This document describes the major changes in Ganeti 2.3 compared to the 2.2 version.
Contents
As for 2.1 and 2.2 we divide the 2.3 design into three areas:
Currently all nodes of a Ganeti cluster are considered as part of the same pool, for allocation purposes: DRBD instances for example can be allocated on any two nodes.
This does cause a problem in cases where nodes are not all equally connected to each other. For example if a cluster is created over two set of machines, each connected to its own switch, the internal bandwidth between machines connected to the same switch might be bigger than the bandwidth for inter-switch connections.
Moreover, some operations inside a cluster require all nodes to be locked together for inter-node consistency, and won’t scale if we increase the number of nodes to a few hundreds.
With this change we’ll divide Ganeti nodes into groups. Nothing will change for clusters with only one node group. Bigger clusters will be able to have more than one group, and each node will belong to exactly one.
To manage node groups and the nodes belonging to them, the following new commands and flags will be introduced:
gnt-group add <group> # add a new node group
gnt-group del <group> # delete an empty node group
gnt-group list # list node groups
gnt-group rename <oldname> <newname> # rename a node group
gnt-node {list,info} -g <group> # list only nodes belonging to a node group
gnt-node modify -g <group> # assign a node to a node group
In clusters with more than one node group, it may be desirable to establish local policies regarding which groups should be preferred when performing allocation of new instances, or inter-group instance migrations.
To help with this, we will provide an alloc_policy attribute for node groups. Such attribute will be honored by iallocator plugins when making automatic decisions regarding instance placement.
The alloc_policy attribute can have the following values:
One operation at the node group level will be initially provided:
gnt-group drain <group>
The purpose of this operation is to migrate all instances in a given node group to other groups in the cluster, e.g. to reclaim capacity if there are enough free resources in other node groups that share a storage pool with the evacuated group.
With the introduction of node groups, instances will be required to live in only one group at a time; this is mostly important for DRBD instances, which will not be allowed to have their primary and secondary nodes in different node groups. To support this, we envision the following changes:
- The iallocator interface will be augmented, and node groups exposed, so that plugins will be able to make a decision regarding the group in which to place a new instance. By default, all node groups will be considered, but it will be possible to include a list of groups in the creation job, in which case the plugin will limit itself to considering those; in both cases, the alloc_policy attribute will be honored.
- If, on the other hand, a primary and secondary nodes are specified for a new instance, they will be required to be on the same node group.
- Moving an instance between groups can only happen via an explicit operation, which for example in the case of DRBD will work by performing internally a replace-disks, a migration, and a second replace-disks. It will be possible to clean up an interrupted group-move operation.
- Cluster verify will signal an error if an instance has nodes belonging to different groups. Additionally, changing the group of a given node will be initially only allowed if the node is empty, as a straightforward mechanism to avoid creating such situation.
- Inter-group instance migration will have the same operation modes as new instance allocation, defined above: letting an iallocator plugin decide the target group, possibly restricting the set of node groups to consider, or specifying a target primary and secondary nodes. In both cases, the target group or nodes must be able to accept the instance network- and storage-wise; the operation will fail otherwise, though in the future we may be able to allow some parameter to be changed together with the move (in the meantime, an import/export will be required in this scenario).
We expect the following changes for cluster management:
- Frequent multinode operations, such as os-diagnose or cluster-verify, will act on one group at a time, which will have to be specified in all cases, except for clusters with just one group. Command line tools will also have a way to easily target all groups, by generating one job per group.
- Groups will have a human-readable name, but will internally always be referenced by a UUID, which will be immutable; for example, nodes will contain the UUID of the group they belong to. This is done to simplify referencing while keeping it easy to handle renames and movements. If we see that this works well, we’ll transition other config objects (instances, nodes) to the same model.
- The addition of a new per-group lock will be evaluated, if we can transition some operations now requiring the BGL to it.
