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HRoller tool

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This is a design document detailing the cluster maintenance scheduler,

HRoller.

Current state and shortcomings

==============================

To enable automating cluster-wide reboots a new htool, called HRoller,

was added to Ganeti starting from version 2.7. This tool helps

parallelizing cluster offline maintenances by calculating which nodes

are not both primary and secondary for a DRBD instance, and thus can be

rebooted at the same time, when all instances are down.

The way this is done is documented in the :manpage:`hroller(1)` manpage.

We would now like to perform online maintenance on the cluster by

rebooting nodes after evacuating their primary instances (rolling

reboots).

Proposed changes

================

New options

-----------

- HRoller should be able to operate on single nodegroups (-G flag) or

  select its target node through some other mean (eg. via a tag, or a

  regexp). (Note that individual node selection is already possible via

  the -O flag, that makes hroller ignore a node altogether).

- HRoller should handle non redundant instances: currently these are

  ignored but there should be a way to select its behavior between "it's

  ok to reboot a node when a non-redundant instance is on it" or "skip

  nodes with non-redundant instances". This will only be selectable

  globally, and not per instance.

- Hroller will make sure to keep any instance which is up in its current

  state, via live migrations, unless explicitly overridden. The

  algorithm that will be used calculate the rolling reboot with live

  migrations is described below, and any override on considering the

  instance status will only be possible on the whole run, and not

  per-instance.

Calculating rolling maintenances

--------------------------------

In order to perform rolling maintenance we need to migrate instances off

the nodes before a reboot. How this can be done depends on the

instance's disk template and status:

Down instances

++++++++++++++

If an instance was shutdown when the maintenance started it will be

considered for avoiding contemporary reboot of its primary and secondary

nodes, but will *not* be considered as a target for the node evacuation.

This allows avoiding needlessly moving its primary around, since it

won't suffer a downtime anyway.

Note that a node with non-redundant instances will only ever be

considered good for rolling-reboot if these are down (or the checking of

status is overridden) *and* an explicit option to allow it is set.

DRBD

++++

Each node must migrate all instances off to their secondaries, and then

can either be rebooted, or the secondaries can be evacuated as well.

Since currently doing a ``replace-disks`` on DRBD breaks redundancy,

it's not any safer than temporarily rebooting a node with secondaries on

them (citation needed). As such we'll implement for now just the

"migrate+reboot" mode, and focus later on replace-disks as well.

In order to do that we can use the following algorithm:

1) Compute node sets that don't contain both the primary and the

   secondary for any instance. This can be done already by the current

   hroller graph coloring algorithm: nodes are in the same set (color)

   if and only if no edge (instance) exists between them (see the

   :manpage:`hroller(1)` manpage for more details).

2) Inside each node set calculate subsets that don't have any secondary

   node in common (this can be done by creating a graph of nodes that

   are connected if and only if an instance on both has the same

   secondary node, and coloring that graph)

3) It is then possible to migrate in parallel all nodes in a subset

   created at step 2, and then reboot/perform maintenance on them, and

   migrate back their original primaries, which allows the computation

   above to be reused for each following subset without N+1 failures

   being triggered, if none were present before. See below about the

   actual execution of the maintenance.

Non-DRBD

++++++++

All non-DRBD disk templates that can be migrated have no "secondary"

concept. As such instances can be migrated to any node (in the same

nodegroup). In order to do the job we can either:

- Perform migrations on one node at a time, perform the maintenance on

  that node, and proceed (the node will then be targeted again to host

  instances automatically, as hail chooses targets for the instances

  between all nodes in a group. Nodes in different nodegroups can be

  handled in parallel.

- Perform migrations on one node at a time, but without waiting for the

  first node to come back before proceeding. This allows us to continue,

  restricting the cluster, until no more capacity in the nodegroup is

  available, and then having to wait for some nodes to come back so that

  capacity is available again for the last few nodes.

- Pre-Calculate sets of nodes that can be migrated together (probably

  with a greedy algorithm) and parallelize between them, with the

  migrate-back approach discussed for DRBD to perform the calculation

  only once.

Note that for non-DRBD disks that still use local storage (eg. RBD and

plain) redundancy might break anyway, and nothing except the first

algorithm might be safe. This perhaps would be a good reason to consider

managing better RBD pools, if those are implemented on top of nodes

storage, rather than on dedicated storage machines.

Future work

===========

Hroller should become able to execute rolling maintenances, rather than

just calculate them. For this to succeed properly one of the following

must happen:

- HRoller handles rolling maintenances that happen at the same time as

  unrelated cluster jobs, and thus recalculates the maintenance at each

  step

- HRoller can selectively drain the cluster so it's sure that only the

  rolling maintenance can be going on

DRBD nodes' ``replace-disks``' functionality should be implemented. Note

that when we will support a DRBD version that allows multi-secondary

this can be done safely, without losing replication at any time, by

adding a temporary secondary and only when the sync is finished dropping

the previous one.

Non-redundant (plain or file) instances should have a way to be moved

off as well via plain storage live migration or ``gnt-instance move``

(which requires downtime).

If/when RBD pools can be managed inside Ganeti, care can be taken so

that the pool is evacuated as well from a node before it's put into

maintenance. This is equivalent to evacuating DRBD secondaries.

Master failovers during the maintenance should be performed by hroller.

This requires RPC/RAPI support for master failover. Hroller should also

be modified to better support running on the master itself and

continuing on the new master.

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