Ganeti monitoring agent

This is a design document detailing the implementation of a Ganeti monitoring agent report system, that can be queried by a monitoring system to calculate health information for a Ganeti cluster.

Current state and shortcomings

There is currently no monitoring support in Ganeti. While we don’t want to build something like Nagios or Pacemaker as part of Ganeti, it would be useful if such tools could easily extract information from a Ganeti machine in order to take actions (example actions include logging an outage for future reporting or alerting a person or system about it).

Proposed changes

Each Ganeti node should export a status page that can be queried by a monitoring system. Such status page will be exported on a network port and will be encoded in JSON (simple text) over HTTP.

The choice of JSON is obvious as we already depend on it in Ganeti and thus we don’t need to add extra libraries to use it, as opposed to what would happen for XML or some other markup format.

Location of agent report

The report will be available from all nodes, and be concerned for all node-local resources. This allows more real-time information to be available, at the cost of querying all nodes.

Information reported

The monitoring agent system will report on the following basic information:

  • Instance status
  • Instance disk status
  • Status of storage for instances
  • Ganeti daemons status, CPU usage, memory footprint
  • Hypervisor resources report (memory, CPU, network interfaces)
  • Node OS resources report (memory, CPU, network interfaces)
  • Information from a plugin system

Format of the report

The report of the will be in JSON format, and it will present an array of report objects. Each report object will be produced by a specific data collector. Each report object includes some mandatory fields, to be provided by all the data collectors:

The name of the data collector that produced this part of the report. It is supposed to be unique inside a report.
The version of the data collector that produces this part of the report. Built-in data collectors (as opposed to those implemented as plugins) should have “B” as the version number.
The format of what is represented in the “data” field for each data collector might change over time. Every time this happens, the format_version should be changed, so that who reads the report knows what format to expect, and how to correctly interpret it.
The time when the reported data were gathered. It has to be expressed in nanoseconds since the unix epoch (0:00:00 January 01, 1970). If not enough precision is available (or needed) it can be padded with zeroes. If a report object needs multiple timestamps, it can add more and/or override this one inside its own “data” section.
A collector can belong to a given category of collectors (e.g.: storage collectors, daemon collector). This means that it will have to provide a minumum set of prescribed fields, as documented for each category. This field will contain the name of the category the collector belongs to, if any, or just the null value.
Two kinds of collectors are possible: Performance reporting collectors and Status reporting collectors. The respective paragraphs will describe them and the value of this field.
This field contains all the data generated by the specific data collector, in its own independently defined format. The monitoring agent could check this syntactically (according to the JSON specifications) but not semantically.

Here follows a minimal example of a report:

    "name" : "TheCollectorIdentifier",
    "version" : "1.2",
    "format_version" : 1,
    "timestamp" : 1351607182000000000,
    "category" : null,
    "kind" : 0,
    "data" : { "plugin_specific_data" : "go_here" }
    "name" : "AnotherDataCollector",
    "version" : "B",
    "format_version" : 7,
    "timestamp" : 1351609526123854000,
    "category" : "storage",
    "kind" : 1,
    "data" : { "status" : { "code" : 1,
                            "message" : "Error on disk 2"
               "plugin_specific" : "data",
               "some_late_data" : { "timestamp" : 1351609526123942720,

Performance reporting collectors

These collectors only provide data about some component of the system, without giving any interpretation over their meaning.

The value of the kind field of the report will be 0.

Status reporting collectors

These collectors will provide information about the status of some component of ganeti, or managed by ganeti.

The value of their kind field will be 1.

The rationale behind this kind of collectors is that there are some situations where exporting data about the underlying subsystems would expose potential issues. But if Ganeti itself is able (and going) to fix the problem, conflicts might arise between Ganeti and something/somebody else trying to fix the same problem. Also, some external monitoring systems might not be aware of the internals of a particular subsystem (e.g.: DRBD) and might only exploit the high level response of its data collector, alerting an administrator if anything is wrong. Still, completely hiding the underlying data is not a good idea, as they might still be of use in some cases. So status reporting plugins will provide two output modes: one just exporting a high level information about the status, and one also exporting all the data they gathered. The default output mode will be the status-only one. Through a command line parameter (for stand-alone data collectors) or through the HTTP request to the monitoring agent (when collectors are executed as part of it) the verbose output mode providing all the data can be selected.