- Master candidate status will be allowed to be spread among groups. For the first version we won’t add any restriction over how this is done, although in the future we may have a minimum number of master candidates which Ganeti will try to keep in each group, for example.
Commands like gnt-cluster command/gnt-cluster copyfile will continue to work on the whole cluster, but it will be possible to target one group only by specifying it.
Commands which allow selection of sets of resources (for example gnt-instance start/gnt-instance stop) will be able to select them by node group as well.
Initially node groups won’t be taggable objects, to simplify the first implementation, but we expect this to be easy to add in a future version should we see it’s useful.
We envision groups as a good place to enhance cluster scalability. In the future we may want to use them as units for configuration diffusion, to allow a better master scalability. For example it could be possible to change some all-nodes RPCs to contact each group once, from the master, and make one node in the group perform internal diffusion. We won’t implement this in the first version, but we’ll evaluate it for the future, if we see scalability problems on big multi-group clusters.
When Ganeti will support more storage models (e.g. SANs, Sheepdog, Ceph) we expect groups to be the basis for this, allowing for example a different Sheepdog/Ceph cluster, or a different SAN to be connected to each group. In some cases this will mean that inter-group move operation will be necessarily performed with instance downtime, unless the hypervisor has block-migrate functionality, and we implement support for it (this would be theoretically possible, today, with KVM, for example).
Assuming the node groups feature will enable bigger clusters, other parts of Ganeti will be impacted even more by the (in effect) bigger clusters.
While many areas will be impacted, one is the most important: the fact that the watcher still needs to be able to repair instance data on the current 5 minutes time-frame (a shorter time-frame would be even better). This means that the watcher itself needs to have parallelism when dealing with node groups.
Also, the iallocator plugins are being fed data from Ganeti but also need access to the full cluster state, and in general we still rely on being able to compute the full cluster state somewhat “cheaply” and on-demand. This conflicts with the goal of disconnecting the different node groups, and to keep the same parallelism while growing the cluster size.
Another issue is that the current capacity calculations are done completely outside Ganeti (and they need access to the entire cluster state), and this prevents keeping the capacity numbers in sync with the cluster state. While this is still acceptable for smaller clusters where a small number of allocations/removal are presumed to occur between two periodic capacity calculations, on bigger clusters where we aim to parallelize heavily between node groups this is no longer true.
As proposed changes, the main change is introducing a cluster state cache (not serialised to disk), and to update many of the LUs and cluster operations to account for it. Furthermore, the capacity calculations will be integrated via a new OpCode/LU, so that we have faster feedback (instead of periodic computation).
A new cluster state cache will be introduced. The cache relies on two main ideas:
Given the above, it makes sense to cache the results of node and instance state (with a focus on the node state) inside the master daemon.
The cache will not be serialised to disk, and will be for the most part transparent to the outside of the master daemon.
The cache will be oriented with a focus on node groups, so that it will be easy to invalidate an entire node group, or a subset of nodes, or the entire cache. The instances will be stored in the node group of their primary node.
Furthermore, since the node and instance properties determine the capacity statistics in a deterministic way, the cache will also hold, at each node group level, the total capacity as determined by the new capacity iallocator mode.
The cache will be updated whenever a query for a node state returns “full” node information (so as to keep the cache state for a given node consistent). Partial results will not update the cache (see next paragraph).
Since there will be no way to feed the cache from outside, and we would like to have a consistent cache view when driven by the watcher, we’ll introduce a new OpCode/LU for the watcher to run, instead of the current separate opcodes (see below in the watcher section).
Updates to a node that change a node’s specs “downward” (e.g. less memory) will invalidate the capacity data. Updates that increase the node will not invalidate the capacity, as we’re more interested in “at least available” correctness, not “at most available”.
If a partial node query is done (e.g. just for the node free space), and the returned values don’t match with the cache, then the entire node state will be invalidated.
