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224 行
7.5 KiB
224 行
7.5 KiB
// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file. See the AUTHORS file for names of contributors.
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#ifndef STORAGE_LEVELDB_DB_DBFORMAT_H_
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#define STORAGE_LEVELDB_DB_DBFORMAT_H_
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#include <cstddef>
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#include <cstdint>
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#include <string>
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#include "leveldb/comparator.h"
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#include "leveldb/db.h"
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#include "leveldb/filter_policy.h"
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#include "leveldb/slice.h"
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#include "leveldb/table_builder.h"
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#include "util/coding.h"
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#include "util/logging.h"
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namespace leveldb {
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// Grouping of constants. We may want to make some of these
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// parameters set via options.
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namespace config {
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static const int kNumLevels = 3;
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// Level-0 compaction is started when we hit this many files.
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static const int kL0_CompactionTrigger = 4;
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// Soft limit on number of level-0 files. We slow down writes at this point.
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static const int kL0_SlowdownWritesTrigger = 8;
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// Maximum number of level-0 files. We stop writes at this point.
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static const int kL0_StopWritesTrigger = 12;
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// Maximum level to which a new compacted memtable is pushed if it
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// does not create overlap. We try to push to level 2 to avoid the
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// relatively expensive level 0=>1 compactions and to avoid some
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// expensive manifest file operations. We do not push all the way to
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// the largest level since that can generate a lot of wasted disk
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// space if the same key space is being repeatedly overwritten.
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static const int kMaxMemCompactLevel = 2;
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// Approximate gap in bytes between samples of data read during iteration.
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static const int kReadBytesPeriod = 1048576;
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} // namespace config
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class InternalKey;
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// Value types encoded as the last component of internal keys.
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// DO NOT CHANGE THESE ENUM VALUES: they are embedded in the on-disk
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// data structures.
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enum ValueType { kTypeDeletion = 0x0, kTypeValue = 0x1 };
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// kValueTypeForSeek defines the ValueType that should be passed when
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// constructing a ParsedInternalKey object for seeking to a particular
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// sequence number (since we sort sequence numbers in decreasing order
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// and the value type is embedded as the low 8 bits in the sequence
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// number in internal keys, we need to use the highest-numbered
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// ValueType, not the lowest).
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static const ValueType kValueTypeForSeek = kTypeValue;
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typedef uint64_t SequenceNumber;
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// We leave eight bits empty at the bottom so a type and sequence#
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// can be packed together into 64-bits.
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static const SequenceNumber kMaxSequenceNumber = ((0x1ull << 56) - 1);
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struct ParsedInternalKey {
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Slice user_key;
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SequenceNumber sequence;
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ValueType type;
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ParsedInternalKey() {} // Intentionally left uninitialized (for speed)
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ParsedInternalKey(const Slice& u, const SequenceNumber& seq, ValueType t)
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: user_key(u), sequence(seq), type(t) {}
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std::string DebugString() const;
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};
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// Return the length of the encoding of "key".
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inline size_t InternalKeyEncodingLength(const ParsedInternalKey& key) {
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return key.user_key.size() + 8;
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}
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// Append the serialization of "key" to *result.
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void AppendInternalKey(std::string* result, const ParsedInternalKey& key);
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// Attempt to parse an internal key from "internal_key". On success,
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// stores the parsed data in "*result", and returns true.
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//
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// On error, returns false, leaves "*result" in an undefined state.
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bool ParseInternalKey(const Slice& internal_key, ParsedInternalKey* result);
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// Returns the user key portion of an internal key.
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inline Slice ExtractUserKey(const Slice& internal_key) {
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assert(internal_key.size() >= 8);
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return Slice(internal_key.data(), internal_key.size() - 8);
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}
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// A comparator for internal keys that uses a specified comparator for
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// the user key portion and breaks ties by decreasing sequence number.