When exporting just the status each status reporting collector will provide, in its data section, at least the following field:


summarizes the status of the component being monitored and consists of two subfields:


It assumes a numeric value, encoded in such a way to allow using a bitset to easily distinguish which states are currently present in the whole cluster. If the bitwise OR of all the status fields is 0, the cluster is completely healty. The status codes are as follows:

The collector can determine that everything is working as intended.
Something is temporarily wrong but it is being automatically fixed by Ganeti. There is no need of external intervention.
The collector has failed to understand whether the status is good or bad. Further analysis is required. Interpret this status as a potentially dangerous situation.
The collector can determine that something is wrong and Ganeti has no way to fix it autonomously. External intervention is required.

A message to better explain the reason of the status. The exact format of the message string is data collector dependent.

The field is mandatory, but the content can be an empty string if the code is 0 (working as intended) or 1 (being fixed automatically).

If the status code is 2, the message should specify what has gone wrong. If the status code is 4, the message shoud explain why it was not possible to determine a proper status.

The data section will also contain all the fields describing the gathered data, according to a collector-specific format.

Instance status

At the moment each node knows which instances are running on it, which instances it is primary for, but not the cause why an instance might not be running. On the other hand we don’t want to distribute full instance “admin” status information to all nodes, because of the performance impact this would have.

As such we propose that:

  • Any operation that can affect instance status will have an optional “reason” attached to it (at opcode level). This can be used for example to distinguish an admin request, from a scheduled maintenance or an automated tool’s work. If this reason is not passed, Ganeti will just use the information it has about the source of the request. This reason information will be structured according to the Ganeti reason trail design document.
  • RPCs that affect the instance status will be changed so that the “reason” and the version of the config object they ran on is passed to them. They will then export the new expected instance status, together with the associated reason and object version to the status report system, which then will export those themselves.

Monitoring and auditing systems can then use the reason to understand the cause of an instance status, and they can use the timestamp to understand the freshness of their data even in the absence of an atomic cross-node reporting: for example if they see an instance “up” on a node after seeing it running on a previous one, they can compare these values to understand which data is freshest, and repoll the “older” node. Of course if they keep seeing this status this represents an error (either an instance continuously “flapping” between nodes, or an instance is constantly up on more than one), which should be reported and acted upon.

The instance status will be on each node, for the instances it is primary for, and its data section of the report will contain a list of instances, with at least the following fields for each instance:

The name of the instance.
The UUID of the instance (stable on name change).
The status of the instance (up/down/offline) as requested by the admin.
The actual status of the instance. It can be up, down, or hung if the instance is up but it appears to be completely stuck.
The uptime of the instance (if it is up, “null” otherwise).
The timestamp of the last known change to the instance state.
The last known reason for state change of the instance, described according to the JSON representation of a reason trail, as detailed in the reason trail design document.
It represents the status of the instance, and its format is the same as that of the status field of Status reporting collectors.

Each hypervisor should provide its own instance status data collector, possibly with the addition of more, specific, fields. The category field of all of them will be instance. The kind field will be 1.

Note that as soon as a node knows it’s not the primary anymore for an instance it will stop reporting status for it: this means the instance will either disappear, if it has been deleted, or appear on another node, if it’s been moved.

The code of the status field of the report of the Instance status data collector will be:

if status is 0 for all the instances it is reporting about.

Storage status

The storage status collectors will be a series of data collectors (drbd, rbd, plain, file) that will gather data about all the storage types for the current node (this is right now hardcoded to the enabled storage types, and in the future tied to the enabled storage pools for the nodegroup).

The name of each of these collector will reflect what storage type each of them refers to.

The category field of these collector will be storage.

The kind field will be 1 (Status reporting collectors).

The data section of the report will provide at least the following fields:

The amount of free space (in KBytes).
The amount of used space (in KBytes).
The total visible space (in KBytes).

