Fencing (computing)

Last updated
An NEC Nehalem cluster Nec-cluster.jpg
An NEC Nehalem cluster

Fencing is the process of isolating a node of a computer cluster or protecting shared resources when a node appears to be malfunctioning. [1] [2]

Contents

As the number of nodes in a cluster increases, so does the likelihood that one of them may fail at some point. The failed node may have control over shared resources that need to be reclaimed and if the node is acting erratically, the rest of the system needs to be protected. Fencing may thus either disable the node, or disallow shared storage access, thus ensuring data integrity.

Basic concepts

A node fence (or I/O fence) is a virtual "fence" that separates nodes which must not have access to a shared resource from that resource. It may separate an active node from its backup. If the backup crosses the fence and, for example, tries to control the same disk array as the primary, a data hazard may occur. Mechanisms such as STONITH are designed to prevent this condition.

Isolating a node means ensuring that I/O can no longer be done from it. Fencing is typically done automatically, by cluster infrastructure such as shared disk file systems, in order to protect processes from other active nodes modifying the resources during node failures. Mechanisms to support fencing, such as the reserve/release mechanism of SCSI, have existed since at least 1985. [3]

Fencing is required because it is impossible to distinguish between a real failure and a temporary hang. If the malfunctioning node is really down, then it cannot do any damage, so theoretically no action would be required (it could simply be brought back into the cluster with the usual join process). However, because there is a possibility that a malfunctioning node could itself consider the rest of the cluster to be the one that is malfunctioning, a split brain condition could ensue, and cause data corruption. Instead, the system has to assume the worst scenario and always fence in case of problems.

Approaches to fencing

There are two classes of fencing methods, one which disables a node itself, the other disallows access to resources such as shared disks. [1] In some cases, it is assumed that if a node does not respond after a given time-threshold it may be assumed as non-operational, although there are counterexamples, e.g. a long paging rampage. [1]

The STONITH method stands for "Shoot The Other Node In The Head", meaning that the suspected node is disabled or powered off. For instance, power fencing uses a power controller to turn off an inoperable node. The node may then restart itself and join the cluster later. However, there are approaches in which an operator is informed of the need for a manual restart for the node. [1]

The resources fencing approach disallows access to resources without powering off the node. This may include:

  • Persistent reservation fencing uses the SCSI3 persistent reservations to block access to shared storage.
  • Fibre Channel fencing disables the fibre channel port
  • Global network block device (GNBD) fencing which disables access to the GNBD server

When the cluster has only two nodes, the reserve/release method may be used as a two node STONITH whereby upon detecting that node B has 'failed', node A will issue the reserve and obtain all resources (e.g. shared disk) for itself. Node B will be disabled if it tries to do I/O (in case it was temporarily hung). On node B the I/O failure triggers some code to kill the node.

Persistent reservation is essentially a match on a key, so the node which has the right key can do I/O, otherwise its I/O fails. Therefore, it is sufficient to change the key on a failure to ensure the right behavior during failure. However, it may not always be possible to change the key on the failed node.

STONITH is an easier and simpler method to implement on multiple clusters, while the various approaches to resources fencing require specific implementation approaches for each cluster implementation. [1]

See also

Related Research Articles

Mutual exclusion

In computer science, mutual exclusion is a property of concurrency control, which is instituted for the purpose of preventing race conditions. It is the requirement that one thread of execution never enters its critical section at the same time that another concurrent thread of execution enters its own critical section, which refers to an interval of time during which a thread of execution accesses a shared resource, such as shared memory.

In computing, iSCSI is an acronym for Internet Small Computer Systems Interface, an Internet Protocol (IP)-based storage networking standard for linking data storage facilities. It provides block-level access to storage devices by carrying SCSI commands over a TCP/IP network. iSCSI is used to facilitate data transfers over intranets and to manage storage over long distances. It can be used to transmit data over local area networks (LANs), wide area networks (WANs), or the Internet and can enable location-independent data storage and retrieval.

OpenSSI is an open-source single-system image clustering system. It allows a collection of computers to be treated as one large system, allowing applications running on any one machine access to the resources of all the machines in the cluster.

In distributed computing, a single system image (SSI) cluster is a cluster of machines that appears to be one single system. The concept is often considered synonymous with that of a distributed operating system, but a single image may be presented for more limited purposes, just job scheduling for instance, which may be achieved by means of an additional layer of software over conventional operating system images running on each node. The interest in SSI clusters is based on the perception that they may be simpler to use and administer than more specialized clusters.

In computing, the Global File System 2 or GFS2 is a shared-disk file system for Linux computer clusters. GFS2 allows all members of a cluster to have direct concurrent access to the same shared block storage, in contrast to distributed file systems which distribute data throughout the cluster. GFS2 can also be used as a local file system on a single computer.

Diskless node

A diskless node is a workstation or personal computer without disk drives, which employs network booting to load its operating system from a server.

STONITH, sometimes called STOMITH, is a technique for fencing in computer clusters.

