doc/start/hardware-recommendations: refresh recommendations for RAM

Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit 6af6127748e7ee7e4c3bcd7e82d618aad07d5a1d)

diff --git a/doc/start/hardware-recommendations.rst b/doc/start/hardware-recommendations.rst
index eac5dc8c9f0be8444e25ac25808f5797e3eea56d..2ad982e39e6823065c7dbe7a57440b47d6158aa6 100644 (file)
--- a/doc/start/hardware-recommendations.rst
+++ b/doc/start/hardware-recommendations.rst
@@ -39,11 +39,29 @@ separate hosts.
 RAM
 ===
 
-Metadata servers and monitors must be capable of serving their data quickly, so
-they should have plenty of RAM (e.g., 1GB of RAM per daemon instance). OSDs do
-not require as much RAM for regular operations (e.g., 500MB of RAM per daemon
-instance); however, during recovery they need significantly more RAM (e.g., ~1GB
-per 1TB of storage per daemon). Generally, more RAM is better.
+Generally, more RAM is better.
+
+Monitors and managers (ceph-mon and ceph-mgr)
+---------------------------------------------
+
+Monitor and manager daemon memory usage generally scales with the size of the
+cluster.  For small clusters, 1-2 GB is generally sufficient.  For
+large clusters, you should provide more (5-10 GB).  You may also want
+to consider tuning settings like ``mon_osd_cache_size`` or
+``rocksdb_cache_size``.
+
+Metadata servers (ceph-mds)
+---------------------------
+
+The metadata daemon memory utilization depends on how much memory its cache is
+configured to consume.  We recommend 1 GB as a minimum for most systems.  See
+``mds_cache_memory``.
+
+OSDs (ceph-osd)
+---------------
+
+By default, OSDs that use the BlueStore backend require 3-5 GB of RAM.  You can
+adjust the amount of memory the OSD consumes with the ``osd_memory_target`` configuration option when BlueStore is in use.  When using the legacy FileStore backend, the operating system page cache is used for caching data, so no tuning is normally needed, and the OSD memory consumption is generally related to the number of PGs per daemon in the system.
 
 
 Data Storage