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Correct typo in replica configs

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Joshua Boniface 2020-11-13 01:23:57 -05:00
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docs/cluster-architecture.md
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@ -80,7 +80,7 @@ PVC Ceph pools make use of the replication mechanism of Ceph to store multiple c
The default replication level for a new pool is `copies=3, mincopies=2`. This will store 3 copies of each object, with a host-level failure domain, and will allow I/O as long as 2 copies are available. Thus, in a cluster of any size, all data is fully available even if a single host becomes unavailable. It will however use 3x the space for each piece of data stored, which must be considered when sizing the disk space for the cluster: a pool in this configuration, running on 3 nodes each with a single 400GB disk, will effectively have 400GB of total space available for use. As mentioned above, new disks must also be added in groups across nodes equal to the total number of `copies` to ensure new space is usable.
Non-default values can also be set at pool creation time. For instance, one could create a `copies=3, mincopies=1` pool, which would allow I/O with two hosts down but leaves the cluster susceptible to a write hole should a disk fail in this state. Alternatively, for more resilience, one could create a `copies=4, mincopies=2` pool, which will allow 2 hosts to fail without a write hole, but would consume 4x the space for each piece of data stored and require new disks to be added in groups of 4 instead. Practically any combination of values is possible, however these 3 are the most relevant for most use-cases, and for most, especially small, clusters, the default is sufficient to provide solid redundancy and guard against host failures until the administrator can respond.
Non-default values can also be set at pool creation time. For instance, one could create a `copies=3, mincopies=1` pool, which would allow I/O with two hosts down but leaves the cluster susceptible to a write hole should a disk fail in this state. Alternatively, for more resilience, one could create a `copies=4, mincopies=3` pool, which will allow 2 hosts to fail without a write hole, but would consume 4x the space for each piece of data stored and require new disks to be added in groups of 4 instead. Practically any combination of values is possible, however these 3 are the most relevant for most use-cases, and for most, especially small, clusters, the default is sufficient to provide solid redundancy and guard against host failures until the administrator can respond.
Replication levels cannot be changed within PVC once a pool is created, however they can be changed via manual Ceph commands on a coordinator should the administrator require this. In any case, the administrator should carefully consider sizing, failure domains, and performance when selecting storage devices to ensure the right level of resiliency versus data usage for their use-case and cluster size.