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Ceph Cluster Upgrade
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Ceph upgrades are done in three steps.

0. Terminology
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Trivial Update:
* each Ceph component can be updated in random order

Non-trivial upgrade:
* each Ceph component must be updated in fixed order, usually (if not specified otherwise in release notes): monitors always first, rest after that, mds always last.
* sometimes specific config settings are required prior or during updates

1. Prerequisites
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* read ceph release notes: https://docs.ceph.com/docs/master/release-notes/
* cluster must be health ok
* set noout on cluster
* set backfil max to minimal values (if needed)
* check that all MDS (including standby!) are up and running
* keep a terminal with 'ceph -w' open during upgrades to monitor cluster messages

2. Host System Updates
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* upgrade all admin*.ceph.bfh.ch hosts first
* then upgrade all storage*.ceph.bfh.ch hosts (while ceph-osds are not running in containers yet, install all system updates except ceph)
* finally upgrade all gateway*.ceph.bfh.ch hosts

For every host upgrade:

* install all updates
* shutdown all containers
* reboot host system
* remove obsolete kernels, if any

check 'ceph -s' if everything is well before proceeding to the next.

3. Container System Updates
===========================

for every ceph container:

* upgrade all packages (ceph package updates do not restart ceph services)
* restart all ceph services

check 'ceph -s' if everything is well before proceeding to the next.