Recovering the file system after catastrophic Monitor store loss
================================================================

During rare occasions, all the monitor stores of a cluster may get corrupted
or lost. To recover the cluster in such a scenario, you need to rebuild the
monitor stores using the OSDs (see :ref:`mon-store-recovery-using-osds`),
and get back the pools intact (active+clean state). However, the rebuilt monitor
stores don't restore the file system maps ("FSMap"). Additional steps are required
to bring back the file system. The steps to recover a multiple active MDS file
system or multiple file systems are yet to be identified. Currently, only the steps
to recover a **single active MDS** file system with no additional file systems
in the cluster have been identified and tested. Briefly the steps are: stop the
MDSs; recreate the FSMap with basic defaults; and allow MDSs to recover from
the journal/metadata stored in the filesystem's pools. The steps are described
in more detail below.

First up, stop all the MDSs of the cluster.

Verify that the MDSs have been stopped. Execute the below command and
check that no active or standby MDS daemons are listed for the file system.

::

    ceph fs dump

Recreate the file system using the recovered file system pools. The new FSMap
will have the filesystem's default settings. However, the user defined file
system settings such as ``standby_count_wanted``, ``required_client_features``,
extra data pools, etc., are lost and need to be reapplied later.

::

    ceph fs new <fs_name> <metadata_pool> <data_pool> --force

The file system cluster ID, fscid, of the file system will not be preserved.
This behaviour may not be desirable for certain applications (e.g., Ceph CSI)
that expect the file system to be unchanged across recovery. To fix this, pass
the desired fscid when recreating the file system.

::

    ceph fs new <fs_name> <metadata_pool> <data_pool> --fscid <fscid> --force

Next, reset the file system. The below command marks the state of the
file system's rank 0 such that eventually when a MDS daemon picks up rank 0 the
daemon reads the existing in-RADOS metadata and doesn't overwrite it.

::

    ceph fs reset <fs_name> --yes-i-really-mean-it

Restart the MDSs. Check that the file system is no longer in degraded state and
one of the MDSs is active.

::

    ceph fs dump

Reapply any other custom file system settings.