Red Hat Enterprise Linux 6 High Availability Add-On Overview Overview of the High Availability Add-On for Red Hat Enterprise Linux Edition 6
Red Hat Enterprise Linux 6 High Availability Add-On Overview Overview of the High Availability Add-On for Red Hat Enterprise Linux Edition 6
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T able of Cont ent s T able of Contents .Int . .roduct . . . . . .ion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. . . . . . . . . . 1. Do c ument Co nventio ns 3 1.1. Typ o g rap hic Co nventio ns 4 1.2. Pull-q uo te Co nventio ns 5 1.3. No tes and Warning s 6 2 . We Need Feed b ac k! 6 . .hapt C . . . .er . .1. .. High . . . . .Availabilit ........y . .Add. . . .O . .n. O . .verview . . . .
Red Hat Ent erprise Linux 6 High Availabilit y Add- O n O verview 7 .1.1. G eneral Rec o mmend atio ns 7 .2. G ues t Clus ters 7 .2.1. Us ing fenc e_s c s i and iSCSI Shared Sto rag e 7 .2.2. G eneral Rec o mmend atio ns 31 32 33 34 . . . . . . . . .Hist Revision . . . ory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 ...........
Int roduct ion Introduction This document provides a high-level overview of the High Availability Add-On for Red Hat Enterprise Linux 6. Although the information in this document is an overview, you should have advanced working knowledge of Red Hat Enterprise Linux and understand the concepts of server computing to gain a good comprehension of the information.
Red Hat Ent erprise Linux 6 High Availabilit y Add- O n O verview 1.1. T ypographic Convent ions Four typographic conventions are used to call attention to specific words and phrases. These conventions, and the circumstances they apply to, are as follows. Mo no -spaced Bo l d Used to highlight system input, including shell commands, file names and paths. Also used to highlight keys and key combinations.
Int roduct ion Whether mono-spaced bold or proportional bold, the addition of italics indicates replaceable or variable text. Italics denotes text you do not input literally or displayed text that changes depending on circumstance. For example: To connect to a remote machine using ssh, type ssh username@ domain.name at a shell prompt. If the remote machine is exampl e. co m and your username on that machine is john, type ssh jo hn@ exampl e. co m.
Red Hat Ent erprise Linux 6 High Availabilit y Add- O n O verview o ut: mutex_unlock(& kvm->lock); return r; } 1.3. Not es and Warnings Finally, we use three visual styles to draw attention to information that might otherwise be overlooked. Note Notes are tips, shortcuts or alternative approaches to the task at hand. Ignoring a note should have no negative consequences, but you might miss out on a trick that makes your life easier.
Chapt er 1 . High Availabilit y Add- O n O verview Chapter 1. High Availability Add-On Overview The High Availability Add-On is a clustered system that provides reliability, scalability, and availability to critical production services. The following sections provide a high-level description of the components and functions of the High Availability Add-On: Section 1.1, “ Cluster Basics” Section 1.2, “ High Availability Add-On Introduction” Section 1.3, “ Cluster Infrastructure” 1.1.
Red Hat Ent erprise Linux 6 High Availabilit y Add- O n O verview Note The cluster types summarized in the preceding text reflect basic configurations; your needs might require a combination of the clusters described. Additionally, the Red Hat Enterprise Linux High Availability Add-On contains support for configuring and managing high availability servers only. It does not support high-performance clusters. 1.2.
Chapt er 1 . High Availabilit y Add- O n O verview cluster infrastructure, you can use other components to suit your clustering needs (for example, setting up a cluster for sharing files on a GFS2 file system or setting up service failover).
Red Hat Ent erprise Linux 6 High Availabilit y Add- O n O verview Chapter 2. Cluster Management with CMAN Cluster management manages cluster quorum and cluster membership. CMAN (an abbreviation for cluster manager) performs cluster management in the High Availability Add-On for Red Hat Enterprise Linux. CMAN is a distributed cluster manager and runs in each cluster node; cluster management is distributed across all nodes in the cluster.
Chapt er 2 . Clust er Management wit h CMAN 2.1.1. Quorum Disks A quorum disk or partition is a section of a disk that's set up for use with components of the cluster project. It has a couple of purposes. Again, I'll explain with an example. Suppose you have nodes A and B, and node A fails to get several of cluster manager's " heartbeat" packets from node B.