Each specific storage type might provide more type-specific fields.

In case of error, the message subfield of the status field of the report of the instance status collector will disclose the nature of the error as a type specific information. Examples of these are “backend pv unavailable” for lvm storage, “unreachable” for network based storage or “filesystem error” for filesystem based implementations.

DRBD status

This data collector will run only on nodes where DRBD is actually present and it will gather information about DRBD devices.

Its kind in the report will be 1 (Status reporting collectors).

Its category field in the report will contain the value storage.

When executed in verbose mode, the data section of the report of this collector will provide the following fields:


Information about the DRBD version number, given by a combination of any (but at least one) of the following fields:

The DRBD driver version.
The API version number.
The protocol version.
The version of the source files.
Git hash of the source files.
Who built the binary, and, optionally, when.

A list of structures, each describing a DRBD device (a minor) and containing the following fields:

The device minor number.
The state of the connection. If it is “Unconfigured”, all the following fields are not present.
The role of the local resource.
The role of the remote resource.
The status of the local disk.
The status of the remote disk.
The replication protocol being used.
The input/output flags.

The performance indicators. This field will contain the following sub-fields:

KiB of data sent on the network.
KiB of data received from the network.
KiB of data written on local disk.
KiB of date read from the local disk.
Number of updates of the activity log.
Number of updates to the bitmap area of the metadata.
Number of open requests to the local I/O subsystem.
Number of requests sent to the partner but not yet answered.
Number of requests received by the partner but still to be answered.
Num of block input/output requests forwarded to DRBD but that have not yet been answered.
(Optional) Number of epoch objects. Not provided by all DRBD versions.
(Optional) Currently used write ordering method. Not provided by all DRBD versions.
(Optional) KiB of storage currently out of sync. Not provided by all DRBD versions.

(Optional) The status of the synchronization of the disk. This is present only if the disk is being synchronized, and includes the following fields:

The percentage of synchronized data.
How far the synchronization is. Written as “x/y”, where x and y are integer numbers expressed in the measurement unit stated in progressUnit
The measurement unit for the progress indicator.
The expected time before finishing the synchronization.
The speed of the synchronization.
The desiderd speed of the synchronization.
The measurement unit of the speed and want values. Expressed as “size/time”.
The name of the Ganeti instance this disk is associated to.

Ganeti daemons status

Ganeti will report what information it has about its own daemons. This should allow identifying possible problems with the Ganeti system itself: for example memory leaks, crashes and high resource utilization should be evident by analyzing this information.

The kind field will be 1 (Status reporting collectors).

Each daemon will have its own data collector, and each of them will have a category field valued daemon.

When executed in verbose mode, their data section will include at least:

The amount of used memory.
The measurement unit used for the memory.
The uptime of the daemon.
CPU usage
How much cpu the daemon is using (percentage).

Any other daemon-specific information can be included as well in the data section.

Hypervisor resources report

Each hypervisor has a view of system resources that sometimes is different than the one the OS sees (for example in Xen the Node OS, running as Dom0, has access to only part of those resources). In this section we’ll report all information we can in a “non hypervisor specific” way. Each hypervisor can then add extra specific information that is not generic enough be abstracted.

The kind field will be 0 (Performance reporting collectors).

Each of the hypervisor data collectory will be of category: hypervisor.

Node OS resources report

Since Ganeti assumes it’s running on Linux, it’s useful to export some basic information as seen by the host system.

The category field of the report will be null.

The kind field will be 0 (Performance reporting collectors).

The data section will include:

The number of available cpus.
A list with one element per cpu, showing its average load.
The current view of memory (free, used, cached, etc.)
A list with one element per filesystem, showing a summary of the total/available space.
A list with one element per network interface, showing the amount of sent/received data, error rate, IP address of the interface, etc.
A map using the name of a component Ganeti interacts (Linux, drbd, hypervisor, etc) as the key and its version number as the value.

Note that we won’t go into any hardware specific details (e.g. querying a node RAID is outside the scope of this, and can be implemented as a plugin) but we can easily just report the information above, since it’s standard enough across all systems.

