ceph/src/rocksdb/java/src/main/java/org/rocksdb/IndexShorteningMode.java

   1 // Copyright (c) 2011-present, Facebook, Inc.  All rights reserved.
   2 //  This source code is licensed under both the GPLv2 (found in the
   3 //  COPYING file in the root directory) and Apache 2.0 License
   4 //  (found in the LICENSE.Apache file in the root directory).
   5 package org.rocksdb;
   6
   7 /**
   8  * This enum allows trading off increased index size for improved iterator
   9  * seek performance in some situations, particularly when block cache is
  10  * disabled ({@link ReadOptions#fillCache()} == false and direct IO is
  11  * enabled ({@link DBOptions#useDirectReads()} == true).
  12  * The default mode is the best tradeoff for most use cases.
  13  * This option only affects newly written tables.
  14  *
  15  * The index contains a key separating each pair of consecutive blocks.
  16  * Let A be the highest key in one block, B the lowest key in the next block,
  17  * and I the index entry separating these two blocks:
  18  * [ ... A] I [B ...]
  19  * I is allowed to be anywhere in [A, B).
  20  * If an iterator is seeked to a key in (A, I], we'll unnecessarily read the
  21  * first block, then immediately fall through to the second block.
  22  * However, if I=A, this can't happen, and we'll read only the second block.
  23  * In kNoShortening mode, we use I=A. In other modes, we use the shortest
  24  * key in [A, B), which usually significantly reduces index size.
  25  *
  26  * There's a similar story for the last index entry, which is an upper bound
  27  * of the highest key in the file. If it's shortened and therefore
  28  * overestimated, iterator is likely to unnecessarily read the last data block
  29  * from each file on each seek.
  30  */
  31 public enum IndexShorteningMode {
  32   /**
  33    * Use full keys.
  34    */
  35   kNoShortening((byte) 0),
  36   /**
  37    * Shorten index keys between blocks, but use full key for the last index
  38    * key, which is the upper bound of the whole file.
  39    */
  40   kShortenSeparators((byte) 1),
  41   /**
  42    * Shorten both keys between blocks and key after last block.
  43    */
  44   kShortenSeparatorsAndSuccessor((byte) 2);
  45
  46   private final byte value;
  47
  48   IndexShorteningMode(final byte value) {
  49     this.value = value;
  50   }
  51
  52   /**
  53    * Returns the byte value of the enumerations value.
  54    *
  55    * @return byte representation
  56    */
  57   byte getValue() {
  58     return value;
  59   }
  60 }