Red Hat Ent erprise Linux 6 High Availabilit y Add- O n O verview Have a two node configuration with the fence devices on a different network path than the path used for cluster communication Have a two node configuration where fencing is at the fabric level - especially for SCSI reservations However, if you have a correct network and fencing configuration in your cluster, a tie-breaker only adds complexity, except in pathological cases.
Chapt er 3. RG Manager Chapter 3. RGManager RGManager manages and provides failover capabilities for collections of cluster resources called services, resource groups, or resource trees. These resource groups are tree-structured, and have parent-child dependency and inheritance relationships within each subtree. How RGManager works is that it allows administrators to define, configure, and monitor cluster services.
Red Hat Ent erprise Linux 6 High Availabilit y Add- O n O verview members within the domain. The highest-ranking member of the domain will run the service whenever it is online. This means that if member A has a higher-rank than member B, the service will migrate to A if it was running on B if A transitions from offline to online. unordered domain: The default behavior, members of the domain have no order of preference; any member may run the service.
Chapt er 3. RG Manager RGManager has three service recovery policies which may be customized by the administrator on a per-service basis. Note These policies also apply to virtual machine resources. 3.2.1. St art Policy RGManager by default starts all services when rgmanager boots and a quorum is present. This behavior may be altered by administrators. autostart (default) - start the service when rgmanager boots and a quorum forms.
Red Hat Ent erprise Linux 6 High Availabilit y Add- O n O verview Note You must specify both parameters together; the use of either parameter by itself is undefined. 3.3. Resource T rees - Basics / Definit ions The following illustrates the structure of a resource tree, with a correpsonding list that defines each area.
Chapt er 3. RG Manager the original start fails, the service behaves as though a relocate operation was requested (see below). If the operation succeeds, the service is placed in the started state. disable — stop the service and place into the disabled state. This is the only permissible operation when a service is in the failed state. relocate — move the service to another node.
Red Hat Ent erprise Linux 6 High Availabilit y Add- O n O verview failed — The service is presumed dead. This state occurs whenever a resource's stop operation fails. Administrator must verify that there are no allocated resources (mounted file systems, etc.) prior to issuing a disable request. The only action which can take place from this state is disable. stopped — When in the stopped state, the service will be evaluated for starting after the next service or node transition.
Chapt er 3. RG Manager live (default) — the virtual machine continues to run while most of its memory contents are copied to the destination host. This minimizes the inaccessibility of the VM (typically well under 1 second) at the expense of performance of the VM during the migration and total amount of time it takes for the migration to complete. pause - the virtual machine is frozen in memory while its memory contents are copied to the destination host.
Red Hat Ent erprise Linux 6 High Availabilit y Add- O n O verview Adding or removing a VM from cluster.conf will not start or stop the VM; it will simply cause rgmanager to start or stop paying attention to the VM Failback (moving to a more preferred node) is performed using migration to minimize downtime. 3.5.4 . Unhandled Behaviors The following conditions and user actions are not supported in RGManager.
Chapt er 4 . Fencing Chapter 4. Fencing Fencing is the disconnection of a node from the cluster's shared storage. Fencing cuts off I/O from shared storage, thus ensuring data integrity. The cluster infrastructure performs fencing through the fence daemon, fenced . When CMAN determines that a node has failed, it communicates to other cluster-infrastructure components that the node has failed. fenced , when notified of the failure, fences the failed node.
Red Hat Ent erprise Linux 6 High Availabilit y Add- O n O verview Fig u re 4 .1.
Chapt er 4 . Fencing Fig u re 4 .2. St o rag e Fen cin g Examp le Specifying a fencing method consists of editing a cluster configuration file to assign a fencingmethod name, the fencing agent, and the fencing device for each node in the cluster. The way in which a fencing method is specified depends on if a node has either dual power supplies or multiple paths to storage.
Red Hat Ent erprise Linux 6 High Availabilit y Add- O n O verview Fig u re 4 .3.
Chapt er 4 . Fencing Fig u re 4 .4 . Fen cin g a N o d e wit h D u al Fib re C h an n el C o n n ect io n s You can configure a node with one fencing method or multiple fencing methods. When you configure a node for one fencing method, that is the only fencing method available for fencing that node.
