Add fixture to drop page cache and SLAB objects

Closes elastic#30

Author: danielmitterdorfer

fixtures/ansible/roles/drop-caches/README.md

@@ -0,0 +1 @@
+Drops the page cache and SLAB objects as documented in https://www.kernel.org/doc/Documentation/sysctl/vm.txt.

fixtures/ansible/roles/drop-caches/tasks/main.yml

@@ -0,0 +1,12 @@
+---
+- name: Run sync to minimize the number of dirty objects on the system
+  command: /bin/sync
+  become: true
+
+# Give sync a little bit of time to finish. For rationale see notes in https://linux.die.net/man/8/sync.
+- pause:
+    seconds: 3
+
+- name: Free slab objects and pagecache
+  command: echo 3 > /proc/sys/vm/drop_caches
+  become: true

night_rally.sh

@@ -29,7 +29,7 @@ NIGHT_RALLY_HOME="$( cd -P "$( dirname "$SOURCE" )" && pwd )"
 # Night Rally is *always* the master for assets
 ASSET_SOURCE="${NIGHT_RALLY_HOME}/external/pages/*"
 
-ANSIBLE_ALL_TAGS=(encryption-at-rest initialize-data-disk trim)
+ANSIBLE_ALL_TAGS=(encryption-at-rest initialize-data-disk trim drop-caches)
 ANSIBLE_SKIP_TAGS=( )
 ANSIBLE_SKIP_TAGS_STRING=""
 SELF_UPDATE=NO