Update README for Ceph EC pools
This updates the README for erasure coded Ceph pools for the case of CephFS. The new text should be as similar as possible for all the charms that support configuration options for EC pools. See the below review for the first of these charms whose README has been updated. https://review.opendev.org/#/c/749824/ Other minor improvements. Change-Id: Ic6543e3241048591818358c972eadecd6ceab50c
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Peter Matulis
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excellent performance, reliability, and scalability.
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The ceph-fs charm deploys the metadata server daemon (MDS) for the Ceph
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distributed file system (CephFS). It is used in conjunction with the
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[ceph-mon][ceph-mon-charm] and the [ceph-osd][ceph-osd-charm] charms.
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Highly available CephFS is achieved by deploying multiple MDS servers (i.e.
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multiple ceph-fs units).
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distributed file system (CephFS). The deployment is done within the context of
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an existing Ceph cluster.
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# Usage
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@ -20,6 +17,11 @@ default values. A YAML file (e.g. `ceph-osd.yaml`) is often used to store
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configuration options. See the [Juju documentation][juju-docs-config-apps] for
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details on configuring applications.
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#### `pool-type`
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The `pool-type` option dictates the storage pool type. See section 'Ceph pool
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type' for more information.
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#### `source`
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The `source` option states the software sources. A common value is an OpenStack
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@ -28,18 +30,70 @@ and the UCA][cloud-archive-ceph]. The underlying host's existing apt sources
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will be used if this option is not specified (this behaviour can be explicitly
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chosen by using the value of 'distro').
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## Ceph pool type
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Ceph storage pools can be configured to ensure data resiliency either through
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replication or by erasure coding. This charm supports both types via the
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`pool-type` configuration option, which can take on the values of 'replicated'
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and 'erasure-coded'. The default value is 'replicated'.
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For this charm, the pool type will be associated with CephFS volumes.
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> **Note**: Erasure-coded pools are supported starting with Ceph Luminous.
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### Replicated pools
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Replicated pools use a simple replication strategy in which each written object
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is copied, in full, to multiple OSDs within the cluster.
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The `ceph-osd-replication-count` option sets the replica count for any object
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stored within the 'ceph-fs-data' cephfs pool. Increasing this value increases
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data resilience at the cost of consuming more real storage in the Ceph cluster.
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The default value is '3'.
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> **Important**: The `ceph-osd-replication-count` option must be set prior to
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adding the relation to the ceph-mon application. Otherwise, the pool's
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configuration will need to be set by interfacing with the cluster directly.
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### Erasure coded pools
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Erasure coded pools use a technique that allows for the same resiliency as
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replicated pools, yet reduces the amount of space required. Written data is
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split into data chunks and error correction chunks, which are both distributed
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throughout the cluster.
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> **Note**: Erasure coded pools require more memory and CPU cycles than
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replicated pools do.
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When using erasure coded pools for CephFS file systems two pools will be
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created: a replicated pool (for storing MDS metadata) and an erasure coded pool
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(for storing the data written into a CephFS volume). The
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`ceph-osd-replication-count` configuration option only applies to the metadata
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(replicated) pool.
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Erasure coded pools can be configured via options whose names begin with the
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`ec-` prefix.
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> **Important**: It is strongly recommended to tailor the `ec-profile-k` and
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`ec-profile-m` options to the needs of the given environment. These latter
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options have default values of '1' and '2' respectively, which result in the
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same space requirements as those of a replicated pool.
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See [Ceph Erasure Coding][cdg-ceph-erasure-coding] in the [OpenStack Charms
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Deployment Guide][cdg] for more information.
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## Deployment
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We are assuming a pre-existing Ceph cluster.
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To deploy a single MDS node:
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To deploy a single MDS node within an existing Ceph cluster:
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juju deploy ceph-fs
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Then add a relation to the ceph-mon application:
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juju add-relation ceph-fs:ceph-mds ceph-mon:mds
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## High availability
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Highly available CephFS is achieved by deploying multiple MDS servers (i.e.
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multiple ceph-fs units).
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## Actions
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This section lists Juju [actions][juju-docs-actions] supported by the charm.
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@ -60,10 +114,10 @@ For general charm questions refer to the OpenStack [Charm Guide][cg].
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<!-- LINKS -->
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[cg]: https://docs.openstack.org/charm-guide
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[cdg]: https://docs.openstack.org/project-deploy-guide/charm-deployment-guide
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[ceph-upstream]: https://ceph.io
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[ceph-mon-charm]: https://jaas.ai/ceph-mon
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[ceph-osd-charm]: https://jaas.ai/ceph-osd
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[juju-docs-actions]: https://jaas.ai/docs/actions
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[juju-docs-config-apps]: https://juju.is/docs/configuring-applications
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[lp-bugs-charm-ceph-fs]: https://bugs.launchpad.net/charm-ceph-fs/+filebug
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[cloud-archive-ceph]: https://wiki.ubuntu.com/OpenStack/CloudArchive#Ceph_and_the_UCA
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[cdg-ceph-erasure-coding]: https://docs.openstack.org/project-deploy-guide/charm-deployment-guide/latest/app-erasure-coding.html
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