1 // Copyright (c) 2011 The Chromium Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style license that can be 3 // found in the LICENSE file. 4 5 // The cache is stored on disk as a collection of block-files, plus an index 6 // file plus a collection of external files. 7 // 8 // Any data blob bigger than kMaxBlockSize (disk_cache/addr.h) will be stored in 9 // a separate file named f_xxx where x is a hexadecimal number. Shorter data 10 // will be stored as a series of blocks on a block-file. In any case, CacheAddr 11 // represents the address of the data inside the cache. 12 // 13 // The index file is just a simple hash table that maps a particular entry to 14 // a CacheAddr value. Linking for a given hash bucket is handled internally 15 // by the cache entry. 16 // 17 // The last element of the cache is the block-file. A block file is a file 18 // designed to store blocks of data of a given size. For more details see 19 // disk_cache/disk_format_base.h 20 // 21 // A new cache is initialized with four block files (named data_0 through 22 // data_3), each one dedicated to store blocks of a given size. The number at 23 // the end of the file name is the block file number (in decimal). 24 // 25 // There are two "special" types of blocks: an entry and a rankings node. An 26 // entry keeps track of all the information related to the same cache entry, 27 // such as the key, hash value, data pointers etc. A rankings node keeps track 28 // of the information that is updated frequently for a given entry, such as its 29 // location on the LRU lists, last access time etc. 30 // 31 // The files that store internal information for the cache (blocks and index) 32 // are at least partially memory mapped. They have a location that is signaled 33 // every time the internal structures are modified, so it is possible to detect 34 // (most of the time) when the process dies in the middle of an update. 35 // 36 // In order to prevent dirty data to be used as valid (after a crash), every 37 // cache entry has a dirty identifier. Each running instance of the cache keeps 38 // a separate identifier (maintained on the "this_id" header field) that is used 39 // to mark every entry that is created or modified. When the entry is closed, 40 // and all the data can be trusted, the dirty flag is cleared from the entry. 41 // When the cache encounters an entry whose identifier is different than the one 42 // being currently used, it means that the entry was not properly closed on a 43 // previous run, so it is discarded. 44 45 #ifndef NET_DISK_CACHE_BLOCKFILE_DISK_FORMAT_H_ 46 #define NET_DISK_CACHE_BLOCKFILE_DISK_FORMAT_H_ 47 48 #include "base/basictypes.h" 49 #include "net/base/net_export.h" 50 #include "net/disk_cache/blockfile/disk_format_base.h" 51 52 namespace disk_cache { 53 54 const int kIndexTablesize = 0x10000; 55 const uint32 kIndexMagic = 0xC103CAC3; 56 const uint32 kCurrentVersion = 0x20000; // Version 2.0. 57 58 struct LruData { 59 int32 pad1[2]; 60 int32 filled; // Flag to tell when we filled the cache. 61 int32 sizes[5]; 62 CacheAddr heads[5]; 63 CacheAddr tails[5]; 64 CacheAddr transaction; // In-flight operation target. 65 int32 operation; // Actual in-flight operation. 66 int32 operation_list; // In-flight operation list. 67 int32 pad2[7]; 68 }; 69 70 // Header for the master index file. 71 struct NET_EXPORT_PRIVATE IndexHeader { 72 IndexHeader(); 73 74 uint32 magic; 75 uint32 version; 76 int32 num_entries; // Number of entries currently stored. 77 int32 num_bytes; // Total size of the stored data. 78 int32 last_file; // Last external file created. 79 int32 this_id; // Id for all entries being changed (dirty flag). 80 CacheAddr stats; // Storage for usage data. 81 int32 table_len; // Actual size of the table (0 == kIndexTablesize). 82 int32 crash; // Signals a previous crash. 83 int32 experiment; // Id of an ongoing test. 84 uint64 create_time; // Creation time for this set of files. 85 int32 pad[52]; 86 LruData lru; // Eviction control data. 87 }; 88 89 // The structure of the whole index file. 90 struct Index { 91 IndexHeader header; 92 CacheAddr table[kIndexTablesize]; // Default size. Actual size controlled 93 // by header.table_len. 94 }; 95 96 // Main structure for an entry on the backing storage. If the key is longer than 97 // what can be stored on this structure, it will be extended on consecutive 98 // blocks (adding 256 bytes each time), up to 4 blocks (1024 - 32 - 1 chars). 99 // After that point, the whole key will be stored as a data block or external 100 // file. 101 struct EntryStore { 102 uint32 hash; // Full hash of the key. 103 CacheAddr next; // Next entry with the same hash or bucket. 104 CacheAddr rankings_node; // Rankings node for this entry. 105 int32 reuse_count; // How often is this entry used. 106 int32 refetch_count; // How often is this fetched from the net. 107 int32 state; // Current state. 108 uint64 creation_time; 109 int32 key_len; 110 CacheAddr long_key; // Optional address of a long key. 111 int32 data_size[4]; // We can store up to 4 data streams for each 112 CacheAddr data_addr[4]; // entry. 113 uint32 flags; // Any combination of EntryFlags. 114 int32 pad[4]; 115 uint32 self_hash; // The hash of EntryStore up to this point. 116 char key[256 - 24 * 4]; // null terminated 117 }; 118 119 COMPILE_ASSERT(sizeof(EntryStore) == 256, bad_EntyStore); 120 const int kMaxInternalKeyLength = 4 * sizeof(EntryStore) - 121 offsetof(EntryStore, key) - 1; 122 123 // Possible states for a given entry. 124 enum EntryState { 125 ENTRY_NORMAL = 0, 126 ENTRY_EVICTED, // The entry was recently evicted from the cache. 127 ENTRY_DOOMED // The entry was doomed. 128 }; 129 130 // Flags that can be applied to an entry. 131 enum EntryFlags { 132 PARENT_ENTRY = 1, // This entry has children (sparse) entries. 133 CHILD_ENTRY = 1 << 1 // Child entry that stores sparse data. 134 }; 135 136 #pragma pack(push, 4) 137 // Rankings information for a given entry. 138 struct RankingsNode { 139 uint64 last_used; // LRU info. 140 uint64 last_modified; // LRU info. 141 CacheAddr next; // LRU list. 142 CacheAddr prev; // LRU list. 143 CacheAddr contents; // Address of the EntryStore. 144 int32 dirty; // The entry is being modifyied. 145 uint32 self_hash; // RankingsNode's hash. 146 }; 147 #pragma pack(pop) 148 149 COMPILE_ASSERT(sizeof(RankingsNode) == 36, bad_RankingsNode); 150 151 } // namespace disk_cache 152 153 #endif // NET_DISK_CACHE_BLOCKFILE_DISK_FORMAT_H_ 154