disk_format.h | Explore in Territory

// Copyright 2011 The Chromium Authors
// 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 (disk_cache/addr.h) will be stored in
// 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. For more details see
// disk_cache/disk_format_base.h
//
// 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.

#ifndef NET_DISK_CACHE_BLOCKFILE_DISK_FORMAT_H_
#define NET_DISK_CACHE_BLOCKFILE_DISK_FORMAT_H_

#include <stddef.h>
#include <stdint.h>
#include <string.h>

#include "net/base/net_export.h"
#include "net/disk_cache/blockfile/disk_format_base.h"

namespace disk_cache {

const int kIndexTablesize = …;
const uint32_t kIndexMagic = …;
const uint32_t kVersion2_0 = …;
const uint32_t kVersion2_1 = …;
const uint32_t kVersion3_0 = …;
const uint32_t kCurrentVersion = …;

struct LruData { … };

// Header for the master index file.
struct NET_EXPORT_PRIVATE IndexHeader { … };

// The structure of the whole index file.
struct Index { … };

// 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 { … };

static_assert …;
const int kMaxInternalKeyLength = …;

// Possible states for a given entry.
enum EntryState { … };

// Flags that can be applied to an entry.
enum EntryFlags { … };

#pragma pack(push, 4)
// Rankings information for a given entry.
struct RankingsNode { … };
#pragma pack(pop)

static_assert …;

}  // namespace disk_cache

#endif  // NET_DISK_CACHE_BLOCKFILE_DISK_FORMAT_H_
chromium/net/disk_cache/blockfile/disk_format.h
