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Ganeti automatic instance allocation |
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==================================== |
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|
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Documents Ganeti version 2.0 |
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|
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.. contents:: |
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|
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Introduction |
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------------ |
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|
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Currently in Ganeti the admin has to specify the exact locations for |
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an instance's node(s). This prevents a completely automatic node |
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evacuation, and is in general a nuisance. |
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|
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The *iallocator* framework will enable automatic placement via |
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external scripts, which allows customization of the cluster layout per |
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the site's requirements. |
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|
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User-visible changes |
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~~~~~~~~~~~~~~~~~~~~ |
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|
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There are two parts of the ganeti operation that are impacted by the |
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auto-allocation: how the cluster knows what the allocator algorithms |
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are and how the admin uses these in creating instances. |
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|
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An allocation algorithm is just the filename of a program installed in |
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a defined list of directories. |
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|
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Cluster configuration |
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~~~~~~~~~~~~~~~~~~~~~ |
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|
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At configure time, the list of the directories can be selected via the |
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``--with-iallocator-search-path=LIST`` option, where *LIST* is a |
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comma-separated list of directories. If not given, this defaults to |
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``$libdir/ganeti/iallocators``, i.e. for an installation under |
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``/usr``, this will be ``/usr/lib/ganeti/iallocators``. |
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|
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Ganeti will then search for allocator script in the configured list, |
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using the first one whose filename matches the one given by the user. |
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|
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Command line interface changes |
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
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|
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The node selection options in instanece add and instance replace disks |
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can be replace by the new ``--iallocator=NAME`` option (shortened to |
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``-I``), which will cause the auto-assignement of nodes with the |
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passed iallocator. The selected node(s) will be show as part of the |
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command output. |
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|
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IAllocator API |
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-------------- |
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|
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The protocol for communication between Ganeti and an allocator script |
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will be the following: |
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|
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#. ganeti launches the program with a single argument, a filename that |
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contains a JSON-encoded structure (the input message) |
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|
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#. if the script finishes with exit code different from zero, it is |
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considered a general failure and the full output will be reported to |
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the users; this can be the case when the allocator can't parse the |
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input message |
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|
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#. if the allocator finishes with exit code zero, it is expected to |
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output (on its stdout) a JSON-encoded structure (the response) |
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|
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Input message |
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~~~~~~~~~~~~~ |
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|
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The input message will be the JSON encoding of a dictionary containing |
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the following: |
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|
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version |
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the version of the protocol; this document |
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specifies version 1 |
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|
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cluster_name |
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the cluster name |
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|
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cluster_tags |
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the list of cluster tags |
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|
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request |
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a dictionary containing the request data: |
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|
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type |
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the request type; this can be either ``allocate`` or ``relocate``; |
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the ``allocate`` request is used when a new instance needs to be |
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placed on the cluster, while the ``relocate`` request is used when |
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an existing instance needs to be moved within the cluster |
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|
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name |
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the name of the instance; if the request is a realocation, then |
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this name will be found in the list of instances (see below), |
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otherwise is the FQDN of the new instance |
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|
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required_nodes |
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how many nodes should the algorithm return; while this information |
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can be deduced from the instace's disk template, it's better if |
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this computation is left to Ganeti as then allocator scripts are |
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less sensitive to changes to the disk templates |
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|
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disk_space_total |
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the total disk space that will be used by this instance on the |
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(new) nodes; again, this information can be computed from the list |
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of instance disks and its template type, but Ganeti is better |
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suited to compute it |
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|
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If the request is an allocation, then there are extra fields in the |
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request dictionary: |
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|
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disks |
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list of dictionaries holding the disk definitions for this |
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instance (in the order they are exported to the hypervisor): |
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|
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mode |
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either ``r`` or ``w`` denoting if the disk is read-only or |
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writable |
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|
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size |
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the size of this disk in mebibytes |
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|
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nics |
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a list of dictionaries holding the network interfaces for this |
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instance, containing: |
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|
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ip |
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the IP address that Ganeti know for this instance, or null |
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|
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mac |
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the MAC address for this interface |
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|
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bridge |
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the bridge to which this interface will be connected |
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|
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vcpus |
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the number of VCPUs for the instance |
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|
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disk_template |
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the disk template for the instance |
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|
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memory |
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the memory size for the instance |
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|
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os |
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the OS type for the instance |
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|
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tags |
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the list of the instance's tags |
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|
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|
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If the request is of type relocate, then there is one more entry in |
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the request dictionary, named ``relocate_from``, and it contains a |
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list of nodes to move the instance away from; note that with Ganeti |
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2.0, this list will always contain a single node, the current |
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secondary of the instance. |
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|
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instances |
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a dictionary with the data for the current existing instance on the |
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cluster, indexed by instance name; the contents are similar to the |
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instance definitions for the allocate mode, with the addition of: |
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|
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should_run |
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if this instance is set to run (but not the actual status of the |
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instance) |
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|
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nodes |
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list of nodes on which this instance is placed; the primary node |
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of the instance is always the first one |
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|
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nodes |
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dictionary with the data for the nodes in the cluster, indexed by |
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the node name; the dict contains: |
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|
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total_disk |
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the total disk size of this node (mebibytes) |
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|
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free_disk |
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the free disk space on the node |
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|
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total_memory |
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the total memory size |
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|
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free_memory |
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free memory on the node; note that currently this does not take |
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into account the instances which are down on the node |
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|
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total_cpus |
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the physical number of CPUs present on the machine; depending on |
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the hypervisor, this might or might not be equal to how many CPUs |