High-availability clusters are groups of computers that support server applications that can be reliably utilized with a minimum amount of down-time. They operate by using high availability software to harness redundant computers in groups or clusters that provide continued service when system components fail. Without clustering, if a server running a particular application crashes, the application will be unavailable until the crashed server is fixed. HA clustering remedies this situation by detecting hardware/software faults, and immediately restarting the application on another system without requiring administrative intervention, a process known as failover. As part of this process, clustering software may configure the node before starting the application on it. For example, appropriate file systems may need to be imported and mounted, network hardware may have to be configured, and some supporting applications may need to be running as well.

The IBM SAN Volume Controller (SVC) is a block storage virtualization appliance that belongs to the IBM System Storage product family. SVC implements an indirection, or "virtualization", layer in a Fibre Channel storage area network (SAN).

Distributed Replicated Block Device Distributed replicated storage system for Linux

DRBD is a distributed replicated storage system for the Linux platform. It is implemented as a kernel driver, several userspace management applications, and some shell scripts. DRBD is traditionally used in high availability (HA) computer clusters, but beginning with DRBD version 9, it can also be used to create larger software defined storage pools with a focus on cloud integration.

The Red Hat Cluster includes software to create a high availability and load balancing cluster. Both can be used on the same system although this use case is unlikely. Both products, the High Availability Add-On and Load Balancer Add-On, are based on open-source community projects. Red Hat Cluster developers contribute code upstream for the community. Computational clustering is not part of cluster suite, but instead provided by Red Hat MRG.

A clustered file system is a file system which is shared by being simultaneously mounted on multiple servers. There are several approaches to clustering, most of which do not employ a clustered file system. Clustered file systems can provide features like location-independent addressing and redundancy which improve reliability or reduce the complexity of the other parts of the cluster. Parallel file systems are a type of clustered file system that spread data across multiple storage nodes, usually for redundancy or performance.

oVirt

oVirt is a free, open-source virtualization management platform. It was founded by Red Hat as a community project on which Red Hat Enterprise Virtualization is based. It allows centralized management of virtual machines, compute, storage and networking resources, from an easy-to-use web-based front-end with platform independent access. KVM on x86-64 and PowerPC64 architecture are the only hypervisors supported, but there is an ongoing effort to support ARM architecture in the future releases.

Computer cluster

A computer cluster is a set of loosely or tightly connected computers that work together so that, in many aspects, they can be viewed as a single system. Unlike grid computers, computer clusters have each node set to perform the same task, controlled and scheduled by software.

NonStop Clusters (NSC) was an add-on package for SCO UnixWare that allowed creation of fault-tolerant single-system image clusters of machines running UnixWare. NSC was one of the first commercially available highly available clustering solutions for commodity hardware.

Storage area network Network which provides access to consolidated, block-level data storage

A storage area network (SAN) or storage network is a computer network which provides access to consolidated, block-level data storage. SANs are primarily used to access storage devices, such as disk arrays and tape libraries from servers so that the devices appear to the operating system as direct-attached storage. A SAN typically is a dedicated network of storage devices not accessible through the local area network (LAN).

Cluster Shared Volumes (CSV) is a feature of Failover Clustering first introduced in Windows Server 2008 R2 for use with the Hyper-V role. A Cluster Shared Volume is a shared disk containing an NTFS or ReFS volume that is made accessible for read and write operations by all nodes within a Windows Server Failover Cluster.

Eucalyptus is a paid and open-source computer software for building Amazon Web Services (AWS)-compatible private and hybrid cloud computing environments, originally developed by the company Eucalyptus Systems. Eucalyptus is an acronym for Elastic Utility Computing Architecture for Linking Your Programs To Useful Systems. Eucalyptus enables pooling compute, storage, and network resources that can be dynamically scaled up or down as application workloads change. Mårten Mickos was the CEO of Eucalyptus. In September 2014, Eucalyptus was acquired by Hewlett-Packard and then maintained by DXC Technology. After DXC stopped developing the product in late 2017, AppScale Systems forked the code and started supporting Eucalyptus customers.

Google Compute Engine (GCE) is the Infrastructure as a Service (IaaS) component of Google Cloud Platform which is built on the global infrastructure that runs Google's search engine, Gmail, YouTube and other services. Google Compute Engine enables users to launch virtual machines (VMs) on demand. VMs can be launched from the standard images or custom images created by users. GCE users must authenticate based on OAuth 2.0 before launching the VMs. Google Compute Engine can be accessed via the Developer Console, RESTful API or command-line interface (CLI).

Kubernetes is an open-source container-orchestration system for automating computer application deployment, scaling, and management.

References

  1. 1 2 3 4 5 "Alan Robertson Resource fencing using STONITH" (PDF). IBM Linux Research Center. Archived from the original (PDF) on 2021-01-05.
  2. Sun Cluster environment: Sun Cluster 2.2 by Enrique Vargas, Joseph Bianco, David Deeths 2001 ISBN page 58
  3. "Small Computer Standards Interface". ANSI X3.131-1986.