Format of the query

The queries to the monitoring agent will be HTTP GET requests on port 1815. The answer will be encoded in JSON format and will depend on the specific accessed resource.

If a request is sent to a non-existing resource, a 404 error will be returned by the HTTP server.

The following paragraphs will present the existing resources supported by the current protocol version, that is version 1.


The root resource. It will return the list of the supported protocol version numbers.

Currently, this will include only version 1.


Not an actual resource per-se, it is the root of all the resources of protocol version 1.

If requested through GET, the null JSON value will be returned.


Returns a list of tuples (kind, category, name) showing all the collectors available in the system.


A list of the reports of all the data collectors, as a JSON list.

Status reporting collectors will provide their output in non-verbose format. The verbose format can be requested by adding the parameter verbose=1 to the request.


Returns the report of the collector [collector_name] that belongs to the specified [category].

The category has to be written in lowercase.

If a collector does not belong to any category, default will have to be used as the value for [category].

Status reporting collectors will provide their output in non-verbose format. The verbose format can be requested by adding the parameter verbose=1 to the request.

Instance disk status propagation

As for the instance status Ganeti has now only partial information about its instance disks: in particular each node is unaware of the disk to instance mapping, that exists only on the master.

For this design doc we plan to fix this by changing all RPCs that create a backend storage or that put an already existing one in use and passing the relevant instance to the node. The node can then export these to the status reporting tool.

While we haven’t implemented these RPC changes yet, we’ll use Confd to fetch this information in the data collectors.

Plugin system

The monitoring system will be equipped with a plugin system that can export specific local information through it.

The plugin system is expected to be used by local installations to export any installation specific information that they want to be monitored, about either hardware or software on their systems.

The plugin system will be in the form of either scripts or binaries whose output will be inserted in the report.

Eventually support for other kinds of plugins might be added as well, such as plain text files which will be inserted into the report, or local unix or network sockets from which the information has to be read. This should allow most flexibility for implementing an efficient system, while being able to keep it as simple as possible.

Data collectors

In order to ease testing as well as to make it simple to reuse this subsystem it will be possible to run just the “data collectors” on each node without passing through the agent daemon.

If a data collector is run independently, it should print on stdout its report, according to the format corresponding to a single data collector report object, as described in the previous paragraphs.

Mode of operation

In order to be able to report information fast the monitoring agent daemon will keep an in-memory or on-disk cache of the status, which will be returned when queries are made. The status system will then periodically check resources to make sure the status is up to date.

Different parts of the report will be queried at different speeds. These will depend on: - how often they vary (or we expect them to vary) - how fast they are to query - how important their freshness is

Of course the last parameter is installation specific, and while we’ll try to have defaults, it will be configurable. The first two instead we can use adaptively to query a certain resource faster or slower depending on those two parameters.

When run as stand-alone binaries, the data collector will not using any caching system, and just fetch and return the data immediately.

Implementation place

The status daemon will be implemented as a standalone Haskell daemon. In the future it should be easy to merge multiple daemons into one with multiple entry points, should we find out it saves resources and doesn’t impact functionality.

The libekg library should be looked at for easily providing metrics in json format.

Implementation order

We will implement the agent system in this order:

  • initial example data collectors (eg. for drbd and instance status).
  • initial daemon for exporting data, integrating the existing collectors
  • plugin system
  • RPC updates for instance status reasons and disk to instance mapping
  • cache layer for the daemon
  • more data collectors

Future work

As a future step it can be useful to “centralize” all this reporting data on a single place. This for example can be just the master node, or all the master candidates. We will evaluate doing this after the first node-local version has been developed and tested.

Another possible change is replacing the “read-only” RPCs with queries to the agent system, thus having only one way of collecting information from the nodes from a monitoring system and for Ganeti itself.

One extra feature we may need is a way to query for only sub-parts of the report (eg. instances status only). This can be done by passing arguments to the HTTP GET, which will be defined when we get to this funtionality.

Finally the autorepair system design. system (see its design) can be expanded to use the monitoring agent system as a source of information to decide which repairs it can perform.