diff options
Diffstat (limited to 'src')
| -rw-r--r-- | src/wal.c | 95 |
1 files changed, 54 insertions, 41 deletions
@@ -141,21 +141,33 @@ ** more index blocks. ** ** The wal-index header contains the total number of frames within the WAL -** in the the mxFrame field. Each index block contains information on -** HASHTABLE_NPAGE frames. Each index block contains two sections, a -** mapping which is a database page number for each frame, and a hash -** table used to look up frames by page number. The mapping section is -** an array of HASHTABLE_NPAGE 32-bit page numbers. The first entry on the -** array is the page number for the first frame; the second entry is the -** page number for the second frame; and so forth. The last index block -** holds a total of (mxFrame%HASHTABLE_NPAGE) page numbers. All index -** blocks other than the last are completely full with HASHTABLE_NPAGE -** page numbers. All index blocks are the same size; the mapping section -** of the last index block merely contains unused entries if mxFrame is -** not an even multiple of HASHTABLE_NPAGE. +** in the the mxFrame field. +** +** Each index block except for the first contains information on +** HASHTABLE_NPAGE frames. The first index block contains information on +** HASHTABLE_NPAGE_ONE frames. The values of HASHTABLE_NPAGE_ONE and +** HASHTABLE_NPAGE are selected so that together the wal-index header and +** first index block are the same size as all other index blocks in the +** wal-index. +** +** Each index block contains two sections, a page-mapping that contains the +** database page number associated with each wal frame, and a hash-table +** that allows users to query an index block for a specific page number. +** The page-mapping is an array of HASHTABLE_NPAGE (or HASHTABLE_NPAGE_ONE +** for the first index block) 32-bit page numbers. The first entry in the +** first index-block contains the database page number corresponding to the +** first frame in the WAL file. The first entry in the second index block +** in the WAL file corresponds to the (HASHTABLE_NPAGE_ONE+1)th frame in +** the log, and so on. +** +** The last index block in a wal-index usually contains less than the full +** complement of HASHTABLE_NPAGE (or HASHTABLE_NPAGE_ONE) page-numbers, +** depending on the contents of the WAL file. This does not change the +** allocated size of the page-mapping array - the page-mapping array merely +** contains unused entries. ** ** Even without using the hash table, the last frame for page P -** can be found by scanning the mapping sections of each index block +** can be found by scanning the page-mapping sections of each index block ** starting with the last index block and moving toward the first, and ** within each index block, starting at the end and moving toward the ** beginning. The first entry that equals P corresponds to the frame @@ -393,6 +405,33 @@ struct Wal { typedef u16 ht_slot; /* +** This structure is used to implement an iterator that loops through +** all frames in the WAL in database page order. Where two or more frames +** correspond to the same database page, the iterator visits only the +** frame most recently written to the WAL (in other words, the frame with +** the largest index). +** +** The internals of this structure are only accessed by: +** +** walIteratorInit() - Create a new iterator, +** walIteratorNext() - Step an iterator, +** walIteratorFree() - Free an iterator. +** +** This functionality is used by the checkpoint code (see walCheckpoint()). +*/ +struct WalIterator { + int iPrior; /* Last result returned from the iterator */ + int nSegment; /* Size of the aSegment[] array */ + struct WalSegment { + int iNext; /* Next slot in aIndex[] not yet returned */ + ht_slot *aIndex; /* i0, i1, i2... such that aPgno[iN] ascend */ + u32 *aPgno; /* Array of page numbers. */ + int nEntry; /* Max size of aPgno[] and aIndex[] arrays */ + int iZero; /* Frame number associated with aPgno[0] */ + } aSegment[1]; /* One for every 32KB page in the WAL */ +}; + +/* ** Define the parameters of the hash tables in the wal-index file. There ** is a hash-table following every HASHTABLE_NPAGE page numbers in the ** wal-index. @@ -404,7 +443,8 @@ typedef u16 ht_slot; #define HASHTABLE_HASH_1 383 /* Should be prime */ #define HASHTABLE_NSLOT (HASHTABLE_NPAGE*2) /* Must be a power of 2 */ -/* The block of page numbers associated with the first hash-table in a +/* +** The block of page numbers associated with the first hash-table in a ** wal-index is smaller than usual. This is so that there is a complete ** hash-table on each aligned 32KB page of the wal-index. */ @@ -470,33 +510,6 @@ static volatile WalIndexHdr *walIndexHdr(Wal *pWal){ } /* -** This structure is used to implement an iterator that loops through -** all frames in the WAL in database page order. Where two or more frames -** correspond to the same database page, the iterator visits only the -** frame most recently written to the WAL (in other words, the frame with -** the largest index). -** -** The internals of this structure are only accessed by: -** -** walIteratorInit() - Create a new iterator, -** walIteratorNext() - Step an iterator, -** walIteratorFree() - Free an iterator. -** -** This functionality is used by the checkpoint code (see walCheckpoint()). -*/ -struct WalIterator { - int iPrior; /* Last result returned from the iterator */ - int nSegment; /* Size of the aSegment[] array */ - struct WalSegment { - int iNext; /* Next slot in aIndex[] not yet returned */ - ht_slot *aIndex; /* i0, i1, i2... such that aPgno[iN] ascend */ - u32 *aPgno; /* Array of page numbers. */ - int nEntry; /* Max size of aPgno[] and aIndex[] arrays */ - int iZero; /* Frame number associated with aPgno[0] */ - } aSegment[1]; /* One for every 32KB page in the WAL */ -}; - -/* ** The argument to this macro must be of type u32. On a little-endian ** architecture, it returns the u32 value that results from interpreting ** the 4 bytes as a big-endian value. On a big-endian architecture, it |