Explanation
The command lxc-checkpoint is used to checkpoint and restore containers. Checkpointing a container means saving the state of the container, including its memory, processes, file descriptors, and network connections, to a file or a directory. Restoring a container means resuming the container from the saved state, as if it was never stopped. Checkpointing and restoring containers can be useful for various purposes, such as live migration, backup, debugging, or snapshotting. The command lxc-checkpoint has the following syntax:
lxc-checkpoint {-n name} {-D path} [-r] [-s] [-v] [-d] [-F]
The options are:
* -n name: Specify the name of the container to checkpoint or restore.
* -D path: Specify the path to the file or directory where the checkpoint data is dumped or restored.
* -r, --restore: Restore the checkpoint for the container, instead of dumping it. This option is incompatible with -s.
* -s, --stop: Optionally stop the container after dumping. This option is incompatible with -r.
* -v, --verbose: Enable verbose criu logging. Only available when providing -r.
* -d, --daemon: Restore the container in the background (this is the default). Only available when providing -r.
* -F, --foreground: Restore the container in the foreground. Only available when providing -r.
The command lxc-checkpoint uses the CRIU (Checkpoint/Restore In Userspace) tool to perform the checkpoint and restore operations. CRIU is a software that can freeze a running application (or part of it) and checkpoint it to a hard drive as a collection of files. It can then use the files to restore and run the application from the point it was frozen at1.
The other statements about the command lxc-checkpoint are not correct. It does not create a clone or an image of a container, nor does it double the memory consumptionof the container. It can work on both running and stopped containers, depending on the options provided. References:
* Linux Containers - LXC - Manpages - lxc-checkpoint.12
* lxc-checkpoint(1) - Linux manual page - man7.org3
* CRIU4

/etc/docker/daemon.json
Explanation
The default path to the Docker daemon configuration file on Linux is /etc/docker/daemon.json. This file is a JSON file that contains the settings and options for the Docker daemon, which is the service that runs on the host operating system and manages the containers, images, networks, and other Docker resources. The
/etc/docker/daemon.json file does not exist by default, but it can be created by the user to customize the Docker daemon behavior. The file can also be specified by using the --config-file flag when starting the Docker daemon. The file must be a valid JSON object and follow the syntax and structure of the dockerd reference docs12. References:
* Docker daemon configuration file - Medium3
* Docker daemon configuration overview | Docker Docs4
* docker daemon | Docker Docs5

Explanation
Cloud-init is a tool that processes configurations and runs through five stages during the initial boot of Linux VMs in a cloud. It allows users to customize a Linux VM as it boots for the first time, by applying user data to the instance. User data can include scripts, commands, packages, files, users, groups, SSH keys, and more.
Cloud-init can also interact with various cloud platforms and services, such as Azure, AWS, OpenStack, and others. The purpose of cloud-init is to prepare the generic image of an laaS instance to fit a specific instance's configuration, such as hostname, network, security, and application settings. References:
* Cloud-init - The standard for customising cloud instances
* Understanding cloud-init - Azure Virtual Machines
* Tutorial - Customize a Linux VM with cloud-init in Azure - Azure Virtual Machines

Explanation
Resource management for full virtualization is the process of allocating and controlling the physical resources of the host system to the virtual machines running on it. The hypervisor is the software layer that performs this task, by providing each virtual machine with a virtual hardware of a defined capacity that limits the resources of the virtual machine. For example, the hypervisor can specify how many virtual CPUs, how much memory, and how much disk space each virtual machine can use. The hypervisor can also enforce resource isolation and prioritization among the virtual machines, to ensure that they do not interfere with each other or consume more resources than they are allowed to. The hypervisor cannot provide fine-grained limits to internal elements of the guest operating system, such as the number of processes, because the hypervisor does not have access to the internal state of the guest operating system. The guest operating system is responsible for managing its own resources within the virtual hardware provided by the hypervisor. For example, the guest operating system can create an arbitrary amount of network sockets, as long as it does not exceed the network bandwidth allocated by the hypervisor. Full virtualization can pose limits to virtual machines, and does not always assign the host system's resources in a first-come-first-serve manner. The hypervisor can use various resource management techniques, such as reservation, limit, share, weight, and quota, to allocate and control the resources of the virtual machines. The hypervisor can also use resource scheduling algorithms, such as round-robin, fair-share, or priority-based, to distribute the resources among the virtual machines according to their needs and preferences. All processes created within the virtual machines are not transparently and equally scheduled in the host system for CPU and I/O usage. The hypervisor can use different scheduling policies, such as proportional-share, co-scheduling, or gang scheduling, to schedule the virtual CPUs of the virtual machines on the physical CPUs of the host system. The hypervisor can alsouse different I/O scheduling algorithms, such as deadline, anticipatory, or completely fair queuing, to schedule the I/O requests of the virtual machines on the physical I/O devices of the host system. The hypervisor can also use different resource accounting and monitoring mechanisms, such as cgroups, perf, or sar, to measure and report the resource consumption and performance of the virtual machines. References:
* Oracle VM VirtualBox: Features Overview
* Resource Management as an Enabling Technology for Virtualization - Oracle
* Introduction to virtualization and resource management in IaaS | Cloud Native Computing Foundation

Explanation
Cloud-init is a tool that allows users to customize the configuration and behavior of cloud instances during the boot process. Cloud-init can process different kinds of data that are passed to the instance via user-data, which is a mechanism provided by various cloud providers to inject data into the instance. Among the kinds of data that cloud-init can process directly from user-data are:
* Shell scripts to execute: Cloud-init can execute user-data that is formatted as a shell script, starting with the #!/bin/sh or #!/bin/bash shebang. The script can contain any commands that are valid in the shell environment of the instance. The script is executed as the root user during the boot process12.
* Lists of URLs to import: Cloud-init can import user-data that is formatted as a list of URLs, separated by newlines. The URLs can point to any valid data source that cloud-init supports, such as shell scripts, cloud-config files, or include files. The URLs are fetched and processed by cloud-init in the order they appear in the list13.
* cloud-config declarations in YAML: Cloud-init can process user-data that is formatted as a cloud-config file, which is a YAML document that contains declarations for various cloud-init modules. The cloud-config file can specify various aspects of the instance configuration, such as hostname, users, packages, commands, services, and more. The cloud-config file must start with the #cloud-config header14.
The other kinds of data listed in the question are not directly processed by cloud-init from user-data. They are either not supported, not recommended, or require additional steps to be processed. These kinds of data are:
* ISO images to boot from: Cloud-init does not support booting from ISO images that are passed as user-data. ISO images are typically used to install an operating system on a physical or virtual machine, not to customize an existing cloud instance. To boot from an ISO image, the user would need to attach it as a secondary disk to the instance and configure the boot order accordingly5.
* Base64-encoded binary files to execute: Cloud-init does not recommend passing binary files as user-data, as they may not be compatible with the instance's architecture or operating system.
Base64-encoding does not change this fact, as it only converts the binary data into ASCII characters. To execute a binary file, the user would need to decode it and make it executable on the instance6.
References:
* User-Data Formats - cloud-init 22.1 documentation
* User-Data Scripts
* Include File
* Cloud Config
* How to Boot From ISO Image File Directly in Windows
* How to run a binary file as a command in the terminal?.