Disk cache: Reference CL for the implementation of file format version 3.

net/disk_cache/disk_format_base.h

Index: net/disk_cache/disk_format_base.h
===================================================================
--- net/disk_cache/disk_format_base.h	(revision 185975)
+++ net/disk_cache/disk_format_base.h	(working copy)
@@ -2,58 +2,23 @@
 // 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.
+// For a general description of the files used by the cache see file_format.h.
 //
-// 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.
+// 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.
 
-#ifndef NET_DISK_CACHE_DISK_FORMAT_H_
-#define NET_DISK_CACHE_DISK_FORMAT_H_
+#ifndef NET_DISK_CACHE_DISK_FORMAT_BASE_H_
+#define NET_DISK_CACHE_DISK_FORMAT_BASE_H_
 
 #include "base/basictypes.h"
 #include "net/base/net_export.h"
@@ -62,106 +27,13 @@
 
 typedef uint32 CacheAddr;
 
-const int kIndexTablesize = 0x10000;
-const uint32 kIndexMagic = 0xC103CAC3;
-const uint32 kCurrentVersion = 0x20000;  // Version 2.0.
+const uint32 kBlockVersion2 = 0x20000;  // Version 2.0.
+const uint32 kBlockCurrentVersion = 0x30000;  // Version 3.0.
 
-struct LruData {
-  int32     pad1[2];
-  int32     filled;          // Flag to tell when we filled the cache.
-  int32     sizes[5];
-  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     pad2[7];
-};
-
-// Header for the master index file.
-struct NET_EXPORT_PRIVATE IndexHeader {
-  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.
-  uint64      create_time;   // Creation time for this set of files.
-  int32       pad[52];
-  LruData     lru;           // Eviction control data.
-};
-
-// 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.
-  uint32      flags;              // Any combination of EntryFlags.
-  int32       pad[4];
-  uint32      self_hash;          // The hash of EntryStore up to this point.
-  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_NORMAL = 0,
-  ENTRY_EVICTED,    // The entry was recently evicted from the cache.
-  ENTRY_DOOMED      // The entry was doomed.
-};
-
-// Flags that can be applied to an entry.
-enum EntryFlags {
-  PARENT_ENTRY = 1,         // This entry has children (sparse) entries.
-  CHILD_ENTRY = 1 << 1      // Child entry that stores sparse data.
-};
-
-#pragma pack(push, 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.
-  uint32      self_hash;        // RankingsNode's hash.
-};
-#pragma pack(pop)
-
-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;
+const int kNumExtraBlocks = 1024;
 
 // Bitmap to track used blocks on a block-file.
 typedef uint32 AllocBitmap[kMaxBlocks / 32];
@@ -173,10 +45,12 @@
 // 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).
+//
+// The only difference between versions 2 and 3 is that the latter stores the
+// header and allocation file in a dedicated file, and the data in another file,
+// instead of using a single file for both things (as does version 2).
 // This Structure is the header of a block-file:
 struct NET_EXPORT_PRIVATE BlockFileHeader {
-  BlockFileHeader();
-
   uint32          magic;
   uint32          version;
   int16           this_file;    // Index of this file.
@@ -259,4 +133,4 @@
 
 }  // namespace disk_cache
 
-#endif  // NET_DISK_CACHE_DISK_FORMAT_H_
+#endif  // NET_DISK_CACHE_DISK_FORMAT_BASE_H_