doc: update bluestore cache settings and clarify data fraction

Fixes: http://tracker.ceph.com/issues/39522
Signed-off-by: Jan Fajerski <jfajerski@suse.com>
(cherry picked from commit 9d8336a7f418fe2bb11361dd74a214403b1e5be7)

diff --git a/doc/rados/configuration/bluestore-config-ref.rst b/doc/rados/configuration/bluestore-config-ref.rst
index d7e70ee92c6bed520edeb5c430af951f10001c3d..507ca5b8c872869bda754032efd67f2a48e4c789 100644 (file)
--- a/doc/rados/configuration/bluestore-config-ref.rst
+++ b/doc/rados/configuration/bluestore-config-ref.rst
@@ -231,12 +231,13 @@ The configured cache memory budget can be used in a few different ways:
 * BlueStore data (i.e., recently read or written object data)
 
 Cache memory usage is governed by the following options:
-``bluestore_cache_meta_ratio``, ``bluestore_cache_kv_ratio``, and
-``bluestore_cache_kv_max``.  The fraction of the cache devoted to data
-is 1.0 minus the meta and kv ratios.  The memory devoted to kv
-metadata (the RocksDB cache) is capped by ``bluestore_cache_kv_max``
-since our testing indicates there are diminishing returns beyond a
-certain point.
+``bluestore_cache_meta_ratio`` and ``bluestore_cache_kv_ratio``.
+The fraction of the cache devoted to data
+is governed by the effective bluestore cache size (depending on
+``bluestore_cache_size[_ssd|_hdd]`` settings and the device class of the primary
+device) as well as the meta and kv ratios.
+The data fraction can be calculated by
+``<effective_cache_size> * (1 - bluestore_cache_meta_ratio - bluestore_cache_kv_ratio)``
 
 ``bluestore_cache_size``
 
@@ -264,14 +265,14 @@ certain point.
 :Description: The ratio of cache devoted to metadata.
 :Type: Floating point
 :Required: Yes
-:Default: ``.01``
+:Default: ``.4``
 
 ``bluestore_cache_kv_ratio``
 
 :Description: The ratio of cache devoted to key/value data (rocksdb).
 :Type: Floating point
 :Required: Yes
-:Default: ``.99``
+:Default: ``.4``
 
 ``bluestore_cache_kv_max``