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Difference between /usr/lib/systemd/system and /etc/systemd/system?

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/usr/lib/systemd/system is a directory that should only contain systemd unit files which were put there by the package manager (YUM/DNF/RPM/APT/etc). Files in /etc/systemd/system are manually placed here by the operator of the system for ad-hoc software installations that are not in the form of a package. This would include tarball type software installations or home grown scripts.
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Systemctl

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Systemctl to see all Active Services :    Systemctl to see all Services : Comparison of service utility with Systemctl. Comparison of chkconfig utility with Systemctl.
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DNF & Repos

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The yum utility that is provided with Oracle Linux 8 is based on Dandified Yum (DNF). We can use dnf to install or upgrade RPM packages, while automatically handling package dependencies and requirements. The dnf command can be used to download the packages from repositories such as those that are available on the Oracle Linux yum server, but we can also set up your own repositories on systems that do not have Internet access. We can also use the dnf command on systems that are registered with the Unbreakable Linux Network (ULN) to install additional software that is limited to Oracle Linux Premier Support customers. The repositories that are available on the Oracle Linux yum server are aligned with the channels that are available from the Unbreakable Linux Network (ULN). Exceptions are any ULN channels that are limited to Oracle Linux Premier Support customers, for example, channels for products such as Ksplice. Compute nodes running Oracle Linux on Oracle Cloud Infrastructure and tha...
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CPU/RAM Slowness

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The three numbers after uptime  i.e load average — 2.03 , 20.17 , and 15.09 —represent the 1-, 5-, and 15-minute load averages on the machine, respectively. A system load average is equal to the average number of processes in a runnable or uninterruptible state. Runnable processes are either currently using the CPU or waiting to do so, and uninterruptible processes are waiting for I/O. A single-CPU system with a load average of 1 means the single CPU is under constant load. If that single-CPU system has a load average of 4,there is four times the load on the system than it can handle, so three out of four processes are waiting for resources. The load average reported on a system is not tweaked based on the number of CPUs you have, so if you have a two-CPU system with a load average of 1, one of your two CPUs is loaded at all times—that is, you are 50% loaded. So a load of 1 on a single-CPU system is the same as a load of 4 on a four-CPU system in terms of the amou...
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LVM

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###Logical Volume Management (LVM)###  Features:   1. Ability to create volume sets and stripe sets   2. LVM masks the underlying physical technology (ATA,ATAPI,IDE,SCSI,SATA,PATA,etc.)   3. LVM represents storage using a hierarchy:    a. Volume groups     a1. Physical volumes (/dev/sda2, /dev/sdb2, etc.)    b. Logical Volumes     b1. File systems   3. LVM physical volumes can be of various sizes   4. Ability to resize volumes on the fly Note: Volume groups join: physical volumes (PVs) and Logical Volumes (LVs) Steps to setup LVM:  1. Create LVM partitions via fdisk or parted   a. fdisk /dev/sda, /dev/sdb, /dev/sdc   b. n   c. p   d. +10G   e. t - change to type '8e' (LVM)   f. w   g. partprobe /dev/sda  2. Create Physical Volumes using 'pvcreate'   ...
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