explain what an inline xattr is and how it relates to omap

The logic of the configuration flags related to xattr is clarified to define what an inline xattr is and when storing in the object map is preferred.

http://tracker.ceph.com/issues/4617 refs #4617

Signed-off-by: Loic Dachary <loic@dachary.org>

diff --git a/doc/rados/configuration/filestore-config-ref.rst b/doc/rados/configuration/filestore-config-ref.rst
index 80aed8e81a4ef8d5a7bc1747f4932107dc501e24..64eaaec1300563003d0638934d44abe33ab0ca84 100644 (file)
--- a/doc/rados/configuration/filestore-config-ref.rst
+++ b/doc/rados/configuration/filestore-config-ref.rst
@@ -21,6 +21,12 @@ Additionally, in some cases, the filesystem may not be as fast as an alternative
 method of storing XATTRs. The following settings may help improve performance
 by using a method of storing XATTRs that is extrinsic to the underlying filesystem.
 
+Ceph XATTRs are stored as ``inline xattr``, using the XATTRs provided
+by the underlying file system, if it does not impose a size limit. If
+there is a size limit ( 4KB total on ext4, for instance ), some Ceph
+XATTRs will be stored in an key/value database ( aka ``omap`` ) when
+the ``filestore max inline xattr size`` or ``filestore max inline
+xattrs`` threshold are reached.
 
 ``filestore xattr use omap``