mds: add "mds traceless replies" debug option

This option specifies (in the range 0-1) the percentage of modifying
operations that should be responded to without including a trace
for the dentries.
In order to implement it, we add a "modify" parameter to
set_trace_dist(), and the callers fill that with
mdr->req->may_write().

Signed-off-by: Greg Farnum <greg@inktank.com>
Reviewed-by: Sam Lang <sam.lang@inktank.com>

3 files changed, 15 insertions, 4 deletions

diff --git a/src/common/config_opts.h b/src/common/config_opts.h
index 5e0449e3606..fcd2f8f8540 100644
--- a/src/common/config_opts.h
+++ b/src/common/config_opts.h
@@ -276,6 +276,8 @@ OPTION(mds_kill_export_at, OPT_INT, 0)
 OPTION(mds_kill_import_at, OPT_INT, 0)
 OPTION(mds_kill_link_at, OPT_INT, 0)
 OPTION(mds_kill_rename_at, OPT_INT, 0)
+OPTION(mds_traceless_replies, OPT_DOUBLE, 0) /* [0-1] amount of time MDS
+				should skip sending the client a trace */
 OPTION(mds_wipe_sessions, OPT_BOOL, 0)
 OPTION(mds_wipe_ino_prealloc, OPT_BOOL, 0)
 OPTION(mds_skip_ino, OPT_INT, 0)
diff --git a/src/mds/Server.cc b/src/mds/Server.cc
index 92f5bf64268..8348be7a68a 100644
--- a/src/mds/Server.cc
+++ b/src/mds/Server.cc
@@ -837,7 +837,7 @@ void Server::early_reply(MDRequest *mdr, CInode *tracei, CDentry *tracedn)
       mdr->cap_releases.erase(tracedn->get_dir()->get_inode()->vino());
 
     set_trace_dist(mdr->session, reply, tracei, tracedn, mdr->snapid,
-		   mdr->client_request->get_dentry_wanted());
+		   mdr->client_request->get_dentry_wanted(), req->may_write());
   }
 
   reply->set_extra_bl(mdr->reply_extra_bl);
@@ -918,7 +918,8 @@ void Server::reply_request(MDRequest *mdr, MClientReply *reply, CInode *tracei,
 	  mdcache->try_reconnect_cap(tracei, session);
       } else {
 	// include metadata in reply
-	set_trace_dist(session, reply, tracei, tracedn, snapid, dentry_wanted);
+	set_trace_dist(session, reply, tracei, tracedn,
+	               snapid, dentry_wanted, req->may_write());
       }
     }
 
@@ -975,8 +976,16 @@ void Server::encode_null_lease(bufferlist& bl)
 void Server::set_trace_dist(Session *session, MClientReply *reply,
 			    CInode *in, CDentry *dn,
 			    snapid_t snapid,
-			    int dentry_wanted)
+			    int dentry_wanted,
+			    bool modified)
 {
+  // skip doing this for debugging purposes?
+  if (modified && g_conf->mds_traceless_replies &&
+      (rand() % 10000 < g_conf->mds_traceless_replies * 10000.0)) {
+    dout(5) << "deliberately skipping trace for " << *reply << dendl;
+    return;
+  }
+
   // inode, dentry, dir, ..., inode
   bufferlist bl;
   int whoami = mds->get_nodeid();
diff --git a/src/mds/Server.h b/src/mds/Server.h
index 79977fc8dd5..4bf3f8604eb 100644
--- a/src/mds/Server.h
+++ b/src/mds/Server.h
@@ -103,7 +103,7 @@ public:
   void reply_request(MDRequest *mdr, MClientReply *reply, CInode *tracei = 0, CDentry *tracedn = 0);
   void set_trace_dist(Session *session, MClientReply *reply, CInode *in, CDentry *dn,
 		      snapid_t snapid,
-		      int num_dentries_wanted);
+		      int num_dentries_wanted, bool modified);
 
   void encode_empty_dirstat(bufferlist& bl);
   void encode_infinite_lease(bufferlist& bl);