X-Git-Url: https://git.proxmox.com/?p=pve-docs.git;a=blobdiff_plain;f=ha-manager.adoc;h=78bbf10ecafd76ed405a77c48b92cc47bf9ea2db;hp=7484ca9ad1d4cacfa3d70c6018a7bb60117c02be;hb=5771d9b0cb6bff7a2a57f8725c3aaa6dc5b25922;hpb=734404b4727bd5bb38418fa69b5e6dcc78a6aa76 diff --git a/ha-manager.adoc b/ha-manager.adoc index 7484ca9..78bbf10 100644 --- a/ha-manager.adoc +++ b/ha-manager.adoc @@ -24,26 +24,229 @@ High Availability include::attributes.txt[] endif::manvolnum[] -'ha-manager' handles management of user-defined cluster services. This -includes handling of user requests including service start, service -disable, service relocate, and service restart. The cluster resource -manager daemon also handles restarting and relocating services in the -event of failures. -HOW IT WORKS +Our modern society depends heavily on information provided by +computers over the network. Mobile devices amplified that dependency, +because people can access the network any time from anywhere. If you +provide such services, it is very important that they are available +most of the time. + +We can mathematically define the availability as the ratio of (A) the +total time a service is capable of being used during a given interval +to (B) the length of the interval. It is normally expressed as a +percentage of uptime in a given year. + +.Availability - Downtime per Year +[width="60%",cols="/lrm_status'. There the CRM may collect +it and let its state machine - respective the commands output - act on it. + +The actions on each service between CRM and LRM are normally always synced. +This means that the CRM requests a state uniquely marked by an UID, the LRM +then executes this action *one time* and writes back the result, also +identifiable by the same UID. This is needed so that the LRM does not +executes an outdated command. +With the exception of the 'stop' and the 'error' command, +those two do not depend on the result produce and are executed +always in the case of the stopped state and once in the case of +the error state. + +.Read the Logs +[NOTE] +The HA Stack logs every action it makes. This helps to understand what +and also why something happens in the cluster. Here its important to see +what both daemons, the LRM and the CRM, did. You may use +`journalctl -u pve-ha-lrm` on the node(s) where the service is and +the same command for the pve-ha-crm on the node which is the current master. + +Cluster Resource Manager +~~~~~~~~~~~~~~~~~~~~~~~~ The cluster resource manager ('pve-ha-crm') starts on each node and waits there for the manager lock, which can only be held by one node at a time. The node which successfully acquires the manager lock gets -promoted to the CRM, it handles cluster wide actions like migrations -and failures. +promoted to the CRM master. + +It can be in three states: + +* *wait for agent lock*: the LRM waits for our exclusive lock. This is + also used as idle sate if no service is configured +* *active*: the LRM holds its exclusive lock and has services configured +* *lost agent lock*: the LRM lost its lock, this means a failure happened + and quorum was lost. + +It main task is to manage the services which are configured to be highly +available and try to always enforce them to the wanted state, e.g.: a +enabled service will be started if its not running, if it crashes it will +be started again. Thus it dictates the LRM the actions it needs to execute. When an node leaves the cluster quorum, its state changes to unknown. If the current CRM then can secure the failed nodes lock, the services @@ -52,12 +255,12 @@ will be 'stolen' and restarted on another node. When a cluster member determines that it is no longer in the cluster quorum, the LRM waits for a new quorum to form. As long as there is no quorum the node cannot reset the watchdog. This will trigger a reboot -after 60 seconds. +after the watchdog then times out, this happens after 60 seconds. -CONFIGURATION +Configuration ------------- -The HA stack is well integrated int the Proxmox VE API2. So, for +The HA stack is well integrated in the Proxmox VE API2. So, for example, HA can be configured via 'ha-manager' or the PVE web interface, which both provide an easy to use tool. @@ -66,18 +269,93 @@ The resource configuration file can be located at '/etc/pve/ha/groups.cfg'. Use the provided tools to make changes, there shouldn't be any need to edit them manually. -RESOURCES/SERVICES AGENTS -------------------------- +Node Power Status +----------------- + +If a node needs maintenance you should migrate and or relocate all +services which are required to run always on another node first. +After that you can stop the LRM and CRM services. But note that the +watchdog triggers if you stop it with active services. -A resource or also called service can be managed by the -ha-manager. Currently we support virtual machines and container. +Package Updates +--------------- -GROUPS +When updating the ha-manager you should do one node after the other, never +all at once for various reasons. First, while we test our software +thoughtfully, a bug affecting your specific setup cannot totally be ruled out. +Upgrading one node after the other and checking the functionality of each node +after finishing the update helps to recover from an eventual problems, while +updating all could render you in a broken cluster state and is generally not +good practice. + +Also, the {pve} HA stack uses a request acknowledge protocol to perform +actions between the cluster and the local resource manager. For restarting, +the LRM makes a request to the CRM to freeze all its services. This prevents +that they get touched by the Cluster during the short time the LRM is restarting. +After that the LRM may safely close the watchdog during a restart. +Such a restart happens on a update and as already stated a active master +CRM is needed to acknowledge the requests from the LRM, if this is not the case +the update process can be too long which, in the worst case, may result in +a watchdog reset. + + +Fencing +------- + +What Is Fencing +~~~~~~~~~~~~~~~ + +Fencing secures that on a node failure the dangerous node gets will be rendered +unable to do any damage and that no resource runs twice when it gets recovered +from the failed node. This is a really important task and one of the base +principles to make a system Highly Available. + +If a node would not get fenced it would be in an unknown state where it may +have still access to shared resources, this is really dangerous! +Imagine that every network but the storage one broke, now while not +reachable from the public network the VM still runs and writes on the shared +storage. If we would not fence the node and just start up this VM on another +Node we would get dangerous race conditions, atomicity violations the whole VM +could be rendered unusable. The recovery could also simply fail if the storage +protects from multiple mounts and thus defeat the purpose of HA. + +How {pve} Fences +~~~~~~~~~~~~~~~~~ + +There are different methods to fence a node, for example fence devices which +cut off the power from the node or disable their communication completely. + +Those are often quite expensive and bring additional critical components in +a system, because if they fail you cannot recover any service. + +We thus wanted to integrate a simpler method in the HA Manager first, namely +self fencing with watchdogs. + +Watchdogs are widely used in critical and dependable systems since the +beginning of micro controllers, they are often independent and simple +integrated circuit which programs can use to watch them. After opening they need to +report periodically. If, for whatever reason, a program becomes unable to do +so the watchdogs triggers a reset of the whole server. + +Server motherboards often already include such hardware watchdogs, these need +to be configured. If no watchdog is available or configured we fall back to the +Linux Kernel softdog while still reliable it is not independent of the servers +Hardware and thus has a lower reliability then a hardware watchdog. + +Configure Hardware Watchdog +~~~~~~~~~~~~~~~~~~~~~~~~~~~ +By default all watchdog modules are blocked for security reasons as they are +like a loaded gun if not correctly initialized. +If you have a hardware watchdog available remove its module from the blacklist +and restart 'the watchdog-mux' service. + + +Groups ------ A group is a collection of cluster nodes which a service may be bound to. -GROUP SETTINGS +Group Settings ~~~~~~~~~~~~~~ nodes:: @@ -96,7 +374,7 @@ the resource won't automatically fail back when a more preferred node (re)joins the cluster. -RECOVERY POLICY +Recovery Policy --------------- There are two service recover policy settings which can be configured @@ -113,12 +391,12 @@ maximal number of tries to relocate the service to a different node. A relocate only happens after the max_restart value is exceeded on the actual node. The default is set to one. -Note that the relocate count state will only reset to zero when the +NOTE: The relocate count state will only reset to zero when the service had at least one successful start. That means if a service is re-enabled without fixing the error only the restart policy gets repeated. -ERROR RECOVERY +Error Recovery -------------- If after all tries the service state could not be recovered it gets @@ -136,7 +414,7 @@ killing its process) * *after* you fixed all errors you may enable the service again -SERVICE OPERATIONS +Service Operations ------------------ This are how the basic user-initiated service operations (via @@ -167,7 +445,7 @@ start and stop commands can be issued to the resource specific tools service state (enabled, disabled). -SERVICE STATES +Service States -------------- stopped::