Adding upstream version 18.2.2.upstream/18.2.2

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+=========================
+Create a Ceph file system
+=========================
+
+Creating pools
+==============
+
+A Ceph file system requires at least two RADOS pools, one for data and one for metadata.
+There are important considerations when planning these pools:
+
+- We recommend configuring *at least* 3 replicas for the metadata pool,
+  as data loss in this pool can render the entire file system inaccessible.
+  Configuring 4 would not be extreme, especially since the metadata pool's
+  capacity requirements are quite modest.
+- We recommend the fastest feasible low-latency storage devices (NVMe, Optane,
+  or at the very least SAS/SATA SSD) for the metadata pool, as this will
+  directly affect the latency of client file system operations.
+- We strongly suggest that the CephFS metadata pool be provisioned on dedicated
+  SSD / NVMe OSDs. This ensures that high client workload does not adversely
+  impact metadata operations. See :ref:`device_classes` to configure pools this
+  way.
+- The data pool used to create the file system is the "default" data pool and
+  the location for storing all inode backtrace information, which is used for hard link
+  management and disaster recovery. For this reason, all CephFS inodes
+  have at least one object in the default data pool. If erasure-coded
+  pools are planned for file system data, it is best to configure the default as
+  a replicated pool to improve small-object write and
+  read performance when updating backtraces. Separately, another erasure-coded
+  data pool can be added (see also :ref:`ecpool`) that can be used on an entire
+  hierarchy of directories and files (see also :ref:`file-layouts`).
+
+Refer to :doc:`/rados/operations/pools` to learn more about managing pools.  For
+example, to create two pools with default settings for use with a file system, you
+might run the following commands:
+
+.. code:: bash
+
+    $ ceph osd pool create cephfs_data
+    $ ceph osd pool create cephfs_metadata
+
+The metadata pool will typically hold at most a few gigabytes of data. For
+this reason, a smaller PG count is usually recommended. 64 or 128 is commonly
+used in practice for large clusters.
+
+.. note:: The names of the file systems, metadata pools, and data pools can
+          only have characters in the set [a-zA-Z0-9\_-.].
+
+Creating a file system
+======================
+
+Once the pools are created, you may enable the file system using the ``fs new`` command:
+
+.. code:: bash
+
+    $ ceph fs new <fs_name> <metadata> <data> [--force] [--allow-dangerous-metadata-overlay] [<fscid:int>] [--recover]
+
+This command creates a new file system with specified metadata and data pool.
+The specified data pool is the default data pool and cannot be changed once set.
+Each file system has its own set of MDS daemons assigned to ranks so ensure that
+you have sufficient standby daemons available to accommodate the new file system.
+
+The ``--force`` option is used to achieve any of the following:
+
+- To set an erasure-coded pool for the default data pool. Use of an EC pool for the
+  default data pool is discouraged. Refer to `Creating pools`_ for details.
+- To set non-empty pool (pool already contains some objects) for the metadata pool.
+- To create a file system with a specific file system's ID (fscid).
+  The --force option is required with --fscid option.
+
+The ``--allow-dangerous-metadata-overlay`` option permits the reuse metadata and
+data pools if it is already in-use. This should only be done in emergencies and
+after careful reading of the documentation.
+
+If the ``--fscid`` option is provided then this creates a file system with a
+specific fscid. This can be used when an application expects the file system's ID
+to be stable after it has been recovered, e.g., after monitor databases are
+lost and rebuilt. Consequently, file system IDs don't always keep increasing
+with newer file systems.
+
+The ``--recover`` option sets the state of file system's rank 0 to existing but
+failed. So when a MDS daemon eventually picks up rank 0, the daemon reads the
+existing in-RADOS metadata and doesn't overwrite it. The flag also prevents the
+standby MDS daemons to join the file system.
+
+For example:
+
+.. code:: bash
+
+    $ ceph fs new cephfs cephfs_metadata cephfs_data
+    $ ceph fs ls
+    name: cephfs, metadata pool: cephfs_metadata, data pools: [cephfs_data ]
+
+Once a file system has been created, your MDS(s) will be able to enter
+an *active* state.  For example, in a single MDS system:
+
+.. code:: bash
+
+    $ ceph mds stat
+    cephfs-1/1/1 up {0=a=up:active}
+
+Once the file system is created and the MDS is active, you are ready to mount
+the file system.  If you have created more than one file system, you will
+choose which to use when mounting.
+
+  - `Mount CephFS`_
+  - `Mount CephFS as FUSE`_
+  - `Mount CephFS on Windows`_
+
+.. _Mount CephFS: ../../cephfs/mount-using-kernel-driver
+.. _Mount CephFS as FUSE: ../../cephfs/mount-using-fuse
+.. _Mount CephFS on Windows: ../../cephfs/ceph-dokan
+
+If you have created more than one file system, and a client does not
+specify a file system when mounting, you can control which file system
+they will see by using the ``ceph fs set-default`` command.
+
+Adding a Data Pool to the File System 
+-------------------------------------
+
+See :ref:`adding-data-pool-to-file-system`.
+
+
+Using Erasure Coded pools with CephFS
+=====================================
+
+You may use Erasure Coded pools as CephFS data pools as long as they have overwrites enabled, which is done as follows:
+
+.. code:: bash
+
+    ceph osd pool set my_ec_pool allow_ec_overwrites true
+    
+Note that EC overwrites are only supported when using OSDs with the BlueStore backend.
+
+You may not use Erasure Coded pools as CephFS metadata pools, because CephFS metadata is stored using RADOS *OMAP* data structures, which EC pools cannot store.
+