By default, all LUs will invalidate the caches for all nodes and instances they lock. If an LU uses the BGL, then it will invalidate the entire cache. In time, it is expected that LUs will be modified to not invalidate, if they are not expected to change the node’s and/or instance’s state (e.g. LUConnectConsole, or LUActivateInstanceDisks).
Invalidation of a node’s properties will also invalidate the capacity data associated with that node.
The cache elements will have an upper bound on their lifetime; the proposal is to make this an hour, which should be a high enough value to cover the watcher being blocked by a medium-term job (e.g. 20-30 minutes).
The cache will be used by default for most queries (e.g. a Luxi call, without locks, for the entire cluster). Since this will be a change from the current behaviour, we’ll need to allow non-cached responses, e.g. via a --cache=off or similar argument (which will force the query).
The cache will also be used for the iallocator runs, so that computing allocation solution can proceed independent from other jobs which lock parts of the cluster. This is important as we need to separate allocation on one group from exclusive blocking jobs on other node groups.
The capacity calculations will also use the cache. This is detailed in the respective sections.
As detailed in the cluster cache section, the watcher also needs improvements in order to scale with the the cluster size.
As a first improvement, the proposal is to introduce a new OpCode/LU pair that runs with locks held over the entire query sequence (the current watcher runs a job with two opcodes, which grab and release the locks individually). The new opcode will be called OpUpdateNodeGroupCache and will do the following:
The reason for the per-node group query is that we don’t want a busy node group to prevent instance maintenance in other node groups. Therefore, the watcher will introduce parallelism across node groups, and it will possible to have overlapping watcher runs. The new execution sequence will be:
Each forked children will do the following:
This new mode of execution means that the master watcher processes might overlap in running, but not the individual per-node group child processes.
This change allows us to keep (almost) the same parallelism when using a bigger cluster with node groups versus two separate clusters.
Currently the watcher only does “small” queries for the node and instance state, and at first sight changing it to use the new OpCode which populates the cache with the entire state might introduce additional costs, which must be payed every five minutes.
However, the OpCodes that the watcher submits are using the so-called dynamic fields (need to contact the remote nodes), and the LUs are not selective—they always grab all the node and instance state. So in the end, we have the same cost, it just becomes explicit rather than implicit.
This ‘grab all node state’ behaviour is what makes the cache worth implementing.
The design above only deals with inter-node group issues. It still makes sense to run instance maintenance for nodes A and B if only node C is locked (all being in the same node group).
This problem is commonly encountered in previous Ganeti versions, and it should be handled similarly, by tweaking lock lifetime in long-duration jobs.
TODO: add more ideas here.
The splitting of node group maintenance to different children which will run in parallel requires that the state file handling changes from monolithic updates to partial ones.
There are two file that the watcher maintains:
For the first file, since it’s used only internally to the watchers, we can move to a per node group configuration.
For the second file, even if it’s used as an external interface, we will need to make some changes to it: because the different node groups can return results at different times, we need to either split the file into per-group files or keep the single file and add a per-instance timestamp (currently the file holds only the instance name and state).
The proposal is that each child process maintains its own node group file, and the master process will, right after querying the node group list, delete any extra per-node group state file. This leaves the consumers to run a simple cat instance-status.group-* to obtain the entire list of instance and their states. If needed, the modify timestamp of each file can be used to determine the age of the results.
Currently, the capacity calculations are done completely outside Ganeti. As explained in the current problems section, this needs to account better for the cluster state changes.
Therefore a new OpCode will be introduced, OpComputeCapacity, that will either return the current capacity numbers (if available), or trigger a new capacity calculation, via the iallocator framework, which will get a new method called capacity.
This method will feed the cluster state (for the complete set of node group, or alternative just a subset) to the iallocator plugin (either the specified one, or the default if none is specified), and return the new capacity in the format currently exported by the htools suite and known as the “tiered specs” (see hspace(1)).
