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// For a general description of the files used by the cache see file_format.h.
6//
7// A block file is a file designed to store blocks of data of a given size. It
8// is able to store data that spans from one to four consecutive "blocks", and
9// it grows as needed to store up to approximately 65000 blocks. It has a fixed
10// size header used for book keeping such as tracking free of blocks on the
11// file. For example, a block-file for 1KB blocks will grow from 8KB when
12// totally empty to about 64MB when completely full. At that point, data blocks
13// of 1KB will be stored on a second block file that will store the next set of
14// 65000 blocks. The first file contains the number of the second file, and the
15// second file contains the number of a third file, created when the second file
16// reaches its limit. It is important to remember that no matter how long the
17// chain of files is, any given block can be located directly by its address,
18// which contains the file number and starting block inside the file.
19
20#ifndef NET_DISK_CACHE_DISK_FORMAT_BASE_H_
21#define NET_DISK_CACHE_DISK_FORMAT_BASE_H_
22
23#include "base/basictypes.h"
24#include "net/base/net_export.h"
25
26namespace disk_cache {
27
28typedef uint32 CacheAddr;
29
30const uint32 kBlockVersion2 = 0x20000;  // Version 2.0.
31
32const uint32 kBlockMagic = 0xC104CAC3;
33const int kBlockHeaderSize = 8192;  // Two pages: almost 64k entries
34const int kMaxBlocks = (kBlockHeaderSize - 80) * 8;
35
36// Bitmap to track used blocks on a block-file.
37typedef uint32 AllocBitmap[kMaxBlocks / 32];
38
39// A block-file is the file used to store information in blocks (could be
40// EntryStore blocks, RankingsNode blocks or user-data blocks).
41// We store entries that can expand for up to 4 consecutive blocks, and keep
42// counters of the number of blocks available for each type of entry. For
43// instance, an entry of 3 blocks is an entry of type 3. We also keep track of
44// where did we find the last entry of that type (to avoid searching the bitmap
45// from the beginning every time).
46// This Structure is the header of a block-file:
47struct BlockFileHeader {
48  uint32          magic;
49  uint32          version;
50  int16           this_file;    // Index of this file.
51  int16           next_file;    // Next file when this one is full.
52  int32           entry_size;   // Size of the blocks of this file.
53  int32           num_entries;  // Number of stored entries.
54  int32           max_entries;  // Current maximum number of entries.
55  int32           empty[4];     // Counters of empty entries for each type.
56  int32           hints[4];     // Last used position for each entry type.
57  volatile int32  updating;     // Keep track of updates to the header.
58  int32           user[5];
59  AllocBitmap     allocation_map;
60};
61
62COMPILE_ASSERT(sizeof(BlockFileHeader) == kBlockHeaderSize, bad_header);
63
64// Sparse data support:
65// We keep a two level hierarchy to enable sparse data for an entry: the first
66// level consists of using separate "child" entries to store ranges of 1 MB,
67// and the second level stores blocks of 1 KB inside each child entry.
68//
69// Whenever we need to access a particular sparse offset, we first locate the
70// child entry that stores that offset, so we discard the 20 least significant
71// bits of the offset, and end up with the child id. For instance, the child id
72// to store the first megabyte is 0, and the child that should store offset
73// 0x410000 has an id of 4.
74//
75// The child entry is stored the same way as any other entry, so it also has a
76// name (key). The key includes a signature to be able to identify children
77// created for different generations of the same resource. In other words, given
78// that a given sparse entry can have a large number of child entries, and the
79// resource can be invalidated and replaced with a new version at any time, it
80// is important to be sure that a given child actually belongs to certain entry.
81//
82// The full name of a child entry is composed with a prefix ("Range_"), and two
83// hexadecimal 64-bit numbers at the end, separated by semicolons. The first
84// number is the signature of the parent key, and the second number is the child
85// id as described previously. The signature itself is also stored internally by
86// the child and the parent entries. For example, a sparse entry with a key of
87// "sparse entry name", and a signature of 0x052AF76, may have a child entry
88// named "Range_sparse entry name:052af76:4", which stores data in the range
89// 0x400000 to 0x4FFFFF.
90//
91// Each child entry keeps track of all the 1 KB blocks that have been written
92// to the entry, but being a regular entry, it will happily return zeros for any
93// read that spans data not written before. The actual sparse data is stored in
94// one of the data streams of the child entry (at index 1), while the control
95// information is stored in another stream (at index 2), both by parents and
96// the children.
97
98// This structure contains the control information for parent and child entries.
99// It is stored at offset 0 of the data stream with index 2.
100// It is possible to write to a child entry in a way that causes the last block
101// to be only partialy filled. In that case, last_block and last_block_len will
102// keep track of that block.
103struct SparseHeader {
104  int64 signature;          // The parent and children signature.
105  uint32 magic;             // Structure identifier (equal to kIndexMagic).
106  int32 parent_key_len;     // Key length for the parent entry.
107  int32 last_block;         // Index of the last written block.
108  int32 last_block_len;     // Lenght of the last written block.
109  int32 dummy[10];
110};
111
112// The SparseHeader will be followed by a bitmap, as described by this
113// structure.
114struct SparseData {
115  SparseHeader header;
116  uint32 bitmap[32];        // Bitmap representation of known children (if this
117                            // is a parent entry), or used blocks (for child
118                            // entries. The size is fixed for child entries but
119                            // not for parents; it can be as small as 4 bytes
120                            // and as large as 8 KB.
121};
122
123// The number of blocks stored by a child entry.
124const int kNumSparseBits = 1024;
125COMPILE_ASSERT(sizeof(SparseData) == sizeof(SparseHeader) + kNumSparseBits / 8,
126               Invalid_SparseData_bitmap);
127
128}  // namespace disk_cache
129
130#endif  // NET_DISK_CACHE_DISK_FORMAT_BASE_H_
131