Added DbLedgerStorage configuration keys with default values in bookkeeper.conf (#123)

conf/bookkeeper.conf

 #
 # Copyright 2016 Yahoo Inc.
-# 
+#
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
 # You may obtain a copy of the License at
-# 
+#
 #     http://www.apache.org/licenses/LICENSE-2.0
-# 
+#
 # Unless required by applicable law or agreed to in writing, software
 # distributed under the License is distributed on an "AS IS" BASIS,
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-# 
+#
 
 ## Bookie settings
 
@@ -43,7 +43,7 @@ journalDirectory=data/bookkeeper/journal
 # could define multi directories to store snapshots, separated by ','
 # For example:
 # ledgerDirectories=/tmp/bk1-data,/tmp/bk2-data
-# 
+#
 # Ideally ledger dirs and journal dir are each in a differet device,
 # which reduce the contention between random i/o and sequential write.
 # It is possible to run with a single disk, but performance will be significantly lower.
@@ -78,7 +78,7 @@ ledgerStorageClass=org.apache.bookkeeper.bookie.storage.ldb.DbLedgerStorage
 # minorCompactionThreshold=0.2
 
 # Interval to run minor compaction, in seconds
-# If it is set to less than zero, the minor compaction is disabled. 
+# If it is set to less than zero, the minor compaction is disabled.
 # minorCompactionInterval=3600
 
 # Threshold of major compaction
@@ -90,8 +90,8 @@ ledgerStorageClass=org.apache.bookkeeper.bookie.storage.ldb.DbLedgerStorage
 majorCompactionThreshold=0.5
 
 # Interval to run major compaction, in seconds
-# If it is set to less than zero, the major compaction is disabled. 
-# majorCompactionInterval=86400 
+# If it is set to less than zero, the major compaction is disabled.
+# majorCompactionInterval=86400
 
 # Set the maximum number of entries which can be compacted without flushing.
 # When compacting, the entries are written to the entrylog and the new offsets
@@ -106,7 +106,7 @@ majorCompactionThreshold=0.5
 # Set the rate at which compaction will readd entries. The unit is adds per second.
 #compactionRate=1000
 
-# Throttle compaction by bytes or by entries. 
+# Throttle compaction by bytes or by entries.
 #isThrottleByBytes=false
 
 # Set the rate at which compaction will readd entries. The unit is adds per second.
@@ -247,7 +247,7 @@ readOnlyModeEnabled=true
 #be written to that partition. If all ledger dir partions are full, then bookie
 #will turn to readonly mode if 'readOnlyModeEnabled=true' is set, else it will
 #shutdown.
-#Valid values should be in between 0 and 1 (exclusive). 
+#Valid values should be in between 0 and 1 (exclusive).
 #diskUsageThreshold=0.95
 
 #Disk check interval in milli seconds, interval to check the ledger dirs usage.
@@ -296,10 +296,36 @@ readBufferSizeBytes=4096
 
 # Stats Provider Class
 #statsProviderClass=org.apache.bookkeeper.stats.CodahaleMetricsProvider
+#codahaleStatsJmxEndpoint=metrics
 
-dbStorage_readAheadCacheBatchSize=1000
-dbStorage_rocksDBEnabled=true
-dbStorage_writeCacheMaxSizeMb=256
+
+## DB Ledger storage configuration
+
+# Size of Write Cache. Memory is allocated from JVM direct memory.
+# Write cache is used to buffer entries before flushing into the entry log
+# For good performance, it should be big enough to hold a sub
+dbStorage_writeCacheMaxSizeMb=512
+
+# Size of Read cache. Memory is allocated from JVM direct memory.
+# This read cache is pre-filled doing read-ahead whenever a cache miss happens
 dbStorage_readAheadCacheMaxSizeMb=256
 
-codahaleStatsJmxEndpoint=metrics
+# How many entries to pre-fill in cache after a read cache miss
+dbStorage_readAheadCacheBatchSize=1000
+
+## RocksDB specific configurations
+## DbLedgerStorage uses RocksDB to store the indexes from
+## (ledgerId, entryId) -> (entryLog, offset)
+
+# Size of RocksDB block-cache. For best performance, this cache
+# should be big enough to hold a significant portion of the index
+# database which can reach ~2GB in some cases
+# dbStorage_rocksDB_blockCacheSize=268435456 # 256 MBytes
+
+# dbStorage_rocksDB_writeBufferSizeMB=64
+# dbStorage_rocksDB_sstSizeInMB=64
+# dbStorage_rocksDB_blockSize=65536
+# dbStorage_rocksDB_bloomFilterBitsPerKey=10
+# dbStorage_rocksDB_numLevels=-1
+# dbStorage_rocksDB_numFilesInLevel0=4
+# dbStorage_rocksDB_maxSizeInLevel1MB=256