Currently, the “tspec” calculations done in hspace require some additional parameters:
For the integration in Ganeti, there are multiple ways to pass these:
Since the first option is not consistent with the intended changes, a combination of the last two is proposed:
Whenever the capacity is requested via different parameters, it will invalidate the cache, even if otherwise the cache is up-to-date.
The new parameters are:
These might also be used in instance creations (to be determined later, after they are introduced).
Input:
Output:
Note that due to the way the tspecs are computed, for any given specification, the total available count is the count for the given entry, plus the sum of counts for higher specifications.
Currently all nodes are, from the point of view of their capabilities, homogeneous. This means the cluster considers all nodes capable of becoming master candidates, and of hosting instances.
This prevents some deployment scenarios: e.g. having a Ganeti instance (in another cluster) be just a master candidate, in case all other master candidates go down (but not, of course, host instances), or having a node in a remote location just host instances but not become master, etc.
Two new capability flags will be added to the node:
In terms of the other flags, master_capable is a stronger version of “not master candidate”, and vm_capable is a stronger version of “drained”.
For the master_capable flag, it will affect auto-promotion code and node modifications.
The vm_capable flag will affect the iallocator protocol, capacity calculations, node checks in cluster verify, and will interact in novel ways with locking (unfortunately).
It is envisaged that most nodes will be both vm_capable and master_capable, and just a few will have one of these flags removed. Ganeti itself will allow clearing of both flags, even though this doesn’t make much sense currently.
Currently all jobs and opcodes have the same priority. Once a job started executing, its thread won’t be released until all opcodes got their locks and did their work. When a job is finished, the next job is selected strictly by its incoming order. This does not mean jobs are run in their incoming order—locks and other delays can cause them to be stalled for some time.
In some situations, e.g. an emergency shutdown, one may want to run a job as soon as possible. This is not possible currently if there are pending jobs in the queue.
Each opcode will be assigned a priority on submission. Opcode priorities are integers and the lower the number, the higher the opcode’s priority is. Within the same priority, jobs and opcodes are initially processed in their incoming order.
Submitted opcodes can have one of the priorities listed below. Other priorities are reserved for internal use. The absolute range is -20..+19. Opcodes submitted without a priority (e.g. by older clients) are assigned the default priority.
- High (-10)
- Normal (0, default)
- Low (+10)
As a change from the current model where executing a job blocks one thread for the whole duration, the new job processor must return the job to the queue after each opcode and also if it can’t get all locks in a reasonable timeframe. This will allow opcodes of higher priority submitted in the meantime to be processed or opcodes of the same priority to try to get their locks. When added to the job queue’s workerpool, the priority is determined by the first unprocessed opcode in the job.
If an opcode is deferred, the job will go back to the “queued” status, even though it’s just waiting to try to acquire its locks again later.
If an opcode can not be processed after a certain number of retries or a certain amount of time, it should increase its priority. This will avoid starvation.
A job’s priority can never go below -20. If a job hits priority -20, it must acquire its locks in blocking mode.
Opcode priorities are synchronised to disk in order to be restored after a restart or crash of the master daemon.
Priorities also need to be considered inside the locking library to ensure opcodes with higher priorities get locks first. See locking priorities for more details.
To support job priorities in the job queue, the worker pool underlying the job queue must be enhanced to support task priorities. Currently tasks are processed in the order they are added to the queue (but, due to their nature, they don’t necessarily finish in that order). All tasks are equal. To support tasks with higher or lower priority, a few changes have to be made to the queue inside a worker pool.
Each task is assigned a priority when added to the queue. This priority can not be changed until the task is executed (this is fine as in all current use-cases, tasks are added to a pool and then forgotten about until they’re done).
A task’s priority can be compared to Unix’ process priorities. The lower the priority number, the closer to the queue’s front it is. A task with priority 0 is going to be run before one with priority 10. Tasks with the same priority are executed in the order in which they were added.
While a task is running it can query its own priority. If it’s not ready yet for finishing, it can raise an exception to defer itself, optionally changing its own priority. This is useful for the following cases:
With these changes, the job queue will be able to implement per-job priorities.
