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Clarify the note about running double replication with replicated disks to mention risks to fault tolerance.

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A.J. Beamon 2020-02-21 14:03:37 -08:00
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documentation/sphinx/source/configuration.rst
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@ -387,7 +387,7 @@ FoundationDB will never use processes on the same machine for the replication of
FoundationDB replicates data to three machines, and at least three available machines are required to make progress. This is the recommended mode for a cluster of five or more machines in a single datacenter.
.. note:: When running in cloud environments with managed disks that are already replicated and persistent, ``double`` replication may still be considered for 5+ machine clusters. This will result in a lower total read throughput, but offers a reasonable tradeoff for cost.
.. note:: When running in cloud environments with managed disks that are already replicated and persistent, ``double`` replication may still be considered for 5+ machine clusters. This will result in lower availability fault tolerance for planned or unplanned failures and lower total read throughput, but offers a reasonable tradeoff for cost.
``three_data_hall`` mode
FoundationDB stores data in triplicate, with one copy on a storage server in each of three data halls. The transaction logs are replicated four times, with two data halls containing two replicas apiece. Four available machines (two in each of two data halls) are therefore required to make progress. This configuration enables the cluster to remain available after losing a single data hall and one machine in another data hall.