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Diffstat (limited to 'doc/rados/operations/stretch-mode.rst')
| -rw-r--r-- | doc/rados/operations/stretch-mode.rst | 30 |
1 files changed, 23 insertions, 7 deletions
diff --git a/doc/rados/operations/stretch-mode.rst b/doc/rados/operations/stretch-mode.rst index e8be5e13e6a..7a4fa46117d 100644 --- a/doc/rados/operations/stretch-mode.rst +++ b/doc/rados/operations/stretch-mode.rst @@ -119,13 +119,29 @@ See https://tracker.ceph.com/issues/68338 for more information. Stretch Mode ============ -Stretch mode is designed to handle deployments in which you cannot guarantee the -replication of data across two data centers. This kind of situation can arise -when the cluster's CRUSH rule specifies that three copies are to be made, but -then a copy is placed in each data center with a ``min_size`` of 2. Under such -conditions, a placement group can become active with two copies in the first -data center and no copies in the second data center. +Stretch mode is designed to handle netsplit scenarios between two data zones as well +as the loss of one data zone. It handles the netsplit scenario by choosing the surviving zone +that has the better connection to the ``tiebreaker monitor``. It handles the loss of one zone by +reducing the ``size`` to ``2`` and ``min_size`` to ``1``, allowing the cluster to continue operating +with the remaining zone. When the lost zone comes back, the cluster will recover the lost data +and return to normal operation. + +Connectivity Monitor Election Strategy +--------------------------------------- +When using stretch mode, the monitor election strategy must be set to ``connectivity``. +This strategy tracks network connectivity between the monitors and is +used to determine which zone should be favored when the cluster is in a netsplit scenario. + +See `Changing Monitor Elections`_ + +Stretch Peering Rule +-------------------- +One critical behavior of stretch mode is its ability to prevent a PG from going active if the acting set +contains only replicas from a single zone. This safeguard is crucial for mitigating the risk of data +loss during site failures because if a PG were allowed to go active with replicas only in a single site, +writes could be acknowledged despite a lack of redundancy. In the event of a site failure, all data in the +affected PG would be lost. Entering Stretch Mode --------------------- @@ -271,7 +287,7 @@ possible, if needed). .. _Changing Monitor elections: ../change-mon-elections Exiting Stretch Mode -===================== +-------------------- To exit stretch mode, run the following command: .. prompt:: bash $ |