M  2.8G   1% /run
tmpfs           2.8G     0  2.8G   0% /sys/fs/cgroup
/dev/sda1       190M  135M   42M  77% /boot
tmpfs           566M     0  566M   0% /run/user/600
tmpfs           566M     0  566M   0% /run/user/0

What I have to do?

A: Based on the output of commands, it looks like you do not have any Volume group set up. You current root filesystem is just based on normal non-lvm partition, /dev/sda3.
You would need to migrate your existing non-LVM partition to LVM partition, if it has enough space. Your /dev/sda4 does not seem to be big enough. If you are able to add additional disk, with at least same size as current /dev/sda3, you can use this procedure for partition migration:
https://www.thegeekdiary.com/centos-rhel-converting-an-existing-root-filesystem-to-lvm-partition/

A: So, I extended size of mounted virtual disk in my hyper-v, then boot livecd CentOS and resized sda3 with unmapped space. Seems easy, but hard to figure out to novice.
