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// Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
// The cache is stored on disk as a collection of block-files, plus an index
// file plus a collection of external files.
// Any data blob bigger than kMaxBlockSize (net/addr.h) will be stored on a
// separate file named f_xxx where x is a hexadecimal number. Shorter data will
// be stored as a series of blocks on a block-file. In any case, CacheAddr
// represents the address of the data inside the cache.
// The index file is just a simple hash table that maps a particular entry to
// a CacheAddr value. Linking for a given hash bucket is handled internally
// by the cache entry.
// The last element of the cache is the block-file. A block file is a file
// designed to store blocks of data of a given size. It is able to store data
// that spans from one to four consecutive "blocks", and it grows as needed to
// store up to approximately 65000 blocks. It has a fixed size header used for
// book keeping such as tracking free of blocks on the file. For example, a
// block-file for 1KB blocks will grow from 8KB when totally empty to about 64MB
// when completely full. At that point, data blocks of 1KB will be stored on a
// second block file that will store the next set of 65000 blocks. The first
// file contains the number of the second file, and the second file contains the
// number of a third file, created when the second file reaches its limit. It is
// important to remember that no matter how long the chain of files is, any
// given block can be located directly by its address, which contains the file
// number and starting block inside the file.
// A new cache is initialized with four block files (named data_0 through
// data_3), each one dedicated to store blocks of a given size. The number at
// the end of the file name is the block file number (in decimal).
// There are two "special" types of blocks: an entry and a rankings node. An
// entry keeps track of all the information related to the same cache entry,
// such as the key, hash value, data pointers etc. A rankings node keeps track
// of the information that is updated frequently for a given entry, such as its
// location on the LRU lists, last access time etc.
// The files that store internal information for the cache (blocks and index)
// are at least partially memory mapped. They have a location that is signaled
// every time the internal structures are modified, so it is possible to detect
// (most of the time) when the process dies in the middle of an update.
// In order to prevent dirty data to be used as valid (after a crash), every
// cache entry has a dirty identifier. Each running instance of the cache keeps
// a separate identifier (maintained on the "this_id" header field) that is used
// to mark every entry that is created or modified. When the entry is closed,
// and all the data can be trusted, the dirty flag is cleared from the entry.
// When the cache encounters an entry whose identifier is different than the one
// being currently used, it means that the entry was not properly closed on a
// previous run, so it is discarded.
#include "base/basictypes.h"
namespace disk_cache {
typedef uint32 CacheAddr;
const int kIndexTablesize = 0x10000;
const uint32 kIndexMagic = 0xC103CAC3;
const uint32 kCurrentVersion = 0x20000; // Version 2.0.
struct LruData {
CacheAddr heads[5];
CacheAddr tails[5];
CacheAddr transaction; // In-flight operation target.
int32 operation; // Actual in-flight operation.
int32 operation_list; // In-flight operation list.
int32 pad[7];
// Header for the master index file.
struct IndexHeader {
uint32 magic;
uint32 version;
int32 num_entries; // Number of entries currently stored.
int32 num_bytes; // Total size of the stored data.
int32 last_file; // Last external file created.
int32 this_id; // Id for all entries being changed (dirty flag).
CacheAddr stats; // Storage for usage data.
int32 table_len; // Actual size of the table (0 == kIndexTablesize).
int32 crash; // Signals a previous crash.
int32 experiment; // Id of an ongoing test.
int32 pad[62];
LruData lru; // Eviction control data.
IndexHeader() {
memset(this, 0, sizeof(*this));
magic = kIndexMagic;
version = kCurrentVersion;
// The structure of the whole index file.
struct Index {
IndexHeader header;
CacheAddr table[kIndexTablesize]; // Default size. Actual size controlled
// by header.table_len.
// Main structure for an entry on the backing storage. If the key is longer than
// what can be stored on this structure, it will be extended on consecutive
// blocks (adding 256 bytes each time), up to 4 blocks (1024 - 32 - 1 chars).
// After that point, the whole key will be stored as a data block or external
// file.
struct EntryStore {
uint32 hash; // Full hash of the key.
CacheAddr next; // Next entry with the same hash or bucket.
CacheAddr rankings_node; // Rankings node for this entry.
int32 reuse_count; // How often is this entry used.
int32 refetch_count; // How often is this fetched from the net.
int32 state; // Current state.
uint64 creation_time;
int32 key_len;
CacheAddr long_key; // Optional address of a long key.
int32 data_size[4]; // We can store up to 4 data streams for each
CacheAddr data_addr[4]; // entry.
int32 pad[6];
char key[256 - 24 * 4]; // null terminated
COMPILE_ASSERT(sizeof(EntryStore) == 256, bad_EntyStore);
const int kMaxInternalKeyLength = 4 * sizeof(EntryStore) -
offsetof(EntryStore, key) - 1;
// Possible states for a given entry.
enum EntryState {
ENTRY_EVICTED, // The entry was recently evicted from the cache.
ENTRY_DOOMED // The entry was doomed.
#pragma pack(push, old, 4)
// Rankings information for a given entry.
struct RankingsNode {
uint64 last_used; // LRU info.
uint64 last_modified; // LRU info.
CacheAddr next; // LRU list.
CacheAddr prev; // LRU list.
CacheAddr contents; // Address of the EntryStore.
int32 dirty; // The entry is being modifyied.
void* pointer; // Pointer to the in-memory entry.
#pragma pack(pop, old)
COMPILE_ASSERT(sizeof(RankingsNode) == 36, bad_RankingsNode);
const uint32 kBlockMagic = 0xC104CAC3;
const int kBlockHeaderSize = 8192; // Two pages: almost 64k entries
const int kMaxBlocks = (kBlockHeaderSize - 80) * 8;
// Bitmap to track used blocks on a block-file.
typedef uint32 AllocBitmap[kMaxBlocks / 32];
// A block-file is the file used to store information in blocks (could be
// EntryStore blocks, RankingsNode blocks or user-data blocks).
// We store entries that can expand for up to 4 consecutive blocks, and keep
// counters of the number of blocks available for each type of entry. For
// instance, an entry of 3 blocks is an entry of type 3. We also keep track of
// where did we find the last entry of that type (to avoid searching the bitmap
// from the beginning every time).
// This Structure is the header of a block-file:
struct BlockFileHeader {
uint32 magic;
uint32 version;
int16 this_file; // Index of this file.
int16 next_file; // Next file when this one is full.
int32 entry_size; // Size of the blocks of this file.
int32 num_entries; // Number of stored entries.
int32 max_entries; // Current maximum number of entries.
int32 empty[4]; // Counters of empty entries for each type.
int32 hints[4]; // Last used position for each entry type.
volatile int32 updating; // Keep track of updates to the header.
int32 user[5];
AllocBitmap allocation_map;
BlockFileHeader() {
memset(this, 0, sizeof(BlockFileHeader));
magic = kBlockMagic;
version = kCurrentVersion;
COMPILE_ASSERT(sizeof(BlockFileHeader) == kBlockHeaderSize, bad_header);
} // namespace disk_cache