RADOS/Ceph support in Ganeti


The project aims to improve Ceph RBD support in Ganeti. It can be primarily divided into following tasks.

  • Use Qemu/KVM RBD driver to provide instances with direct RBD support. [implemented as of Ganeti 2.10]
  • Allow Ceph RBDs’ configuration through Ganeti. [unimplemented]
  • Write a data collector to monitor Ceph nodes. [unimplemented]


Ceph RBD

Ceph is a distributed storage system which provides data access as files, objects and blocks. As part of this project, we’re interested in integrating ceph’s block device (RBD) directly with Qemu/KVM.

Primary components/daemons of Ceph. - Monitor - Serve as authentication point for clients. - Metadata - Store all the filesystem metadata (Not configured here as they are not required for RBD) - OSD - Object storage devices. One daemon for each drive/location.

RBD support in Ganeti

Currently, Ganeti supports RBD volumes on a pre-configured Ceph cluster. This is enabled through RBD disk templates. These templates allow RBD volume’s access through RBD Linux driver. The volumes are mapped to host as local block devices which are then attached to the instances. This method incurs an additional overhead. We plan to resolve it by using Qemu’s RBD driver to enable direct access to RBD volumes for KVM instances.

Also, Ganeti currently uses RBD volumes on a pre-configured ceph cluster. Allowing configuration of ceph nodes through Ganeti will be a good addition to its prime features.

Qemu/KVM Direct RBD Integration

A new disk param access is introduced. It’s added at cluster/node-group level to simplify prototype implementation. It will specify the access method either as userspace or kernelspace. It’s accessible to StartInstance() in hv_kvm.py. The device path, rbd:<pool>/<vol_name>, is generated by RADOSBlockDevice and is added to the params dictionary as kvm_dev_path.

This approach ensures that no disk template specific changes are required in hv_kvm.py allowing easy integration of other distributed storage systems (like Gluster).

Note that the RBD volume is mapped as a local block device as before. The local mapping won’t be used during instance operation in the userspace access mode, but can be used by administrators and OS scripts.

Updated commands

$ gnt-instance info

access:userspace/kernelspace will be added to Disks category. This output applies to KVM based instances only.

Ceph configuration on Ganeti nodes

This document proposes configuration of distributed storage pool (Ceph or Gluster) through ganeti. Currently, this design document focuses on configuring a Ceph cluster. A prerequisite of this setup would be installation of ceph packages on all the concerned nodes.

At Ganeti Cluster init, the user will set distributed-storage specific options which will be stored at cluster level. The Storage cluster will be initialized using gnt-storage. For the prototype, only a single storage pool/node-group is configured.

Following steps take place when a node-group is initialized as a storage cluster.

  • Check for an existing ceph cluster through /etc/ceph/ceph.conf file on each node.
  • Fetch cluster configuration parameters and create a distributed storage object accordingly.
  • Issue an ‘init distributed storage’ RPC to group nodes (if any).
  • On each node, ceph cli tool will run appropriate services.
  • Mark nodes as well as the node-group as distributed-storage-enabled.

The storage cluster will operate at a node-group level. The ceph cluster will be initiated using gnt-storage. A new sub-command init-distributed-storage will be added to it.

The configuration of the nodes will be handled through an init function called by the node daemons running on the respective nodes. A new RPC is introduced to handle the calls.

A new object will be created to send the storage parameters to the node - storage_type, devices, node_role (mon/osd) etc.

A new node can be directly assigned to the storage enabled node-group. During the ‘gnt-node add’ process, required ceph daemons will be started and node will be added to the ceph cluster.

Only an offline node can be assigned to storage enabled node-group. gnt-node --readd needs to be performed to issue RPCs for spawning appropriate services on the newly assigned node.

Updated Commands

Following are the affected commands.:

$ gnt-cluster init -S ceph:disk=/dev/sdb,option=value...

During cluster initialization, ceph specific options are provided which apply at cluster-level.:

$ gnt-cluster modify -S ceph:option=value2...

For now, cluster modification will be allowed when there is no initialized storage cluster.:

$ gnt-storage init-distributed-storage -s{--storage-type} ceph \

Ensure that no other node-group is configured as distributed storage cluster and configure ceph on the specified node-group. If there is no node in the node-group, it’ll only be marked as distributed storage enabled and no action will be taken.:

$ gnt-group assign-nodes <group> <node>

It ensures that the node is offline if the node-group specified is distributed storage capable. Ceph configuration on the newly assigned node is not performed at this step.:

$ gnt-node --offline

If the node is part of storage node-group, an offline call will stop/remove ceph daemons.:

$ gnt-node add --readd

If the node is now part of the storage node-group, issue init distributed storage RPC to the respective node. This step is required after assigning a node to the storage enabled node-group:

$ gnt-node remove

A warning will be issued stating that the node is part of distributed storage, mark it offline before removal.

Future Work

Due to the loopback bug in ceph, one may run into daemon hang issues while performing writes to a RBD volumes through block device mapping. This bug is applicable only when the RBD volume is stored on the OSD running on the local node. In order to mitigate this issue, we can create storage pools on different nodegroups and access RBD volumes on different pools. http://tracker.ceph.com/issues/3076