Red Hat Ent erprise Linux 6 High Availabilit y Add- O n O verview Chapter 5. Lock Management Lock management is a common cluster-infrastructure service that provides a mechanism for other cluster infrastructure components to synchronize their access to shared resources. In a Red Hat cluster, D LM (D istributed Lock Manager) is the lock manager. A lock manager is a traffic cop who controls access to resources in the cluster, such as access to a GFS file system.
Chapt er 5. Lock Management listed above. The choice of an open source or closed source environment is up to the user. However, the D LM’s main limitation is the amount of testing performed with different environments. 5.2. Lock St at es A lock state indicates the current status of a lock request. A lock is always in one of three states: Granted — The lock request succeeded and attained the requested mode.
Red Hat Ent erprise Linux 6 High Availabilit y Add- O n O verview Chapter 6. Configuration and Administration Tools The cluster configuration file, /etc/cl uster/cl uster. co nf specifies the High Availability Add-On configuration.The configuration file is an XML file that describes the following cluster characteristics: Cluster name — Specifies the cluster name, cluster configuration file revision level, and basic fence timing properties used when a node joins a cluster or is fenced from the cluster.
Chapt er 6 . Configurat ion and Administ rat ion T ools provides support for the ccs cluster configuration command, which allows an administrator to create, modify and view the cluster.conf cluster configuration file. Refer to the Cluster Administration manual for information about configuring and managing the High Availability Add-On with the ccs comand. Note system-co nfi g -cl uster is not available in RHEL 6.
Red Hat Ent erprise Linux 6 High Availabilit y Add- O n O verview Chapter 7. Virtualization and High Availability Various virtualization platforms are supported in conjunction with Red Hat Enterprise Linux 6 using the High Availability and Resilient Storage Add-Ons. There are two supported use cases for virtualization in conjunction with Red Hat Enterprise Linux High Availability Add-on. This refers to RHEL Cluster/HA running on bare metal hosts that are themselves usable as virtualization platforms.
Chapt er 7 . Virt ualiz at ion and High Availabilit y RHEL 5 supports two virtualization platforms. Xen has been supported since RHEL 5.0 release. In RHEL 5.4 KVM was introduced. RHEL 6 only supports KVM as a virtualization platform. RHEL 5 AP Cluster supports both KVM and Xen for use in running virtual machines that are managed by the host cluster infrastructure. RHEL 6 HA supports KVM for use in running virtual machines that are managed by the host cluster infrastructure.
Red Hat Ent erprise Linux 6 High Availabilit y Add- O n O verview Host hardware should be provisioned such that they are capable of absorbing relocated guests from multiple other failed hosts without causing a host to overcommit memory or severely overcommit virtual CPUs. If enough failures occur to cause overcommit of either memory of virtual CPUs this can lead to severe performance degradation and potentially cluster failure.
Chapt er 7 . Virt ualiz at ion and High Availabilit y Using the Qpid Management Framework (QMF) provided by the libvirt-qpid package. This utilizes QMF to track guest migrations without requiring a full host cluster to be present. Shared storage can be provided by either iSCSI or KVM shared block devices backed by either host block storage or by file backed storage (raw images). Red Hat Enterprise Virtualization Management (RHEV-M) versions 2.2+ and 3.0 currently support RHEL 5.6+ and RHEL 6.
Red Hat Ent erprise Linux 6 High Availabilit y Add- O n O verview The iSCSI server software shipped with RHEL does not support SCSI 3 persistent reservations, therefore it cannot be used with fence_scsi. It is suitable for use as a shared storage solution in conjunction with other fence devices like fence_vmware or fence_rhevm, however.
Revision Hist ory Revision History R evisio n 1- 13 Wed O ct 8 2014 GA Release for Red Hat Enterprise Linux 6.6 St even Levin e R evisio n 1- 12 T h u Au g 7 2014 Beta Release for Red Hat Enterprise Linux 6.6 St even Levin e R evisio n 1- 11 Resolves: #852720 Small editorial issues Fri Au g 1 2014 St even Levin e R evisio n 1- 10 Fri Ju n 6 2014 D raft for Red Hat Enterprise Linux 6.6 St even Levin e R evisio n 1- 7 Wed N o v 20 2013 Release for GA of Red Hat Enterprise Linux 6.
