doc/cephfs: mention RADOS object size limit

Reflect the recent change in this limit
for 12.x.

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

diff --git a/doc/cephfs/file-layouts.rst b/doc/cephfs/file-layouts.rst
index a268016577349cbca36aa3b10f0eba325fbee37d..4124e1e061df22339121f92ffb2ac1d567012488 100644 (file)
--- a/doc/cephfs/file-layouts.rst
+++ b/doc/cephfs/file-layouts.rst
@@ -34,6 +34,14 @@ stripe_count
 object_size
     Integer in bytes.  File data is chunked into RADOS objects of this size.
 
+.. tip::
+
+    RADOS enforces a configurable limit on object sizes: if you increase CephFS
+    object sizes beyond that limit then writes may not succeed.  The OSD
+    setting is ``rados_max_object_size``, which is 128MB by default.
+    Very large RADOS objects may prevent smooth operation of the cluster,
+    so increasing the object size limit past the default is not recommended.
+
 Reading layouts with ``getfattr``
 ---------------------------------