doc/cephfs: add info on using EC pools with CephFS

Signed-off-by: John Spray <john.spray@redhat.com>

diff --git a/doc/cephfs/createfs.rst b/doc/cephfs/createfs.rst
index 005ede86e52981fbaadaab5405b0bdaf5763cdd2..7bd4511b2d89fd52934e0e7beed126347cf89d60 100644 (file)
--- a/doc/cephfs/createfs.rst
+++ b/doc/cephfs/createfs.rst
@@ -2,14 +2,8 @@
 Create a Ceph filesystem
 ========================
 
-.. tip::
-
-    The ``ceph fs new`` command was introduced in Ceph 0.84.  Prior to this release,
-    no manual steps are required to create a filesystem, and pools named ``data`` and
-    ``metadata`` exist by default.
-
-    The Ceph command line now includes commands for creating and removing filesystems,
-    but at present only one filesystem may exist at a time.
+Creating pools
+==============
 
 A Ceph filesystem requires at least two RADOS pools, one for data and one for metadata.
 When configuring these pools, you might consider:
@@ -29,6 +23,9 @@ might run the following commands:
     $ ceph osd pool create cephfs_data <pg_num>
     $ ceph osd pool create cephfs_metadata <pg_num>
 
+Creating a filesystem
+=====================
+
 Once the pools are created, you may enable the filesystem using the ``fs new`` command:
 
 .. code:: bash
@@ -60,3 +57,17 @@ choose which to use when mounting.
 
 .. _Mount CephFS: ../../cephfs/kernel
 .. _Mount CephFS as FUSE: ../../cephfs/fuse
+
+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.
+