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// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file. See the AUTHORS file for names of contributors.
#include <stdio.h>
#include "leveldb/comparator.h"
#include "leveldb/db.h"
#include "leveldb/filter_policy.h"
#include "leveldb/slice.h"
#include "leveldb/table_builder.h"
#include "util/coding.h"
#include "util/logging.h"
namespace leveldb {
// Grouping of constants. We may want to make some of these
// parameters set via options.
namespace config {
static const int kNumLevels = 7;
// Level-0 compaction is started when we hit this many files.
static const int kL0_CompactionTrigger = 4;
// Soft limit on number of level-0 files. We slow down writes at this point.
static const int kL0_SlowdownWritesTrigger = 8;
// Maximum number of level-0 files. We stop writes at this point.
static const int kL0_StopWritesTrigger = 12;
// Maximum level to which a new compacted memtable is pushed if it
// does not create overlap. We try to push to level 2 to avoid the
// relatively expensive level 0=>1 compactions and to avoid some
// expensive manifest file operations. We do not push all the way to
// the largest level since that can generate a lot of wasted disk
// space if the same key space is being repeatedly overwritten.
static const int kMaxMemCompactLevel = 2;
// Approximate gap in bytes between samples of data read during iteration.
static const int kReadBytesPeriod = 1048576;
} // namespace config
class InternalKey;
// Value types encoded as the last component of internal keys.
// DO NOT CHANGE THESE ENUM VALUES: they are embedded in the on-disk
// data structures.
enum ValueType {
kTypeDeletion = 0x0,
kTypeValue = 0x1
// kValueTypeForSeek defines the ValueType that should be passed when
// constructing a ParsedInternalKey object for seeking to a particular
// sequence number (since we sort sequence numbers in decreasing order
// and the value type is embedded as the low 8 bits in the sequence
// number in internal keys, we need to use the highest-numbered
// ValueType, not the lowest).
static const ValueType kValueTypeForSeek = kTypeValue;
typedef uint64_t SequenceNumber;
// We leave eight bits empty at the bottom so a type and sequence#
// can be packed together into 64-bits.
static const SequenceNumber kMaxSequenceNumber =
((0x1ull << 56) - 1);
struct ParsedInternalKey {
Slice user_key;
SequenceNumber sequence;
ValueType type;
ParsedInternalKey() { } // Intentionally left uninitialized (for speed)
ParsedInternalKey(const Slice& u, const SequenceNumber& seq, ValueType t)
: user_key(u), sequence(seq), type(t) { }
std::string DebugString() const;
// Return the length of the encoding of "key".
inline size_t InternalKeyEncodingLength(const ParsedInternalKey& key) {
return key.user_key.size() + 8;
// Append the serialization of "key" to *result.
extern void AppendInternalKey(std::string* result,
const ParsedInternalKey& key);
// Attempt to parse an internal key from "internal_key". On success,
// stores the parsed data in "*result", and returns true.
// On error, returns false, leaves "*result" in an undefined state.
extern bool ParseInternalKey(const Slice& internal_key,
ParsedInternalKey* result);
// Returns the user key portion of an internal key.
inline Slice ExtractUserKey(const Slice& internal_key) {
assert(internal_key.size() >= 8);
return Slice(, internal_key.size() - 8);
inline ValueType ExtractValueType(const Slice& internal_key) {
assert(internal_key.size() >= 8);
const size_t n = internal_key.size();
uint64_t num = DecodeFixed64( + n - 8);
unsigned char c = num & 0xff;
return static_cast<ValueType>(c);
// A comparator for internal keys that uses a specified comparator for
// the user key portion and breaks ties by decreasing sequence number.
class InternalKeyComparator : public Comparator {
const Comparator* user_comparator_;
explicit InternalKeyComparator(const Comparator* c) : user_comparator_(c) { }
virtual const char* Name() const;
virtual int Compare(const Slice& a, const Slice& b) const;
virtual void FindShortestSeparator(
std::string* start,
const Slice& limit) const;
virtual void FindShortSuccessor(std::string* key) const;
const Comparator* user_comparator() const { return user_comparator_; }
int Compare(const InternalKey& a, const InternalKey& b) const;
// Filter policy wrapper that converts from internal keys to user keys
class InternalFilterPolicy : public FilterPolicy {
const FilterPolicy* const user_policy_;
explicit InternalFilterPolicy(const FilterPolicy* p) : user_policy_(p) { }
virtual const char* Name() const;
virtual void CreateFilter(const Slice* keys, int n, std::string* dst) const;
virtual bool KeyMayMatch(const Slice& key, const Slice& filter) const;
// Modules in this directory should keep internal keys wrapped inside
// the following class instead of plain strings so that we do not
// incorrectly use string comparisons instead of an InternalKeyComparator.
class InternalKey {
std::string rep_;
InternalKey() { } // Leave rep_ as empty to indicate it is invalid
InternalKey(const Slice& user_key, SequenceNumber s, ValueType t) {
AppendInternalKey(&rep_, ParsedInternalKey(user_key, s, t));
void DecodeFrom(const Slice& s) { rep_.assign(, s.size()); }
Slice Encode() const {
return rep_;
Slice user_key() const { return ExtractUserKey(rep_); }
void SetFrom(const ParsedInternalKey& p) {
AppendInternalKey(&rep_, p);
void Clear() { rep_.clear(); }
std::string DebugString() const;
inline int InternalKeyComparator::Compare(
const InternalKey& a, const InternalKey& b) const {
return Compare(a.Encode(), b.Encode());
inline bool ParseInternalKey(const Slice& internal_key,
ParsedInternalKey* result) {
const size_t n = internal_key.size();
if (n < 8) return false;
uint64_t num = DecodeFixed64( + n - 8);
unsigned char c = num & 0xff;
result->sequence = num >> 8;
result->type = static_cast<ValueType>(c);
result->user_key = Slice(, n - 8);
return (c <= static_cast<unsigned char>(kTypeValue));
// A helper class useful for DBImpl::Get()
class LookupKey {
// Initialize *this for looking up user_key at a snapshot with
// the specified sequence number.
LookupKey(const Slice& user_key, SequenceNumber sequence);
// Return a key suitable for lookup in a MemTable.
Slice memtable_key() const { return Slice(start_, end_ - start_); }
// Return an internal key (suitable for passing to an internal iterator)
Slice internal_key() const { return Slice(kstart_, end_ - kstart_); }
// Return the user key
Slice user_key() const { return Slice(kstart_, end_ - kstart_ - 8); }
// We construct a char array of the form:
// klength varint32 <-- start_
// userkey char[klength] <-- kstart_
// tag uint64
// <-- end_
// The array is a suitable MemTable key.
// The suffix starting with "userkey" can be used as an InternalKey.
const char* start_;
const char* kstart_;
const char* end_;
char space_[200]; // Avoid allocation for short keys
// No copying allowed
LookupKey(const LookupKey&);
void operator=(const LookupKey&);
inline LookupKey::~LookupKey() {
if (start_ != space_) delete[] start_;
} // namespace leveldb
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