micro improvements for rocksdb options (#8433)

CHANGELOG

@@ -1,6 +1,15 @@
 devel
 -----
 
+* renamed hidden startup option `--rocksdb.delayed_write_rate` to the more
+  consistent `--rocksdb.delayed-write-rate`. When the old option name is
+  used, the arangod startup will be aborted with a descriptive error message.
+
+* if not explicitly configured, make agency nodes start removing their unused 
+  WAL files a few seconds after the completed server startup already. This is
+  because on agency nodes, unused WAL files do not need to be retained for
+  potential replication clients to read them. 
+
 * added option `--all-databases` to arangodump and arangorestore
 
   When set to true, this makes arangodump dump all available databases

Documentation/Books/Manual/ReleaseNotes/UpgradingChanges35.md

@@ -6,6 +6,12 @@ upgrading to ArangoDB 3.5, and adjust any client programs if necessary.
 
 The following incompatible changes have been made in ArangoDB 3.5:
 
+Startup options
+---------------
+
+The hidden startup option `--rocksdb.delayed_write_rate` was renamed to the more
+consistent `--rocksdb.delayed-write-rate`. When the old option name is used, the 
+arangod startup will be aborted with a descriptive error message.
 
 Web interface
 -------------

arangod/RocksDBEngine/RocksDBEngine.cpp

@@ -22,6 +22,7 @@
 /// @author Jan Christoph Uhde
 ////////////////////////////////////////////////////////////////////////////////
 
+#include "ApplicationFeatures/ApplicationServer.h"
 #include "ApplicationFeatures/RocksDBOptionFeature.h"
 #include "Basics/Exceptions.h"
 #include "Basics/FileUtils.h"
@@ -356,6 +357,13 @@ void RocksDBEngine::start() {
   if (!isEnabled()) {
     return;
   }
+  
+  if (ServerState::instance()->isAgent() &&
+      !application_features::ApplicationServer::server->options()->processingResult().touched("rocksdb.wal-file-timeout-initial")) {
+    // reduce --rocksb.wal-file-timeout-initial to 15 seconds for agency nodes
+    // as we probably won't need the WAL for WAL tailing and replication here
+    _pruneWaitTimeInitial = 15; 
+  }
 
   LOG_TOPIC(TRACE, arangodb::Logger::ENGINES)
       << "rocksdb version " << rest::Version::getRocksDBVersion()
@@ -2328,4 +2336,4 @@ void RocksDBEngine::releaseTick(TRI_voc_tick_t tick) {
 
 // -----------------------------------------------------------------------------
 // --SECTION--                                                       END-OF-FILE
-// -----------------------------------------------------------------------------
+// -----------------------------------------------------------------------------

lib/ApplicationFeatures/ApplicationServer.h

@@ -230,6 +230,9 @@ class ApplicationServer {
 
   // return VPack options
   VPackBuilder options(std::unordered_set<std::string> const& excludes) const;
+  
+  // return the program options object
+  std::shared_ptr<options::ProgramOptions> options() const { return _options; }
 
   // return the server state
   ServerState state() const { return _state; }

lib/ApplicationFeatures/RocksDBOptionFeature.cpp

@@ -153,13 +153,14 @@ void RocksDBOptionFeature::collectOptions(std::shared_ptr<ProgramOptions> option
                      new UInt64Parameter(&_maxTotalWalSize));
 
   options->addOption(
-      "--rocksdb.delayed_write_rate",
+      "--rocksdb.delayed-write-rate",
       "limited write rate to DB (in bytes per second) if we are writing to the "
       "last mem-table allowed and we allow more than 3 mem-tables, or if we "
       "have surpassed a certain number of level-0 files and need to slowdown "
       "writes",
       new UInt64Parameter(&_delayedWriteRate),
       arangodb::options::makeFlags(arangodb::options::Flags::Hidden));
+  options->addOldOption("rocksdb.delayed_write_rate", "rocksdb.delayed-write-rate");
 
   options->addOption("--rocksdb.min-write-buffer-number-to-merge",
                      "minimum number of write buffers that will be merged "