In order to support priorities in Ganeti’s own lock classes, locking.SharedLock and locking.LockSet, the internal structure of the former class needs to be changed. The last major change in this area was done for Ganeti 2.1 and can be found in the respective design document.
The plain list ([]) used as a queue is replaced by a heap queue, similar to the worker pool. The heap or priority queue does automatic sorting, thereby automatically taking care of priorities. For each priority there’s a plain list with pending acquires, like the single queue of pending acquires before this change.
When the lock is released, the code locates the list of pending acquires for the highest priority waiting. The first condition (index 0) is notified. Once all waiting threads received the notification, the condition is removed from the list. If the list of conditions is empty it’s removed from the heap queue.
Like before, shared acquires are grouped and skip ahead of exclusive acquires if there’s already an existing shared acquire for a priority. To accomplish this, a separate dictionary of shared acquires per priority is maintained.
To simplify the code and reduce memory consumption, the concept of the “active” and “inactive” condition for shared acquires is abolished. The lock can’t predict what priorities the next acquires will use and even keeping a cache can become computationally expensive for arguable benefit (the underlying POSIX pipe, see pipe(2), needs to be re-created for each notification anyway).
The following diagram shows a possible state of the internal queue from a high-level view. Conditions are shown as (waiting) threads. Assuming no modifications are made to the queue (e.g. more acquires or timeouts), the lock would be acquired by the threads in this order (concurrent acquires in parentheses): threadE1, threadE2, (threadS1, threadS2, threadS3), (threadS4, threadS5), threadE3, threadS6, threadE4, threadE5.
[
(0, [exc/threadE1, exc/threadE2, shr/threadS1/threadS2/threadS3]),
(2, [shr/threadS4/threadS5]),
(10, [exc/threadE3]),
(33, [shr/threadS6, exc/threadE4, exc/threadE5]),
]
Currently Ganeti does not support IPv6. This is true for nodes as well as instances. Due to the fact that IPv4 exhaustion is threateningly near the need of using IPv6 is increasing, especially given that bigger and bigger clusters are supported.
In Ganeti 2.3 we introduce additionally to the ordinary pure IPv4 setup a hybrid IPv6/IPv4 mode. The latter works as follows:
The reason for this hybrid setup is that key components that Ganeti depends on do not or only partially support IPv6. More precisely, Xen does not support instance migration via IPv6 in version 3.4 and 4.0. Similarly, KVM does not support instance migration nor VNC access for IPv6 at the time of this writing.
This led to the decision of not supporting pure IPv6 Ganeti clusters, as very important cluster operations would not have been possible. Using IPv4 as secondary address does not affect any of the goals of the IPv6 support: since secondary addresses do not need to be publicly accessible, they need not be globally unique. In other words, one can practically use private IPv4 secondary addresses just for intra-cluster communication without propagating them across layer 3 boundaries.
Currently common utility functions are kept in the utils module. Since this module grows bigger and bigger network-related functions are moved to a separate module named netutils. Additionally all these utilities will be IPv6-enabled.
As mentioned above there will be two different setups in terms of IP addressing: pure IPv4 and hybrid IPv6/IPv4 address. To choose that a new cluster init parameter –primary-ip-version is introduced. This is needed as a given name can resolve to both an IPv4 and IPv6 address on a dual-stack host effectively making it impossible to infer that bit.
Once a cluster is initialized and the primary IP version chosen all nodes that join have to conform to that setup. In the case of our IPv6/IPv4 setup all nodes must have a secondary IPv4 address.
Furthermore we store the primary IP version in ssconf which is consulted every time a daemon starts to determine the default bind address (either 0.0.0.0 or ::. In a IPv6/IPv4 setup we need to bind the Ganeti daemon listening on network sockets to the IPv6 address.
