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doc/glossary: add "Placement Groups" definition

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Add a definition of "Placement Groups" to the Glossary.

Co-authored-by: Anthony D'Atri <anthony.datri@gmail.com>
Signed-off-by: Zac Dover <zac.dover@proton.me>
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Zac Dover 2023-04-22 10:55:38 +02:00
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doc/glossary.rst
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@ -307,6 +307,20 @@
state of a multi-site configuration. When the period is updated,
the "epoch" is said thereby to have been changed.
Placement Groups (PGs)
Placement groups (PGs) are subsets of each logical Ceph pool.
Placement groups perform the function of placing objects (as a
group) into OSDs. Ceph manages data internally at
placement-group granularity: this scales better than would
managing individual (and therefore more numerous) RADOS
objects. A cluster that has a larger number of placement groups
(for example, 100 per OSD) is better balanced than an otherwise
identical cluster with a smaller number of placement groups.
Ceph's internal RADOS objects are each mapped to a specific
placement group, and each placement group belongs to exactly
one Ceph pool.
:ref:`Pool<rados_pools>`
A pool is a logical partition used to store objects.