Checkpoint/Restore In Userspace (CRIU) (pronounced ''kree-oo'', ), is a software tool for the

Linux Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, which ...

operating system. Using this tool, it is possible to freeze a running application (or part of it) and checkpoint it to persistent storage as a collection of files. One can then use the files to restore and run the application from the point it was frozen at. The distinctive feature of the CRIU project is that it is mainly implemented in

user space A modern computer operating system usually segregates virtual memory into user space and kernel space. Primarily, this separation serves to provide memory protection and hardware protection from malicious or errant software behaviour. Kernel ...

, rather than in the kernel.

History

The initial version of CRIU software was presented to the Linux developers community by Pavel Emelyanov, the

OpenVZ OpenVZ (Open Virtuozzo) is an operating-system-level virtualization technology for Linux. It allows a physical server to run multiple isolated operating system instances, called containers, virtual private servers (VPSs), or virtual environments ...

kernel team leader, on 15 July 2011. In September 2011, the project was presented at the Linux Plumbers Conference. In general, most of the attendees took a positive view of the project, which is proven by the fact that a number of kernel patches required for implementing the project were included in the mainline kernel. Andrew Morton, however, was a bit skeptical: The project is currently under active development, with monthly release cycle for stable releases.

Use

The CRIU tool is being developed as part of the OpenVZ project, with the aim of replacing the in-kernel checkpoint/restore. Though its main focus is to support the migration of containers, allowing users to check-point and restore the current state of running processes and process groups. The tool can currently be used on

x86-64 x86-64 (also known as x64, x86_64, AMD64, and Intel 64) is a 64-bit version of the x86 instruction set, first released in 1999. It introduced two new modes of operation, 64-bit mode and compatibility mode, along with a new 4-level paging mod ...

and

ARM In human anatomy, the arm refers to the upper limb in common usage, although academically the term specifically means the upper arm between the glenohumeral joint (shoulder joint) and the elbow joint. The distal part of the upper limb between the ...

systems and supports the following features: * Processes: their

hierarchy A hierarchy (from Greek: , from , 'president of sacred rites') is an arrangement of items (objects, names, values, categories, etc.) that are represented as being "above", "below", or "at the same level as" one another. Hierarchy is an important ...

, PIDs, user and group authenticators ( UID, GID, SID, etc.), system capabilities, threads, and running and stopped states * Application memory: memory-mapped files and

shared memory In computer science, shared memory is memory that may be simultaneously accessed by multiple programs with an intent to provide communication among them or avoid redundant copies. Shared memory is an efficient means of passing data between progr ...

* Open files *

Pipes Pipe(s), PIPE(S) or piping may refer to: Objects * Pipe (fluid conveyance), a hollow cylinder following certain dimension rules ** Piping, the use of pipes in industry * Smoking pipe ** Tobacco pipe * Half-pipe and quarter pipe, semi-circula ...

and FIFOs *

Unix domain sockets A Unix domain socket aka UDS or IPC socket (inter-process communication socket) is a data communications endpoint for exchanging data between processes executing on the same host operating system. It is also referred to by its address family AF_UN ...

Network socket A network socket is a software structure within a network node of a computer network that serves as an endpoint for sending and receiving data across the network. The structure and properties of a socket are defined by an application programming ...

s, including

TCP TCP may refer to: Science and technology * Transformer coupled plasma * Tool Center Point, see Robot end effector Computing * Transmission Control Protocol, a fundamental Internet standard * Telephony control protocol, a Bluetooth communication s ...

sockets in ESTABLISHED state (see below) * System V IPC * Timers * Signals * Terminals * Linux kernel-specific system calls:

inotify 

inotify (inode notify) is a Linux kernel subsystem created by John McCutchan, which monitors changes to the filesystem, and reports those changes to applications. It can be used to automatically update directory views, reload configuration files,  ...

, signalfd, eventfd and

epoll 

epoll is a Linux kernel system call for a scalable I/O event notification mechanism, first introduced in version 2.5.44 of the Linux kernel. Its function is to monitor multiple file descriptors to see whether I/O is possible on any of them. It is  ...

, no kernel patching is required because all of the required functionality has already been merged into the

Linux kernel mainline The Linux kernel is a free and open-source, monolithic, modular, multitasking, Unix-like operating system kernel. It was originally authored in 1991 by Linus Torvalds for his i386-based PC, and it was soon adopted as the kernel for the GNU oper ...

since kernel version 3.11, which was released on September 2, 2013.

TCP connection migration

One of the initial project goals was to support the migration of TCP connections, the biggest challenge being to suspend and then restore only one side of a connection. This was necessary for performing the live migration of containers (along with all their active network connections) between physical servers, the main scenario of using the checkpoint/restore feature in OpenVZ. To cope with this problem, a new feature, "TCP repair mode", was implemented. The feature was included in version 3.5 of the

and provides users with additional means to disassemble and reconstruct TCP sockets without the necessity of exchanging network packets with the opposite side of the connection.

Similar projects

The following projects provide functionality similar to CRIU: * OpenVZ * DMTCP * BLCR * Linux C/R

History

Use

TCP connection migration

Similar projects

References

Further reading