When adding a new node to a IPv6/IPv4 cluster it must have a IPv6 address to be used as primary and a IPv4 address used as secondary. As explained above, every time a daemon is started we use the cluster primary IP version to determine to which any address to bind to. The only exception to this is when a node is added to the cluster. In this case there is no ssconf available when noded is started and therefore the correct address needs to be passed to it.
Since the gethostbyname*() functions do not support IPv6 name resolution will be done by using the recommended getaddrinfo().
| Component | IPv6 Status | Planned Version |
|---|---|---|
| Xen instance migration | Not supported | Xen 4.1: libxenlight |
| KVM instance migration | Not supported | Unknown |
| KVM VNC access | Not supported | Unknown |
In Ganeti 2.2 we introduced privilege separation for the RAPI daemon. This was done directly in the daemon’s code in the process of daemonizing itself. Doing so leads to several potential issues. For example, a file could be opened while the code is still running as root and for some reason not be closed again. Even after changing the user ID, the file descriptor can be written to.
To address these shortcomings, daemons will be started under the target user right away. The start-stop-daemon utility used to start daemons supports the --chuid option to change user and group ID before starting the executable.
The intermediate solution for the RAPI daemon from Ganeti 2.2 will be removed again.
Files written by the daemons may need to have an explicit owner and group set (easily done through utils.WriteFile).
All SSH-related code is removed from the ganeti.bootstrap module and core components and moved to a separate script. The core code will simply assume a working SSH setup to be in place.
In order to separate the permissions of file sets we separate them into the following 3 overall security domain chunks:
So for point 3 this tables shows the correlation of the sets to groups and their users:
| Set | Group | Users | Description |
|---|---|---|---|
| A | gntrapi | gntrapi, gntmasterd | Share data between gntrapi and gntmasterd |
| B | gntadmins | gntrapi, gntmasterd, users | Shared between users who needs to call gntmasterd |
| C | gntconfd | gntconfd, gntmasterd | Share data between gntconfd and gntmasterd |
| D | gntmasterd | gntmasterd | masterd only; Currently only to redistribute the configuration, has access to all files under lib/ganeti |
| E | gntdaemons | gntmasterd, gntrapi, gntconfd | Shared between the various Ganeti daemons to exchange data |
The following commands needs still root to fulfill their functions:
gnt-cluster {init|destroy|command|copyfile|rename|masterfailover|renew-crypto}
gnt-node {add|remove}
gnt-instance {console}
Here’s how we propose to change the filesystem hierarchy and their permissions.
Assuming it follows the defaults: gnt${daemon} for user and the groups from the section Security Domains:
${localstatedir}/lib/ganeti/ (0755; gntmasterd:gntmasterd)
cluster-domain-secret (0600; gntmasterd:gntmasterd)
config.data (0640; gntmasterd:gntconfd)
hmac.key (0440; gntmasterd:gntconfd)
known_host (0644; gntmasterd:gntmasterd)
queue/ (0700; gntmasterd:gntmasterd)
archive/ (0700; gntmasterd:gntmasterd)
* (0600; gntmasterd:gntmasterd)
* (0600; gntmasterd:gntmasterd)
rapi.pem (0440; gntrapi:gntrapi)
rapi_users (0640; gntrapi:gntrapi)
server.pem (0440; gntmasterd:gntmasterd)
ssconf_* (0444; root:gntmasterd)
uidpool/ (0750; root:gntmasterd)
watcher.data (0600; root:gntmasterd)
${localstatedir}/run/ganeti/ (0770; gntmasterd:gntdaemons)
socket/ (0750; gntmasterd:gntadmins)
ganeti-master (0770; gntmasterd:gntadmins)
${localstatedir}/log/ganeti/ (0770; gntmasterd:gntdaemons)
master-daemon.log (0600; gntmasterd:gntdaemons)
rapi-daemon.log (0600; gntrapi:gntdaemons)
conf-daemon.log (0600; gntconfd:gntdaemons)
node-daemon.log (0600; gntnoded:gntdaemons)