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Chrome Disk Cache Format

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Revision as of 09:09, 22 June 2014 by Joachim Metz (Talk | contribs) (Data block file format (data_#))

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Cache files

The cache is stored in multiple:

Filename Description
index The index file
data_# Data block files
f_###### (Separate) data stream file

Cache address

The cache address is 4 bytes in size and consists of:

offset size value description
If file type is 0 (Separate file)
0.0 28 bits File number
The value represents the value of # in f_######
Else
0.0 16 bits Block number
2.0 8 bits File number (or file selector)
The value represents the value of # in data_#
3.0 2 bits Block size
The number of contiguous blocks where 0 represents 1 block and 3 represents 4 blocks.
3.2 2 bits Reserved
Common
3.4 3 bits File type
3.7 1 bit Initialized flag

File types

Value Description
0 (Separate) data stream file
1 (Rankings) block data file (36 byte block data file)
2 256 byte block data file
3 1024 byte block data file
4 4096 byte block data file
6 Unknown; seen on Mac OS X 0x6f430074

Examples

Value Description
0x00000000 Not initialized
0x8000002a Data stream file: f_00002a
0xa0010003 Block data file: data_1, block number 3, 1 block of size

Index file format (index)

Overview:

  • File header
  • least recently used (LRU) data (or eviction control data)
  • index table

File header

  • TODO*

Data block file format (data_#)

Overview:

  • File header
  • array of blocks

File header

The index file header (struct BlockFileHeader) is 8192 bytes in size and consists of:

offset size value description
0 4 "\xc3\xca\x04\xc1" Signature
4 2 Minor version
6 2 Major version
8 2 File number (or file index)
The value represents the value of # in data_#
10 2 Next file number (or next file index)
The value represents the value of # in data_#
12 4 Block size (or cache entry) size
16 4 Number of entries
20 4 Maximum number of entries
24 4 x 4 = 16 Array of empty entry counters
The counters of empty entries for each type
40 4 x 4 = 16 Array of last used position (hints)
The last used position for each type
56 4 Updating
Value to keep track of updates to the header
60 5 x 4 = 20 User
80 2028 x 4 = 8112 Block allocation bitmap

Format versions

Value Description
2.0
2.1

Cache entry

The cache entry (struct EntryStore) is 256 bytes in size and consists of:

offset size value description
0 4 Hash
The hash of the key
4 4 Next entry cache address
The next entry with the same hash or bucket
8 4 Rankings node cache address
12 4 Reuse count
Value that indicates how often this entry was (re-)used
16 4 Refetch count
Value that indicates how often this entry was (re-)fetched (or downloaded)
20 4 Cache entry state
24 8 Creation time
32 4 Key data size
36 4 Long key data cache address
The value is 0 if no long key data is present
40 4 x 4 = 16 Array of data stream sizes
56 4 x 4 = 16 Array of data stream cache addresses
72 4 Cache entry flags
76 4 x 4 = 16 Padding
92 4 Self hash
The hash of the first 92 bytes of the cache entry is this used as a checksum?
96 160 Key data
Contains an UTF-8 encoded string with an end-of-string character with the original URL

Array indexes

The data stream array indexes correspond to:

Value Description
0 HTTP response headers
1 Page contents (Payload)
2
3

Cache entry state

Value Identifier Description
0 ENTRY_NORMAL Normal cache entry
1 ENTRY_EVICTED The cache entry was recently evicted
2 ENTRY_DOOMED The cache entry was doomed

Cache entry flags

Value Identifier Description
0x00000001 PARENT_ENTRY Parent cache entry
0x00000002 CHILD_ENTRY Child cache entry

Rankings node

The rankings node (struct RankingsNode) is 36 bytes in size and consists of:

offset size value description
0 8 Last used
Contains LRU information?
8 8 Last modified
Contains LRU information?
16 4 Next rankings node cache address
20 4 Previous rankings node cache address
24 4 Cache entry cache address
28 4 Is dirty flag
32 4 Self hash
The hash of the first 32 bytes of the rankings node is this used as a checksum?

Data stream

See: gzip

See Also

External Links