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class InternalKeyComparator : public Comparator {
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private:
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const Comparator* user_comparator_;
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public:
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explicit InternalKeyComparator(const Comparator* c) : user_comparator_(c) {}
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const char* Name() const override;
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int Compare(const Slice& a, const Slice& b) const override;
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void FindShortestSeparator(std::string* start,
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const Slice& limit) const override;
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void FindShortSuccessor(std::string* key) const override;
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const Comparator* user_comparator() const { return user_comparator_; }
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int Compare(const InternalKey& a, const InternalKey& b) const;
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};
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// Filter policy wrapper that converts from internal keys to user keys
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class InternalFilterPolicy : public FilterPolicy {
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private:
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const FilterPolicy* const user_policy_;
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public:
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explicit InternalFilterPolicy(const FilterPolicy* p) : user_policy_(p) {}
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const char* Name() const override;
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void CreateFilter(const Slice* keys, int n, std::string* dst) const override;
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bool KeyMayMatch(const Slice& key, const Slice& filter) const override;
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};
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// Modules in this directory should keep internal keys wrapped inside
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// the following class instead of plain strings so that we do not
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// incorrectly use string comparisons instead of an InternalKeyComparator.
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class InternalKey {
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private:
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std::string rep_;
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public:
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InternalKey() {} // Leave rep_ as empty to indicate it is invalid
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InternalKey(const Slice& user_key, SequenceNumber s, ValueType t) {
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AppendInternalKey(&rep_, ParsedInternalKey(user_key, s, t));
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}
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bool DecodeFrom(const Slice& s) {
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rep_.assign(s.data(), s.size());
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return !rep_.empty();
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}
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Slice Encode() const {
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assert(!rep_.empty());
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return rep_;
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}
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Slice user_key() const { return ExtractUserKey(rep_); }
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void SetFrom(const ParsedInternalKey& p) {
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rep_.clear();
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AppendInternalKey(&rep_, p);
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}
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void Clear() { rep_.clear(); }
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std::string DebugString() const;
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};
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inline int InternalKeyComparator::Compare(const InternalKey& a,
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const InternalKey& b) const {
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return Compare(a.Encode(), b.Encode());
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}
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inline bool ParseInternalKey(const Slice& internal_key,
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ParsedInternalKey* result) {
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const size_t n = internal_key.size();
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if (n < 8) return false;
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uint64_t num = DecodeFixed64(internal_key.data() + n - 8);
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uint8_t c = num & 0xff;
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result->sequence = num >> 8;
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result->type = static_cast<ValueType>(c);
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result->user_key = Slice(internal_key.data(), n - 8);
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return (c <= static_cast<uint8_t>(kTypeValue));
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}
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// A helper class useful for DBImpl::Get()
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class LookupKey {
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public:
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// Initialize *this for looking up user_key at a snapshot with
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// the specified sequence number.
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LookupKey(const Slice& user_key, SequenceNumber sequence);
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LookupKey(const LookupKey&) = delete;
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LookupKey& operator=(const LookupKey&) = delete;
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~LookupKey();
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// Return a key suitable for lookup in a MemTable.
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Slice memtable_key() const { return Slice(start_, end_ - start_); }
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// Return an internal key (suitable for passing to an internal iterator)
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Slice internal_key() const { return Slice(kstart_, end_ - kstart_); }
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// Return the user key
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Slice user_key() const { return Slice(kstart_, end_ - kstart_ - 8); }
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private:
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// We construct a char array of the form:
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// klength varint32 <-- start_
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// userkey char[klength] <-- kstart_
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// tag uint64
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// <-- end_
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// The array is a suitable MemTable key.
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// The suffix starting with "userkey" can be used as an InternalKey.
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const char* start_;
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const char* kstart_;
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const char* end_;
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char space_[200]; // Avoid allocation for short keys
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};
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inline LookupKey::~LookupKey() {
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if (start_ != space_) delete[] start_;
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}
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} // namespace leveldb
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#endif // STORAGE_LEVELDB_DB_DBFORMAT_H_
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