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the node operating system sees; |
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|
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primary_ip |
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the primary IP address of the node |
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|
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secondary_ip |
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the secondary IP address of the node (the one used for the DRBD |
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replication); note that this can be the same as the primary one |
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|
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tags |
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list with the tags of the node |
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|
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Respone message |
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~~~~~~~~~~~~~~~ |
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|
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The response message is much more simple than the input one. It is |
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also a dict having three keys: |
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|
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success |
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a boolean value denoting if the allocation was successfull or not |
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|
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info |
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a string with information from the scripts; if the allocation fails, |
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this will be shown to the user |
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|
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nodes |
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the list of nodes computed by the algorithm; even if the algorithm |
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failed (i.e. success is false), this must be returned as an empty |
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list; also note that the length of this list must equal the |
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``requested_nodes`` entry in the input message, otherwise Ganeti |
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will consider the result as failed |
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|
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Examples |
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-------- |
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|
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Input messages to scripts |
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~~~~~~~~~~~~~~~~~~~~~~~~~ |
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|
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Input message, new instance allocation:: |
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|
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{ |
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"cluster_tags": [], |
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"request": { |
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"required_nodes": 2, |
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"name": "instance3.example.com", |
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"tags": [ |
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"type:test", |
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"owner:foo" |
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], |
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"type": "allocate", |
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"disks": [ |
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{ |
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"mode": "w", |
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"size": 1024 |
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}, |
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{ |
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"mode": "w", |
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"size": 2048 |
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} |
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], |
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"nics": [ |
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{ |
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"ip": null, |
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"mac": "00:11:22:33:44:55", |
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"bridge": null |
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} |
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], |
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"vcpus": 1, |
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"disk_template": "drbd", |
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"memory": 2048, |
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"disk_space_total": 3328, |
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"os": "etch-image" |
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}, |
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"cluster_name": "cluster1.example.com", |
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"instances": { |
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"instance1.example.com": { |
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"tags": [], |
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"should_run": false, |
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"disks": [ |
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{ |
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"mode": "w", |
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"size": 64 |
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}, |
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{ |
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"mode": "w", |
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"size": 512 |
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} |
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], |
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"nics": [ |
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{ |
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"ip": null, |
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"mac": "aa:00:00:00:60:bf", |
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"bridge": "xen-br0" |
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} |
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], |
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"vcpus": 1, |
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"disk_template": "plain", |
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"memory": 128, |
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"nodes": [ |
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"nodee1.com" |
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], |
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"os": "etch-image" |
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}, |
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"instance2.example.com": { |
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"tags": [], |
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"should_run": false, |
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"disks": [ |
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{ |
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"mode": "w", |
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"size": 512 |
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}, |
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{ |
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"mode": "w", |
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"size": 256 |
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} |
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], |
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"nics": [ |
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{ |
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"ip": null, |
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"mac": "aa:00:00:55:f8:38", |
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"bridge": "xen-br0" |
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} |
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], |
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"vcpus": 1, |
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"disk_template": "drbd", |
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"memory": 512, |
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"nodes": [ |
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"node2.example.com", |
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"node3.example.com" |
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], |
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"os": "etch-image" |
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} |
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}, |
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"version": 1, |
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"nodes": { |
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"node1.example.com": { |
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"total_disk": 858276, |
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"primary_ip": "192.168.1.1", |
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"secondary_ip": "192.168.2.1", |
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"tags": [], |
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"free_memory": 3505, |
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"free_disk": 856740, |
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"total_memory": 4095 |
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}, |
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"node2.example.com": { |
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"total_disk": 858240, |
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"primary_ip": "192.168.1.3", |
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"secondary_ip": "192.168.2.3", |
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"tags": ["test"], |
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"free_memory": 3505, |
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"free_disk": 848320, |
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"total_memory": 4095 |
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}, |
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"node3.example.com.com": { |
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"total_disk": 572184, |
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"primary_ip": "192.168.1.3", |
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"secondary_ip": "192.168.2.3", |
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"tags": [], |
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"free_memory": 3505, |
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"free_disk": 570648, |
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"total_memory": 4095 |
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} |
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} |
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} |
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|
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Input message, reallocation. Since only the request entry in the input |
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message is changed, we show only this changed entry:: |
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|
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"request": { |
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"relocate_from": [ |
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"node3.example.com" |
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], |
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"required_nodes": 1, |
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"type": "relocate", |
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"name": "instance2.example.com", |
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"disk_space_total": 832 |
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}, |
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|
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|
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Response messages |
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~~~~~~~~~~~~~~~~~ |
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Successful response message:: |
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|
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{ |
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"info": "Allocation successful", |
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"nodes": [ |
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"node2.example.com", |
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"node1.example.com" |
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], |
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"success": true |
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} |
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|
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Failed response message:: |
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|
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{ |
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"info": "Can't find a suitable node for position 2 (already selected: node2.example.com)", |
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"nodes": [], |
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"success": false |
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} |
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|
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Command line messages |
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~~~~~~~~~~~~~~~~~~~~~ |
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:: |
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|
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# gnt-instance add -t plain -m 2g --os-size 1g --swap-size 512m --iallocator dumb-allocator -o etch-image instance3 |
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Selected nodes for the instance: node1.example.com |
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* creating instance disks... |
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[...] |
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|
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# gnt-instance add -t plain -m 3400m --os-size 1g --swap-size 512m --iallocator dumb-allocator -o etch-image instance4 |
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Failure: prerequisites not met for this operation: |
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Can't compute nodes using iallocator 'dumb-allocator': Can't find a suitable node for position 1 (already selected: ) |
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|
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# gnt-instance add -t drbd -m 1400m --os-size 1g --swap-size 512m --iallocator dumb-allocator -o etch-image instance5 |
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Failure: prerequisites not met for this operation: |
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Can't compute nodes using iallocator 'dumb-allocator': Can't find a suitable node for position 2 (already selected: node1.example.com) |